DN81 Lighting handbook Introduction Light Emitting Diodes (LEDs) are becoming more and more popular in general illumination. They offer benefits in energy efficiency, long life and ruggedness. LEDs are low voltage devices and both safe and easy to use. However, LEDs are ‘current driven’ devices and simply applying a voltage to drive them is not a good method of control. Current control schemes are essential to maintain constant brightness. Additionally, LEDs offer longevity which means that they can often work in excess of 50,000 hours. LED drivers play a key role in achieving maximum working life. In particular, switching regulators maximize electrical and thermal efficiency. Zetex provides a comprehensive range of high brightness LED drivers to suit a wide range of applications. These high efficiency drivers meet all these stringent requirements.. In this handbook, a broad range of design notes are included for customers to select the right device and application circuits. Test results and bill of materials are also included to provide a convenient means to achieve optimum solutions. Individual datasheets for all the devices mentioned in these these devices can be found on www.zetex.com. All the designs have been built and evaluated. However, users should satisfy themselves of the suitability for their specific application. Issue 7 - March 2008 © Zetex Semiconductors plc 2006 www.zetex.com Table of contents, ordered by application Flashlight, portable lighting DN61 Dual cell powered ZXSC310 solution for a 1W high power white LED. . . . . . . . . . . . . . . . 3 DN64 ZXSC310 Solution flashlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 DN67 ZXSC400 solution for 1W high powered LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 DN68 ZXSC310 High power torch reference design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 DN70 ZXSC400 Driving 2 serial high power LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 DN71 ZXSC400 Solution for Luxeon® V Star high powered LED . . . . . . . . . . . . . . . . . . . . . . . . 37 DN73 ZXSC300 Step down converter for 3W LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 DN78 ZXSC310 with reverse polarity protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 DN79 ZXSC400 1W LED driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 DN84 ZXSC400 Driving 3W high power LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 DN85 ZXSC400 1W/3W buck LED drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 General lighting, illumination, signage DN62 ZXSC310 Solution to drive 3 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 DN63 ZXSC310 Solution to drive 8 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 DN65 ZXSC310 Solution for emergency light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 DN69 ZXSC310 Garden light reference design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 DN75 ZXSC310 Solar powered garden light reference design . . . . . . . . . . . . . . . . . . . . . . . . . . 45 DN86 Reduced component count and compact reference design for MR16 replacement lamps using multiple 1W LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 DN83 LED MR16 Lamp solution using the ZXLD1350 LED driver . . . . . . . . . . . . . . . . . . . . . . . . 57 AN44 A high power LED driver for low voltage halogen replacement . . . . . . . . . . . . . . . . . . . . 81 Backlight DN62 ZXSC310 Solution to drive 3 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 DN63 ZXSC310 Solution to drive 8 LEDs connected in series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 DN66 An OLED bias supply for a clamshell handset sub display . . . . . . . . . . . . . . . . . . . . . . . . 15 DN72 ZXLD1101 Driving 8 series LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 DN76 ZXLD1100 and ZXLD1101 driving from 3 to 6 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Photoflash DN74 ZXSC400 Photoflash LED reference design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Miscellaneous AN47 Getting more out of the ZXLD1350 - dimming techniques . . . . . . . . . . . . . . . . . . . . . . . . 87 DN48 Getting more out of the ZXLD1350 - high output current . . . . . . . . . . . . . . . . . . . . . . . . . 95 AN50 Feed forward compensation for ZXSC300 LED driver . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Issue 7 - March 2008 © Zetex Semiconductors plc 2008 1 www.zetex.com Intentionally left blank www.zetex.com 2 Issue 7 - March 2008 © Zetex Semiconductors plc 2008 DN61 Dual cell powered ZXSC310 solution for a 1W high power white LED Khagendra Thapa, Principal Systems Engineer, Zetex Semiconductors Description High power LEDs are increasingly being used in lighting applications (general illumination, portable, signage/security, traffic, automotive, architectural) as lumens, and efficacy of high power LEDs are increasing while the cost per lumens is decreasing. Low cost, small and simple solutions are important in applications such as flashlight, signage and illuminations where 1W high power LED is powered from a low voltage supply as in single and dual cell batteries. Figure 1 shows a typical simple low cost solution with a ZXSC310 driving a 1W LED with a typical forward voltage of 3.4V at 300mA from a dual cell battery. A dual cell supply will have a voltage range of 1.8V to 2.5V for NiCd and NiMH type batteries and up to 3V for alkaline type batteries. The component values are tabulated (see Tables 1 and 2), depending on the range of voltage which is defined by the battery chemistry. VIN 1.5V to 2.5/3V C2 Stdn ZXSC310 RSENSE GND Figure 1 Typical dual cell battery powered 1W LED drive circuit ZXSC310 is a constant current boost converter in a small SOT23-5 package. It has a typical drive current of 2.3mA at 1.8V. The drive current at 25°C is 1.5mA minimum at 1.5V supply. The bipolar transistor switch, Q1, should have adequate voltage and peak switching current ratings, a very high transistor gain (hfe), a very low saturation voltage (VCE) and a small device package size with an adequate thermal capability. The transistor, Q1 in this application, is a low saturation voltage transistor, ZXTN25012EFL, with a very high gain of 700 at 1A collector current at 25°C to match the drive current from the Drive pin of the ZXSC310. Note: If transistors with lower gain are used, then at lower temperatures, it may not support a full switching current and therefore proper operation may not start or may take few seconds to start. The Schottky diode should have an adequate peak switching current rating and a very low forward voltage. The Zetex ZXSC1000 Schottky diode, SD1, has a low forward voltage. If operation at higher temperature is required then the low leakage, low forward voltage, Zetex ZLLS1000 can be used. The choice of inductor, L1, depends on the desired switching frequency, the LED current, the input voltage, forward voltage of the Schottky diode, SD1, and the LED forward voltage. Note: The LED current output is dependent on the input voltage, the LED forward voltage, the sense resistor and the inductor value. Issue 5 - August 2007 © Zetex Semiconductors plc 2006 3 www.zetex.com DN61 Dual cell NiCd/NiMH battery solution A dual cell NiCd/NiMH battery voltage range is 1.8V to 2.5V. Table 1 shows the component values for a dual cell NICd/NiMH battery powered ZXSC310 solution for a 1W high power white LED. The efficiency and the LED current versus the input voltage performance are shown in Figures 2 and 3. Efficiency vs Input voltage LED Current vs Supply voltage 100 0.4 90 Luxion LED Current (A) Efficiency (%) 80 70 With low im pedance pow er supply: Voltage ram ping dow n 60 50 With dual AA size 1300m Ahr NiMH battery 40 30 20 Luxion TM With dual AA size 1300mAhr NiMH battery 0.1 With low impedance power supply: voltage ramping down White LED 0 2.5 2.25 2 1.75 1.5 White LED With low impedance power supply: voltage ramping up 0.2 10 0 2.5 TM 0.3 2.25 1.25 2 1.75 1.5 1.25 Input voltage (V) Input voltage (V) Figure 2 Figure 3 Efficency vs. input supply voltage LED current vs. input supply voltage Reference Part no. Value Manufacturer Contact details U1 ZXSC310E5 LED driver Zetex www.zetex.com Q1 ZXTN25012EFH high gain, low VCE(sat) Zetex www.zetex.com SD1 ZHCS1000 or ZLLS1000 low forward voltage VF Zetex www.zetex.com L1 DO3316P-103 10H, 2A Coilcraft www.coilcraft.com RSENSE Generic 33m⍀ Generic NA R1 Generic 10k⍀ Generic NA C1 Generic 1F, 6.3V, X7R Generic NA C2 Generic 6.8F, 6.3V Generic NA LED1 LXHL-NW98 White LED; 3.4V Lumileds www.lumileds.com Table 1 Bill of materials for dual cell NiCd/NiMH battery powered single 1W LED driver www.zetex.com 4 Issue 5 - August 2007 © Zetex Semiconductors plc 2006 DN61 Dual cell alkaline battery solution The dual cell alkaline battery has a voltage range of up to 3V. Table 2 shows the component values for a dual cell alkaline battery powered ZXSC310 solution for a 1W high power white LED. The efficiency and the LED current versus the input voltage performance are shown in Figures 4 and 5. Efficiency vs Input voltage LED Current vs Supply voltage 100 0.4 90 Luxion TM White LED 70 LED Current (A) Efficiency (%) 80 With dual AA size 1300mAhr NiMH battery 60 50 With low impedance power supply: Voltage ramping down 40 30 20 Luxion TM White LED 0.3 With low impedance power supply: voltage ramping down 0.2 With dual AA size Alkaline battery 0.1 With low impedance power supply: voltage ramping up 10 0 3 2.75 2.5 2.25 2 1.75 1.5 1.25 0 1 3 Input voltage (V) Figure 4 2.75 2.5 2.25 2 1.75 1.5 1.25 Efficiency vs. input signal voltage Figure 5 LED current vs. input supply voltage Reference Part no. Value Manufacturer Contact details U1 ZXSC310E5 LED driver Zetex www.zetex.com Q1 ZXTN25012EFL high gain, low VCE(sat) Zetex www.zetex.com SD1 ZHCS1000 or ZLLS1000 low forward voltage Zetex VF www.zetex.com L1 DO3316P-103 10uH, 2A Coilcraft www.coilcraft.com RSENSE Generic 50m⍀ Generic NA R1 Generic 10k⍀ Generic NA C1 Generic 1F, 6.3V, X7R Generic NA C2 Generic 6.8F, 6.3V Generic NA LED1 LXHL-NW98 White LED Lumileds www.lumileds.com Table 2 1 Input voltage (V) Bill of materials for dual cell alkaline battery powered 1W LED driver Dimming and shutdown In Figure 1, the shutdown pin, Stdn, can be tied to VCC pin for normal operation. If the shutdown pin is taken to ground, the ZXSC310 enters standby mode with a low quiescent current of 5A. The shutdown pin can also be used for PWM dimming by connecting a PWM signal. The LED current is then dependent on PWM duty ratio. Thermal management The LED junction temperature should be maintained within the specified maximum or dederating curve, whichever is lower, by use of proper thermal management for lumens maintenance and LED protection. Size 0805 for the sense resistor is adequate. Issue 5 - August 2007 © Zetex Semiconductors plc 2006 5 www.zetex.com DN61 Boot-strap operation In boot-strap mode, the supply to the VCC is from the output stage (cathode of SD1) to maintain the supply to the ZXSC310 at a reasonably constant voltage even when the battery voltage reduces. This improves the ZXSC310 drive pin current capability due to the reasonably constant voltage of 3.4V typical (or the forward voltage of the LED) at the VCC pin, even though the battery voltage may drop below 1.5V. The boot-strap allows the ZXSC310 to continue driving the LED even with battery supply drops below 0.8V after the initial successful start-up. The boot-strap mode is recommended for a single cell alkaline/NiMH/NiCd battery. The boot-strap mode can also be used in throw-away (single use) dual cell alkaline batteries to draw as much energy as possible before discarding the battery. Figures 6 and 7 show the efficiency and LED current versus battery voltage for a boot-strap mode of operation with an AA size dual cell alkaline battery. Efficiency vs Input voltage in boot-strap mode LED Current vs Supply voltage in boot-strap mode 0.4 100 Luxion 90 70 LED Current (A) Efficiency (%) 80 With dual size With dual AAAA size Alkaline batteryin in boot-strap m mode Alkaline battery boost-starp ode 60 50 With pedance pow er supply: supply: voltage voltage Withlow lowimimpedance power ram ping dow n ininboost-strap m ode ramping down boot-strap mode 40 30 Luxion 10 White LED With low impedance power supply: voltage ramping down in boot-strap mode 0.2 With dual AA size Alkaline battery in boot-strap mode 0.1 20 TM TM 0.3 With low impedance power supply: voltage ramping up in boot-strap mode White LED 0 0 3 2.75 2.5 2.25 2 1.75 1.5 1.25 1 0.75 3 0.5 Figure 6 Efficiency vs. input supply voltage 2.75 2.5 2.25 2 1.75 1.5 1.25 1 0.75 0.5 Input voltage (V) Input voltage (V) Figure 7 LED current vs. input supply voltage Note: To prevent rechargeable batteries entering a deep discharge state, ZXSC310 devices can be shut down (by pulling the shutdown pin low to the ground) by an external circuit when the rechargeable battery voltage falls below its recommended minimum voltage. The boot-strap mode is not recommended with a ZXSC310 for dual/three cell NiCd/NiMH rechargeable batteries without a under voltage protection. www.zetex.com 6 Issue 5 - August 2007 © Zetex Semiconductors plc 2006 DN62 ZXSC310 Solution to drive 3 LEDs connected in series Description This solution is optimized for an input voltage range of 4.3V to 3V. The LED current is set to 15mA VIN = 4.3V and 8mA at VIN = 3V. ZXSC310 Issue 3 - July 2006 © Zetex Semiconductors plc 2006 Figure 1 Schematic diagram Figure 2 Performance graphs 7 www.zetex.com DN62 Reference Part no. Value Manufacturer Contact details U1 ZXSC310E5 NA Zetex www.zetex.com Q1 FMMT618 NA Zetex www.zetex.com D1 ZHCS1000 1A Zetex www.zetex.com R1 Generic 510m⍀ Generic NA R2 Generic 510F Generic NA C1 Generic 2.2F Generic NA L1 DO1608P-103 10H Coilcraft www.coilcraft.com LED1-3 NSPMW500BS White LED Nichia www.nichia.com Table 1 www.zetex.com Bill of materials 8 Issue 3 - July 2006 © Zetex Semiconductors plc 2006 DN63 ZXSC310 Solution to drive 8 LEDs connected in series Khagendra Thapa, Principal Systems Engineer, Zetex Semiconductors Description Low cost, small simple and low power multi-LED drive solutions are important in applications including LCD backlight, key illuminations and effects for handheld devices (e.g. cell phones), signage and indicators. The LED current is generally between 10mA to 30mA and is powered from a single cell Li-Ion or three cell alkaline/NiMH/NiCad batteries. For battery powered applications low shutdown quiescent current is important to conserve battery life. Figure 1 shows a simple low cost boost convertor, ZXSC310, driving eight series connected LEDs. ZXSC310 is in a small SOT23-5 package. The design solution is for an application with an input voltage range of 4.5V to 2.5V (e.g. a single cell Li-Ion can have a voltage range of 4.3V to 2.6V) with LED current optimized at 20mA typical, at 4.0V supply. The LED current at 4V is chosen to match the 20mA typical forward current of the LED used. D1 ZHCS1000 VIN 4.5V to 2.5V L1 68H LED1 U1 Q1 ZXTN25040DFH VCC LED2 Drv Stdn Stdn L1 68H ISENSE GND C1 100nF LED7 ZXSC310 R1 100k⍀ RSENSE 200m⍀ LED8 GND Figure 1 Schematic diagram With a single cell Li-Ion battery, the circuit in Figure 1 can drive 3 or more series connected LEDs, the maximum number of LEDs limited by the breakdown voltage of the bipolar transistor Q1. Depending on the number of LEDs connected in series, the sense resistor, RSENSE, will have to be adjusted to obtain the required LED current at a certain supply voltage. The ZXSC310 can be shutdown by pulling the Stdn pin low. The quiescent current in the shutdown mode is typically 5A. If shutdown feature is not required tie the Stdn pin to the VCC pin. Figure 2 shows the efficiency and the LED current against supply voltage. The LED current decreases with the supply voltage. This helps to draw less current from a discharged battery. The bill of materials for the circuit in Figure 1 is shown in Table 1. Issue 5 - August 2007 © Zetex Semiconductors plc 2007 9 www.zetex.com DN63 Efficiency vs input voltage LED current vs supply voltage 100 20 90 LED Current (mA) Efficiency (%) 80 70 60 50 40 30 20 3 LEDs in Series 10 0 4.5 4 3.5 3 15 10 5 3 LEDs in Series 0 4.5 2.5 4 Input voltage (V) 3.5 3 2.5 Input voltage (V) Figure 2 Performance graphs Ref. Part no. Value Manufacturer Contact details U1 Q1 ZXSC310E5 NA ZXTN25040DFH NPN, VCEO = 40V D1 ZHCS1000 Zetex Zetex www.zetex.com www.zetex.com 1A, low forward voltage VF Zetex www.zetex.com RSENSE Generic 200m⍀ Generic NA R1 C1 C2 L1 LED1-8 100k⍀ 100nF, 6.3V, X7R 2.2F, 35V 68H White LED Generic Generic Generic Coilcraft Nichia NA NA NA www.coilcraft.com www.nichia.com Generic Generic Generic DO1608P-683 NSPMW500BS Table 1 www.zetex.com Bill of materials 10 Issue 5 - August 2007 © Zetex Semiconductors plc 2007 DN64 ZXSC310 Solution flashlight Description A solution is provided for flashlight driving 4 white LEDs connected in series from a 2 alkaline cell input. Issue 3 - July 2006 © Zetex Semiconductors plc 2006 Figure 1 Schematic diagram Figure 2 Performance graphs 11 www.zetex.com DN64 Reference Part no. Value Manufacturer Contact details U1 ZXSC310E5 LED driver Zetex www.zetex.com Q1 FMMT618 2.5A, low VCE(sat) Zetex www.zetex.com D1 ZHCS1000 1A, low VF Zetex www.zetex.com L1 LPO2506OB-683 68H, 0.4A Coilcraft www.coilcraft.com R1 Generic 130m⍀ Generic NA C1 Generic 2.2F Generic NA LED1 Learn-4753A White LED LG Innotek www.iginnotek.com Table 1 www.zetex.com Bill of materials 12 Issue 3 - July 2006 © Zetex Semiconductors plc 2006 DN65 ZXSC310 Solution for emergency light Description This solution is provided for an emergency light driving 8 white LEDs connected in series from a 4 cell input. ZXSC310 Issue 3 - July 2006 © Zetex Semiconductors plc 2006 Figure 1 Schematic diagram Figure 2 Performance graphs 13 www.zetex.com DN65 Reference Part no. Value Manufacturer Contact details U1 ZXSC310E5 LED driver Zetex www.zetex.com Q1 FMMT619 2A, low VCE(sat) Zetex www.zetex.com D1 ZHCS1000 1A, low VF Zetex www.zetex.com L1 LPO2506OB-683 68H, 0.4A Coilcraft www.coilcraft.com R1 Generic 82m⍀ Generic NA C1 Generic 2.2F Generic NA LED1 NSPW500BS White LED Nichia www.nichia.com Table 1 www.zetex.com Bill of matrials 14 Issue 3 - July 2006 © Zetex Semiconductors plc 2006 DN66 An OLED bias supply for a clamshell handset sub display Author - Kit Latham, Applications Engineer, Zetex Semiconductors Description Portable applications such as cell phones are becoming increasingly complex with more and more features designed into every generation. One popular feature is to replace the STN sub display with an OLED sub display. OLED displays have infinite contrast ratio and are selfilluminating. This gives the handset manufacturer two key advantages, the first is lower power consumption and the second is a slimmer display. One disadvantage with OLED sub displays over LCD sub displays is the higher leakage current when not in use, which is the majority of the time. The way to overcome this issue is to disconnect the OLED sub display when the handset is dormant. The ZXLB1600 is a boost converter that can provide the power requirements for OLED sub display with the additional feature of a fully integrated isolation switch which disconnects the input from output when the ZXLB1600 is shutdown, making it ideally suited to OLED biasing. The schematic diagram in Figure 1 shows a full color OLED bias supply for clamshell handset sub display. U2 BAT54S L1 Output U1 ZXLB1600 VBATT C3 SW C1 N/C LBF SENSE C2 N/C FB EN N/C R1 LX VIN ADJ LBT N/C GND Figure 1 R2 Schematic diagram Note: For applications where OLED leakage is not an issue and the ZXLB1600 isolation switch is not needed, the SW pin can be shorted to the VIN pin, giving a further 3% to 5% improvement in efficiency. Issue 5 - July 2007 © Zetex Semiconductors plc 2007 15 www.zetex.com DN66 The materials list and associated performance characteristics provide an OLED biasing solution for the following sub display specification: • Input voltage: 4.2V to 3.0V • Output voltage: 12V • Output current: 20mA (max.) • Output ripple: 50mVpk-pk (max.) Reference Value Part number Manufacturer Contact details Comments U1 ZXLB1600X10 Zetex www.zetex.com OLED bias IC U2 BAT54S Zetex www.zetex.com Dual Schottky diode L1 22H NPIS32Q220MTRF NIC www.niccomp.com Low profile R1 715k⍀ Generic Generic NA 0603 size R2 82k⍀ Generic Generic NA 0603 size C1 10F/6V3 NMC0805X7R106M16 NIC www.niccomp.com 0805 size C2(1) 10F/16V NMC1206X7R106M16 NIC www.niccomp.com 1206 size C3 82pF/16V NMC0603NPO820J50 NIC www.niccomp.com 0603 size Table 1 Bill of materials NOTES: (1) For a lower profile, two 4.7F 0805 capacitors can be used by connecting in parallel. www.zetex.com 16 Issue 5 - July 2007 © Zetex Semiconductors plc 2007 DN66 Typical operating characteristics (For typical application circuit where VIN = 3V, VOUT = 12V, IOUT = 20mA unless otherwise stated) Series1 Series1 140 12 120 10 8 V output mA 100 80 60 6 40 4 20 2 0 3 3.2 3.4 3.6 Volts 3.8 4 0 4.2 3 3.2 3.4 3.6 V inout 3.8 4 4.2 Output voltage v. input voltage Input I v. input voltage Series1 Series1 80 20 70 60 15 mA % 50 40 10 30 20 5 10 0 0 3 3.2 3.4 3.6 Vin 3.8 4 4.2 Figure 2 Issue 5 - July 2007 © Zetex Semiconductors plc 2007 3 3.2 3.4 3.6 Vin 3.8 4 4.2 Efficiency v. input voltage Output current v. input current voltage Performance graphs 17 www.zetex.com DN66 Typical operating waveforms (For typical application circuit where VIN = 3V, VOUT = 12V, IOUT = 20mA unless otherwise stated) Figure 3 www.zetex.com Typical operating waveforms 18 Issue 5 - July 2007 © Zetex Semiconductors plc 2007 DN66 1.6V input with a 2mA load with a 25V DC output. LX drive 1.6V input with a 6mA load. LX drive, output is now unregulated. 5.5V input with an 18mA load producing 28V DC output LX drive Issue 5 - July 2007 © Zetex Semiconductors plc 2007 19 www.zetex.com DN66 5.5V output with no load producing 28V output This shows the fixed output LX drive waveform which can be as wide as 10sec. At 5.5V input and an output of 28V, this graph shows the typical output regulation to be <10% and ripple < 1V from no load to full load. Issue 5 - July 2007 © Zetex Semiconductors plc 2007 20 www.zetex.com DN66 Additional notes Adjusting output voltage 1) R1 and R2 When connected without external resistors R1 and R1, the ZXLB1600 will produce a nominal output voltage of 28V. This is because the chip has an internal high value resistor divider which is shunted by R1 and R2 externally if low value resistors are used. The relationship between R1 and R2 and VOUT is: VOUT(DC) = (R1+R2)/R1 x 1.23V The following table gives suggested E24/E96 resistor values for various output voltages. Required output voltage 5V 12V 18V 20V 22V 25V Table 2 External resistor across R2 280k 715k 1M 1.15M 1.15M 1.2 M External resistor across R1 91k 82k 75k 75k 68.1k 62k Resistor values for output voltages 2) Output adjustment by external voltage The internal voltage reference (Pin ADJ) may be overdriven by an external control voltage to set the output voltage. The relationship between applied voltage (VADJ) and output voltage (VOUT) is: VOUT = 22.86 x VADJ Note that the output can be set to any value between the input voltage and the maximum operating voltage in this way. However, some non-linearity in the above expression may occur at values of VADJ below approximately 0.5V. Also note that when driving the ADJ pin, the control voltage must have sufficiently low impedance to sink the bias current of the internal reference (10A max). 3) PWM output adjustment A Pulse Width Modulated (PWM) signal can be applied to the EN pin in order to adjust the output voltage to a value below the value set in 1) or 2). This method of adjustment permits the device to be turned on and the output voltage set by a single logic signal applied to the EN pin. No external resistors or capacitors are required and the amplitude of the control signal is not critical, providing it conforms to the limits defined in the electrical characteristics. Two modes of adjustment are possible as described below: Filtered 'DC' mode If a PWM signal of 10kHz or higher is applied to the EN pin, the device will remain active when the EN pin is low. However, the input to the internal low pass filter will be switched alternately from VREF to ground, with a duty cycle (D) corresponding to that of the PWM signal. This will present a filtered dc voltage equal to the duty cycle multiplied by VREF to the control loop and will produce a dc output voltage lower than the maximum set value. This voltage is given by: VOUT = 28 x D A square wave signal applied to the EN pin, for example, will turn the device on and produce a nominal regulated output of 14V. Issue 5 - July 2007 © Zetex Semiconductors plc 2007 21 www.zetex.com DN66 Gated mode The ZXLB1600 contains a timing circuit that switches the device on a few microseconds after the application of a rising edge to EN and turns it back off again nominally 120s after the falling edge of EN. So, if a lower frequency of 1kHz or less is applied to the EN pin, the device will be gated on and off at a duty cycle (D) corresponding to that of the input signal. The average output voltage is then given by: VOUT(avg) ~ 28 x D Output voltage can be adjusted all the way down to the input voltage by means of PWM control, but for best results, the duty cycle range should be kept within the specified range of 0.4 to 1. Lower duty cycles may result in increased output ripple and non-linearity in the relationship between duty cycle and output voltage. If a greater control range, or reduced ripple is required, the nominal output can be adjusted by one of the other methods before the PWM signal is applied. Negative output The ZXLB1600 can be used to provide a negative output voltage (in addition to the normal positive output) as shown in the application circuit below. In this circuit, the external resistors R3 an R4 are used to set the output voltage to 22V as described in the previous section. These resistors and output capacitor C2 have relatively low values in this circuit in order to give a short time constant. This improves the regulation of the negative voltage. BAT54S L1 22H D1 1 2 D2 3 R3 210k VBATT (3V nom.) on C1 10F 6V C2 10nF 35V ZXLB1600 off R4 12k GND C4 100nF 50V 2 C3 1F 35V BAT54S D4 1 Figure 4 www.zetex.com D3 3 Negative output -22V @ 5mA Title??????? 22 Issue 5 - July 2007 © Zetex Semiconductors plc 2007 DN66 Capacitor selection A low ESR ceramic capacitor grounded close to the GND pin of the package is recommended at the output of the device. Surface mount types offer the best performance due to their lower inductance. A minimum value of 1F is advised, although higher values will lower switching frequency and improve efficiency especially at lower load currents. A higher value will also minimize ripple when using the device to provide an adjustable dc output voltage. A good quality, low ESR capacitor should also be used for input decoupling, as the ESR of this capacitor is effectively in series with the source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. A minimum value of 3.3F is acceptable if the input source is close to the device, but higher values are recommended at lower input voltages, when the source impedance is high. The input capacitor should be mounted as close as possible to the IC. For maximum stability over temperature, capacitors with X7R dielectric are recommended, as these have a much smaller temperature coefficient than other types. Inductor selection The choice of inductor will depend on available board space as well as required performance. Small value inductors have the advantage of smaller physical size and may offer lower series resistance and higher saturation current compared to larger values. A disadvantage of smaller inductors is that they result in higher frequency switching, which in turn causes reduced efficiency due to switch losses. Higher inductor values can provide better performance at lower supply voltages. However, if the inductance is too high, the output power will be limited by the internal oscillator, which will prevent the coil current from reaching its peak value. This condition will arise whenever the ramp time ILX(peak) x L/VIN exceeds the preset 10s maximum 'on' time limit for the LX output. The ZXLB1600 has been optimized for use with inductor values in the range 10H to 100H. The typical characteristics show how efficiency and available output current vary with input voltage and inductance. The inductor should be mounted as close to the device as possible with low resistance connections to the LX and SW pins. Suitable coils for use with the ZXLB1600 are those in the NPIS range listed from NIC components or LP02506 and DO1608 series, made by Coilcraft if preferred. Diode selection The rectifier diode (D1) should be a fast low capacitance switching type with low reverse leakage at the working voltage. It should also have a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. Small Schottky diodes such as the BAT54 are suitable for use with the ZXLB1600 and this diode will give good all round performance over the output voltage and current range. At lower output voltages, a larger Schottky diode such as the ZHCS500 or MBR0540 will provide a smaller forward drop and higher efficiency. At higher output voltages, where forward drop is less important, a silicon switching diode such as the 1N4148 can be used, however this will give lower efficiency but will have better leakage characteristics than a Schottky device. The BAT54S device specified in the application circuit contains a second diode (D2) as one half of a series connected pair. This second diode is used here to clamp possible negative excursions (due to coil ringing) from driving the drain of the output transistor below -0.5V. This prevents internal coupling effects, which might otherwise affect output regulation. The table below gives some typical characteristics for various diodes. Issue 5 - July 2007 © Zetex Semiconductors plc 2007 23 www.zetex.com DN66 Diode Forward voltage at 100mA (V) Peak current (mA) Continuous current (mA) Reverse leakage (µA) BAT54 530 300 200 2 ZHCS500 300 1000 500 15 MBR0540 390 1000 500 1 1N4148 950 450 200 0.025 Table 3 Typical diode characteristics Increased efficiency If isolation of the coil from the supply is not needed, the high side of this can be connected directly to VIN to improve efficiency. This prevents power loss in the internal PMOS switch and typical efficiency gains of 5% can be achieved. (See efficiency vs. load curves). Some applications may require the coil to be fed from a separate supply with a different voltage to VIN. In this case, the SW pin should be left floating. Layout considerations PCB tracks should be kept as short as possible to minimize ground bounce and the ground pin of the device should be soldered directly to the ground plane. It is particularly important to mount the coil and the input/output capacitors close to the device to minimize parasitic resistance and inductance, which will degrade efficiency and increase output ripple. The FB and LBT pins are high impedance inputs, so PCB track lengths to these should also be kept as short as possible to reduce noise pickup. Output ripple is typically only 50mV p-p, but a small feed-forward capacitor (~100pF) connected from the FB pin to the output may help to reduce this further. Capacitance from the FB pin to ground should be avoided, but a capacitor can be connected from the LBT pin to ground to reduce noise pickup into the low battery comparator if required. Low Battery Detection Circuit (LBDC) The device contains an independent low battery detection circuit that remains powered when the device is shutdown. The detection threshold is set internally to a default value of 1.98V, but can be adjusted by means of external resistors as described below. Low Battery Threshold adjustment (LBT) The internal potential divider network R3/R4 sets the detection threshold. This is accessible at the LBT pin and can be shunted by means of external resistors to set different nominal threshold voltages. The potential divider defines threshold voltage according to the relationship: VLBT = (R3+R4)/R4 x 1.21V When using external resistors, these should be chosen with lower values than the internal resistors to minimize errors caused by the +/-25% absolute value variation of the internal resistors. The internal resistors have high values in order to minimize these errors. Low Battery Flag output (LBF) This is an open drain output that switches low when the battery voltage falls below the detection threshold. An external pull-up resistor can be connected to this pin to allow it to interface to any voltage up to a maximum of 29V. Current in the pull-up resistor should be limited to a value below IBLOL. www.zetex.com 24 Issue 5 - July 2007 © Zetex Semiconductors plc 2007 DN67 ZXSC400 solution for 1W high powered LED Mike Farley, Field Applications Engineer. December 2003 Description The ZXSC400, although designed for small LEDs in LCD backlighting, is sufficiently flexible to provide an efficient 1W solution producing a nominal 350mA constant current source from 2 NiMH or NiCd cells. Issue 3 - July 2006 © Zetex Semiconductors plc 2006 Figure 1 Schematic diagram Figure 2 Performance graphs 25 www.zetex.com DN67 Reference Part number U1 Value Manufacturer Contact details ZXSC400E6 Zetex www.zetex.com Q1 FMMT617 Zetex www.zetex.com D1 ZHCS2000 Zetex www.zetex.com D2 LXHL-NW98 Lumileds www.lumileds.com L1 DO1608C-332 3.3H Coilcraft www.coilcraft.com C1 GRM42-6X5R226K6.3 22F Murata www.murata.com C2 GRM42-6X5R226K6.3 22F Murata www.murata.com R1(1) 17m⍀ Generic NA R2 0.82⍀ Generic NA Table 1 Bill of materials NOTES: (1) Actual in-circuit value, see notes overleaf Figure 3 Open circuit protection Additional BoM AD1 - 5V6 R3 - 1K⍀ www.zetex.com 26 Issue 3 - July 2006 © Zetex Semiconductors plc 2006 DN67 VIN L1 Q1 + C1 D1 U1 R1 GND R2 Figure 4 K’ D2 + A’ D2 C2 Layout suggestion Note For these approximate layout dimensions, R1 is 15m⍀. See note 3. Notes: 1. D1 can be exchanged with a SOT23 4. Open circuit protection can be added as ZHCS1000 with a loss of 5% efficiency. shown below. The voltage rating of the small signal Zener diode ZD1 is not critical. 2. Inductor DCR (DC resistance) strongly It must be greater than the maximum influences efficiency, keep below 0.1⍀. forward voltage of the LED and less than 3. R1 is small and it is strongly advised to take the maximum VCE rating of the switching track resistance into account. A proven transistor, 15V in the case of the FMMT617. method is to source a 1A current from the The supply current in the open circuit Sense pin to the GND pin and check for 16condition is around 2mA. 17mV. This resistor can be made from a 22m⍀ in parallel with a 47m⍀ (or a single 15m⍀ resistor if available) with the PCB trace contributing the difference. Issue 3 - July 2006 © Zetex Semiconductors plc 2006 27 www.zetex.com DN67 Intentionally left blank www.zetex.com 28 Issue 3 - July 2006 © Zetex Semiconductors plc 2006 DN68 ZXSC310 High power torch reference design Description This design note shows a typical ZXSC310 LED driver circuit for a high powered LED torch. The input voltage ranges from 0.7V to 1.6V with a maximum output current of 335mA at 1.4V input. A typical schematic diagram is shown in Figure 1. D1 L1 VIN U1 Q1 Vcc Vdrive C1 C2 Stdn C3 Isense Gnd ZXSC310 Figure 1 R1 Schematic diagram Reference Value Part number Manufacturer Contact details Comments U1 ZXSC310E5 Zetex www.zetex.com LED driver in SOT23-5 Q1 FMMT617 Zetex www.zetex.com Low sat. NPN in SOT23 Zetex www.zetex.com 2A Schottky in SOT23 D1 2A ZHCS2000 L1 7.5H DO3316P-153x2 Coilcraft R1 19.5m⍀ Generic Generic NA C1 1F Generic Generic NA C2 220F Generic Generic NA C3 100F Generic Generic NA Table 1 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 www.coilcraft.com ISAT = 3A 0805 size Bill of materials 29 www.zetex.com DN68 Figure 2 www.zetex.com Performance graphs 30 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN69 ZXSC310 Garden light reference design Description This design note shows a typical ZXSC310 LED driver circuit for a solar power garden light. The input voltage ranges from1.7V to 2.5V with a maximum output current of 160mA at 2.4V input. A typical schematic diagram is shown in Figure 1. L1 1.7V to 2.5V D1 U1 Q1 VCC VDRIVE C1 Stdn ISENSE Gnd ZXSC310 R1 Figure 1 Ref Value Schematic diagram Part number Manufacturer Contact details Comments U1 ZXSC310E5 Zetex www.zetex.com LED driver in SOT23-5 Q1 FMMT617 Zetex www.zetex.com Low sat NPN in SOT23 Zetex www.zetex.com 0.5A Schottky in SOT23 D1 500mA ZHCS500 L1 15H DO3316P-153 Coilcraft www.coilcraft.com ISAT =3A R1 70m⍀ Generic Generic NA C1 100F Generic Generic NA Table 1 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 0805 size Bill of materials 31 www.zetex.com DN69 Total output current Table 2 shows the maximum available output current and the current per LED for a given number of LEDs. An LED forward voltage of 3.5V is assumed. Total LED Current (mA) 4 LEDs 5 LEDs 6 LEDs 176 44 35 29 163 41 33 27 153 38 31 25 141 35 28 23 131 33 26 22 119 30 24 20 110 27 22 18 97 24 19 16 89 22 18 15 80 20 16 13 70 18 14 12 61 15 12 10 Table 2 www.zetex.com Total output current 32 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN69 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) Figure 2 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 Performance graphs 33 www.zetex.com DN69 Intentionally left blank www.zetex.com 34 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN70 ZXSC400 Driving 2 serial high power LEDs Description This design note shows the ZXSC400 driving 2 serial LEDs. The input voltage ranges from 2V to 3.6V with a maximum output current of 360mA from 2.6V input. Figure 1 shows a typical constant current solution with the ZXSC400 driving two 1W LEDs in series. The wide input voltage range allows the use of different battery cell combinations. This could be dual alkaline cells with voltage starting from 3V down to 2V or triple NiCad/NiMH cells with voltage starting from 3.6V down to 2.7V. D1 L1 VIN=1.8V to 3.6V Q1 C2 U1 C1 Vcc Vdrive Stdn Isense Gnd VFB ZXSC400 R1 Figure 1 Ref. Value R2 Schematic diagram Part number Manufacturer Comments U1 ZXSC400E6 Zetex LED driver in SOT23-6 Q1 ZXTN25012EFH Zetex Low sat. NPN transistor in SOT23 D1 2A ZHCS2000 Zetex 2A Schottky in SOT23 L1 22H Generic Generic ISAT = 2A R1 18m⍀ Generic Generic 0805 size R2 820m⍀ Generic Generic 0805 size R3 1K⍀ Generic Generic 0805 size C1 22uF/10V Generic Generic C2 100uF/10V Generic Generic C3 220nF/10V Generic Generic Table 1 Issue 3 - July 2007 © Zetex Semiconductors plc 2007 0805 size Bill of materials 35 www.zetex.com DN70 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) 100 0.4 Output Current (A) Efficiency (%) 90 80 70 60 50 0.2 0.0 3.6 3.4 3.2 3 2.8 2.6 2.4 2.2 2 3.6 3.4 3.2 Input Voltage (V) 1.6 8 1.2 6 0.8 0.4 0.0 3.4 3.2 3 2.8 2.6 2.6 2.4 2.2 2 2.4 2.2 2.2 2 4 2 0 2 3.6 3.4 3.2 Input Voltage (V) 3 2.8 2.6 2.4 Input Voltage (V) Input Voltage vs Input Current Figure 2 www.zetex.com 2.8 Input Voltage vs Output Current Output Voltage (V) Input Current (A) Input Voltage vs Efficiency 3.6 3 Input Voltage (V) Input Voltage vs Output Voltage Performance graphs 36 Issue 3 - July 2007 © Zetex Semiconductors plc 2007 DN71 ZXSC400 Solution for Luxeon® V Star high powered LED Description This design note shows the ZXSC400 driving a Luxeon® V Star LED. The input voltage ranges from 4.2V to 5.4 V with a maximum output current of 700mA at 5V input. A typical schematic diagram is shown in Figure 1. D1 L1 VIN = 4.4V to 5.4V Q1 C2 U1 C1 VCC VDRIVE Stdn ISENSE GND VFB R4 ZXSC400 C3 R1 Figure 1 R2 R3 Schematic diagram # Ref. Value Part number Manufacturer Comments U1 ZXSC400E6 Zetex LED driver in SOT23-6 Q1 ZXTN25012EFH Zetex Low sat. NPN in SOT23 D1 2A ZHCS2000 Zetex 2A Schottky in SOT23 L1 22H Generic Generic ISAT = 2A R1 18m⍀ Generic Generic 0805 size R2, R3 820m⍀ Generic Generic 0805 size R4 1k⍀ Generic Generic 0805 size C1 22F/10V Generic Generic C2 100F/10V Generic Generic C3 100nF/10V Generic Generic Table 1 Issue 6 - July 2007 © Zetex Semiconductors plc 2007 0805 size Bill of materials 37 www.zetex.com DN71 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) 100 0.4 Output Current (A) Efficiency (%) 90 80 70 60 50 0.2 0.0 3.6 3.4 3.2 3 2.8 2.6 2.4 2.2 2 3.6 3.4 3.2 Input Voltage (V) 1.6 8 1.2 6 0.8 0.4 0.0 3.4 3.2 3 2.8 2.6 2.6 2.4 2.2 2 2.4 2.2 2.2 2 4 2 0 2 3.6 3.4 3.2 Input Voltage (V) 3 2.8 2.6 2.4 Input Voltage (V) Input Voltage vs Input Current Figure 2 www.zetex.com 2.8 Input Voltage vs Output Current Output Voltage (V) Input Current (A) Input Voltage vs Efficiency 3.6 3 Input Voltage (V) Input Voltage vs Output Voltage Performance graphs 38 Issue 6 - July 2007 © Zetex Semiconductors plc 2007 DN72 ZXLD1101 Driving 8 series LEDs Description This design note shows the ZXLD1101 driving 8 series connected LEDs. The input voltage ranges from 4.2V to 5.2V with a maximum output current of 24mA at 5V input. A typical schematic diagram is shown in Figure 1. L1 D1 VIN C1 x8 LEDs C2 VIN LX EN ZXLD1101 C3 FB GND Figure 1 Ref. Value U1 R1 Schematic diagram Part number Manufacturer Comments ZXLD1101E6 Zetex LED Driver in SOT23-6 1A Schottky in SOT23 D1 1A ZHCS1000 Zetex L1 33H Generic Generic R1(1) 0⍀ Generic Generic C1 100F Generic Generic C2 1F Generic Generic C3 10F Generic Generic Table 1 0805 size Bill of materials NOTES: (1) R1 is set to zero. It shows the maximum output power characteristic of the LED driver. A regulated LED current below the maximum value can be set by: ILED = VFB/R1, where VFB = 0.1V. Issue 2 - July 2006 © Zetex Semiconductors plc 2006 39 www.zetex.com DN72 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) Figure 2 www.zetex.com Performance graphs 40 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN73 ZXSC300 Step down converter for 3W LED Description This design note shows the ZXSC300 or ZXSC310 driving a 3W LED. The input voltage ranges from 6.2V to 3.8V with a maximum output current of 1.11A at 6V input. A typical schematic diagram is shown in Figure 1. Figure 1 Ref. Value Schematic diagram Part number Manufacturer Comments U1 ZXSC300/310 Zetex LED Driver in SOT23-5 Q1 ZXMN2A01F Zetex SOT23 MOSFET D1 1A ZHCS1000 Zetex 1A Schottky in SOT23 L1 22H Generic Generic ISAT = 3A R1 20m⍀ Generic Generic 0805 size C1 100F Generic Generic C2 100F Generic Generic Table 1 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 Bill of materials 41 www.zetex.com DN73 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) Figure 2 www.zetex.com Performance graphs 42 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN74 ZXSC400 Photoflash LED reference design Description This design note shows the ZXSC400 driving a Photoflash LED. The input voltage is 3V with a maximum pulsed output current of 1A for 2ms. A typical schematic diagram is shown in Figure 1. C3 U2 L1 SW1 VBATT U1 VCC VDRIVE STDN ISENSE GND C1 SW2 C2 R2 VFB ZXSC400 R3 R1 R4 Charging mode: SW1 closed, SW2 open Discharging mode: SW1 open, SW2 closed Figure 1 Schematic diagram Operation In charging mode, SW1 is closed and SW2 is open the ZXSC400 is configured as a typical boost converter, charging capacitor C2 up the regulated output voltage set by the ratio of R1 and R2. This is typically 16V. The peak current of the converter (current drawn from the battery) is controlled by R3 plus R4, and is typically 280mA for this application. When C2 is charged to 16V the SW1 is opened and SW2 is closed, converting the ZXSC400 to a step down converter to provide a 1A constant current for 2ms to the photoflash LED. During step down operation, current flows from C2, through the photoflash LED, L1, U2 and is returned to C2 through R3. This means that the peak current is set at a higher value than in charging mode, typically 1A. When the current reaches it's peak value, U2 is switched off and current flows from L1 through the Schottky diode in U2, to the photoflash LED. This cyclic process is repeated until C2 is discharged. Issue 2 - July 2006 © Zetex Semiconductors plc 2006 43 www.zetex.com DN74 Ref Value Part number Manufacturer Comments U1 ZXSC400E6 Zetex LED Driver in SOT23-6 U2 ZX3CDBS1M832 Zetex Dual NPN and Schottky L1 12H Generic Generic ISAT=1A R1 10k⍀ Generic Generic 0805 size R2 510k⍀ Generic Generic 0805 size R3 22m⍀ Generic Generic 0805 size R4 100m⍀ Generic Generic 0805 size C1 1F Generic Generic C2 150F Generic Generic C3 1F Generic Generic Table 1 Bill of materials Typical operating waveforms (For typical application circuit where VBATT = 3V and Tamb = 25°C unless otherwise stated) Figure 2 www.zetex.com Performance graphs 44 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN75 ZXSC310 Solar powered garden light reference design Description This design note shows a typical ZXSC310 LED driver circuit for a solar powered garden light. The input voltage ranges from 0.4V to 1.6V with a maximum output current of 43mA at 1V input. A typical schematic diagram is shown in Figure 1. D1 L1 VIN U1 C3 Q1 VCC VDRIVE C1 C2 Stdn ISENSE Gnd ZXSC310 R1 Figure 1 Ref. Value Schematic diagram Part Number Manufacturer Comments U1 ZXSC310E5 Zetex LED Driver in SOT23-5 Q1 FMMT617 Zetex Low sat NPN in SOT23 ZHCS1000 Zetex 1A Schottky in SOT23 0805 size D1 1A L1 37H R1 100m⍀ Generic Generic C1 1F Generic Generic C2 22F Generic Generic C3 10F Generic Generic Table 1 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 Bill of materials 45 www.zetex.com DN75 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) Figure 2 www.zetex.com Performance graphs 46 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN76 ZXLD1100 and ZXLD1101 driving from 3 to 6 LEDs Description This design note shows the ZXLD1100 and ZXLD1101 driving series connected LEDs. The input voltage range is 2.5V to 5.5V. The same circuit can be used for up to 6 LEDs. The ZXLD1100 contains onchip open circuit LED protection. This function would require an additional Zener and resistor with the ZXLD1101. Note: LED current is set to 15mA Figure 1 Ref. Value Package Part number Schematic diagrams Manufacturer Contact details Notes U1 TSOT23-5 ZXLD1101ET5 Zetex www.zetex.com LED driver IC U1 SC706 ZXLD1100H6 Zetex www.zetex.com LED driver IC Zetex www.zetex.com 400mA Schottky diode D1 400mA SOD323 ZHCS400 L1 10H CMD4D11-100MC Sumida R1 6.8⍀ 0603 Generic Generic R21 100k⍀ 0603 Generic Generic C1 1F 0603 Generic Generic C2 1F 0603 Generic Generic NSCW215 Nichia LEDs Table 1 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 www.sumida.com 1mm height profile www.nichia.com 6pcs per board Bill of materials 47 www.zetex.com DN76 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) Figure 2 www.zetex.com Performance graphs 48 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN78 ZXSC310 with reverse polarity protection Ray Liu - Applications Engineer, Zetex Semiconductors Description The schematic diagram shown in Figure 1 is a typical example of the ZXSC310 used in a LED flashlight application. The input voltage can either be one or two alkaline cells. If the battery is put in the flashlight the wrong way, the reverse polarity can damage the ZXSC310 and switching transistor, Q1. Implementing a mechanical reverse protection method can be expensive, and not always reliable. This paper describes methods of electronic reverse protection, without efficiency loss, for the ZXSC series ICs and related LED flashlight application circuits. Circuit problems caused by the reverse polarity battery If a negative voltage appears at the input terminal of Figure 1 then reverse current will flow from the ground pin of the ZXSC310 to the VCC terminal and back to the battery. This current is high and will damage the ZXSC310. Some of this reverse current will also flow through the VDRIVE terminal of the ZXSC310 and into Q1 base-collector completing the circuit to the battery. The reverse current through base-collector of Q1 turns the transistor on in the reverse direction and causes high current to flow from ground, through emitter-collector to the battery, resulting in battery drainage and possible damage to the switching transistor, Q1. A common method of reverse polarity protection A common method of reverse protection is to add a Schottky diode in series with the battery positive. The problem with this method of reverse protection is that there is a loss of efficiency due to the forward voltage drop of the diode, typically 5% to 10% depending upon input voltage, reducing the usable battery life. The proposed method of reverse protection for the ZXSC series IC's gives full protection with no loss of efficiency. D1 L1 VIN U1 Q2 VCC C2 VDRIVE Stdn ISENSE GND ZXSC310 Figure 1 Issue 3 - August 2007 © Zetex Semiconductors plc 2007 R1 Schematic diagram 49 www.zetex.com DN78 Reverse protection without efficiency loss By adding current limiting resistor and Schottky diode, the reverse current flow can be eliminated without a loss of efficiency. Flashlight circuit with bootstrap For the bootstrap circuit in Figure 2, the current through the ZXSC310 is blocked by the reversed biased Schottky diode, D1. The current from VDRIVE, which turns on Q1 in the reverse direction, is diverted via D2 back to the battery so that Q1 does not turn on. R2 is a current limiting resistor to control this VDRIVE current. This value is typically set to 100⍀ to 500⍀ to minimize battery current drain without affecting the normal operation of the circuit. D1 L1 VIN D2 U1 VCC C1 Q1 R2 VDRIVE Stdn ISENSE GND C2 R1 ZXSC310 Figure 2 Ref Part number Manufacturer Comments U1 ZXSC310E5 Zetex LED driver in SOT23-5 Q1 ZXTN25012EFL Zetex Low sat. NPN in SOT23 D1 Value 750mA BAT750 Zetex 750mA Schottky in SOT23 200mA BAT54 Zetex 200mA Schottky in SOT23 68H Generic Generic ISAT>0.4A, R<0.8⍀ 270m⍀ Generic Generic 0805 size 100⍀ Generic Generic 0805 size C1 10F/6.3V Generic Generic C2 22F/6.3V Generic Generic D2 (1) L1 R1 R2 (1) Table 1 Bill of materials NOTES: (1) Add for reverse protection www.zetex.com 50 Issue 3 - August 2007 © Zetex Semiconductors plc 2007 DN78 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) Without Protection With Protection Without Protection 100 50 Output Current (mA) 90 Efficiency (%) With Protection 60 80 70 60 40 30 20 10 50 0 3 2.6 2.2 1.8 1.4 1 3 2.6 Input Voltage (V) 2.2 1.8 1.4 1 Input Voltage (V Input Voltage vs Output Curren Input Voltage vs Efficiency Without Protection With Protection Without Protection 100 With Protection 4 80 Output Voltage (V) Input Current (mA) 60 40 20 0 -20 -40 -60 3 2 1 -80 -100 3 2 1 0 -1 -2 0 2.8 -3 2.6 2.4 2.2 2 1.8 1.6 1.4 Input Voltage (V) Input Voltage (V) Input Voltage vs Input Current Input Voltage vs Output Voltage Figure 3 Issue 3 - August 2007 © Zetex Semiconductors plc 2007 1.2 1 Performance graphs 51 www.zetex.com DN78 Other circuit examples using reverse polarity protection Flashlight circuit without bootstrap The circuit shown in Figure 4 is for an LED flashlight application without bootstrap. As described previously, reverse current can flow from the GND terminal to VCC and back to the battery. To block this current path an extra diode, D2b, is added. It is recommended that a Schottky diode be used for this application to maximize the start-up input voltage from VCC(MAX) to VCC(MIN) + D2b VF, 3V to 1V. The Schottky diode, D2a, and resistor, R2, work in the same way as described in the bootstrap circuit in Figure 2. A dual Schottky diode, BAT54S, is recommended for D2 in order to achieve low component count. L1 D1 VIN U1 VCC C1 R2 C2 Stdn GND ISENSE ZXSC310 Figure 4 www.zetex.com Q1 VDRIVE R1 LED flashlight application without bootstrap 52 Issue 3 - August 2007 © Zetex Semiconductors plc 2007 DN78 Other circuit examples using reverse polarity protection Flashlight circuit without bootstrap Figure 5 is a step down converter with reverse polarity protection. The main application for this circuit is a four alkaline cell flashlight driving a high powered LED. Again the protection circuit operates as described above. A dual Schottky diode, BAT54S, is recommended for D2 in order to achieve low component count. C2 L1 D1 VIN D2a D2b R2 VCC VDRIVE Q1 Stdn C1 ISENSE GND ZXSC310 R1 Figure 5 Issue 3 - August 2007 © Zetex Semiconductors plc 2007 Step down converter with reverse polarity protection 53 www.zetex.com DN78 Intentionally left blank Issue 3 - August 2007 © Zetex Semiconductors plc 2007 54 www.zetex.com DN79 ZXSC400 1W LED driver Neil Wolstenholme, Applications Engineer, Zetex Semiconductors plc Description ZXSC400 is configured to the reference design below. The target application is a 1W white LED driven from a two cell NiCd/NiMH or alkaline battery input for flashlights and high powered LED driving. The supply voltage for ZXSC400 reference design is: VIN = 1.8V ~ 3V. L1 D1 22uH ZHCS1000 VIN Q1 LED1 FMMT617 U1 ZXSC400 C1 100uF/6V3 VCC VDRIVE STDN ISENSE GND LXHL-PW01 C2 VFB 100uF/6V3 R1 R2 22mohm 0.82ohm GND Figure 1 Schematic diagram Ref. Value Package Part number Manufacturer Contact details Notes U1 N/A SOT23-6 ZXSC400E6 Zetex www.zetex.com Boost converter Q1 N/A SOT23 FMMT617 Zetex www.zetex.com Low sat NPN transistor D1 40V/1A SOT23 ZHCS1000 Zetex www.zetex.com 40V/1A Schottky diode L1 22uH/2.5A N/A Coilcraft www.coilcraft.com 22H/2.5A SMT inductor R1 22m⍀ 0805 Generic NA 0805 5% tolerance R2 0.82⍀ 0805 Generic NA 0805 5% tolerance C1 100F/6V3 1812 18126D107KAT2A AVX www.avx.com 100F/6V3/X5R/1812 C2 100F/6V3 1812 18126D107KAT2A AVX www.avx.com 100F/6V3/X5R/1812 LXHL-PW01 www.lumileds.com 1W white LED emitter LED1 1W N/A DO3316P-223 Table 1 Issue 1 - May 2005 © Zetex Semiconductors plc 2005 Lumileds Bill of materials 55 www.zetex.com DN79 Performance Increasing efficiency On ZXSC400 reference design, R1 is set to 22mΩ to ensure that the LED current is regulated over the full input voltage range of 3V ~ 1.8V. For improved efficiency R1 can be changed to a 33mΩ resistor but LED current will not be regulated below 2V. See Figure 2, Performance graphs. 400 VSTDN=VIN ILED=350mA 90 LED Current (mA) Efficiency (%) 100 R1=33mohms 80 70 R1=22mohms 60 50 3.0 2.8 2.6 2.4 2.2 2.0 VSTDN=VIN 380 R1=22mohms 360 340 R1=33mohms 320 300 3.0 1.8 2.8 2.6 2.4 2.2 2.0 1.8 Input voltage (V) Input Voltage (V) Efficiency vs Input Voltage LED Current vs Input Voltage VSTDN=VIN 800 VSTDN=VIN R1=22mohms 600 400 3.0 LED Voltage (V) Input Current (mA) 4.0 1000 R1=33mohms 2.8 2.6 2.4 2.2 2.0 3.8 R1=33mohms 3.6 3.4 R1=22mohms 3.2 3.0 3.0 1.8 2.8 Input Voltage (V) Input Current vs Input Voltage Figure 2 www.zetex.com 2.6 2.4 2.2 2.0 1.8 Input Voltage (V) LED Voltage vs Input Voltage Performance graphs 56 Issue 1 - May 2005 © Zetex Semiconductors plc 2005 DN83 LED MR16 Lamp solution using the ZXLD1350 LED driver Neil Chadderton, Colin Davies, Roger Heap, Zetex Semiconductors Introduction Lighting class LEDs are now available that deliver the brightness, efficacy, lifetime, color temperature, and white point stability required for general illumination. As a result, these LEDs are being adopted into most general lighting applications including roadway, parking area and indoor directional lighting. LED-based luminaires reduce Total-Cost-of-Ownership (TCO) in these applications through maintenance avoidance and reduced energy costs. MR16 lamps are one variety of Multifaceted Reflector (MR) lamps that have traditionally employed a halogen filament capsule as the light source. They are used in many retail and consumer lighting applications where their size, configurability, spot-lighting capability and aesthetics provide utility and creativity. Their low efficiency, heat generation (an issue for illuminating heat sensitive subjects and materials) and halogen capsule handling issues are typically cited among the disadvantages of the technology. They typically operate from 12V AC or 12V DC, though designs for 6V to 24V are also popular and as such require a step-down transformer to allow use from offline supplies. This is usually effected with conventional electromagnetic or electronic transformers. With the advancement of HB (High Brightness) LED technologies, MR16 lamps can now be realized with an alternate light source. This hybrid solution can yield a cost effective, long-life, maintenance free, cooler operating unit which has not been previously possible. Figure 1 MR16 Lamps (Incandescent A-lamp on far right shown for size comparison) Issue 2 - August 2007 © Zetex Semiconductors plc 2007 57 www.zetex.com DN83 Description This design note describes a driver solution developed using the Zetex ZXLD1350 LED driver IC to drive three CREE® XLamp XR-E High Brightness (HB) LEDs. The ZXLD1350 features can be summarized as: • Wide input voltage range • 7V to 30V; internal 30V NDMOS switch • Up to 350mA output current (the ZXLD1360 can provide up to 1A output current) • Capable of driving up to 8 series connected 1 Watt LEDs • High efficiency (see datasheet - but >90% with 8 LEDs) • Low quiescent current: (100A typical) • 1A max shutdown current • Brightness control using DC voltage or PWM (low or high frequency) • Internal PWM filter for high frequency PWM signals • Optional soft-start; up to 1MHz switching frequency The Cree XR-E LED is a lighting class device that provides energy savings for many traditional technologies such as the MR16 halogen lamp. The XR-E LED is capable of operating at forward currents of up to 1A without any noticeable shifts in chromaticity. The XR-E is ideally suited for direct replacement of MR16 when used in clusters of three at a forward current of 300mA—1000mA. They are specified at 80 lumens and 70 lumens per watt at 350mA (136 lumens at 700mA). These lighting class LEDs offer efficient, directional light that offers a lumen maintenance of 70% at 50,000 hours, in addition to significantly reducing power consumption. The circuit diagram of the ZXLD1350 effected MR16 lamp solution is shown in Figure 2. Table 1 provides the bill of materials. A full bridge (D1—D4) is employed using 1A DC rated, low leakage Schottky diodes to allow AC or DC input supplies. A thermistor circuit is incorporated to reduce the output current of the circuit to provide thermal feedback control, which allows the circuit to a) match the thermal de-rating requirements of the LEDs to ensure lumen maintenance expectations are achieved and b) prevent overheating. The thermistor must be thermally coupled to the LEDs to ensure accurate and responsive tracking. Adjustment of the thermal feedback circuit can be accomplished by the choice of the thermistor R3 - which sets the slope of the current vs. temperature response, and resistor R2 - which determines the temperature threshold point for the control circuit. R1 and D5 provide a reference voltage for the thermal control circuit. Q1 is a low VCE(sat) PNP transistor. Schottky diode D9 is again a low leakage 1A rated device in a SOT23 package - its low forward voltage and low reverse current ensure high efficiency and thermal stability in the main switching circuit. C3 may be added to reduce the amplitude of the current ramp waveform experienced by the LED string but in many applications this isn't required as the integrating nature of human sight cannot perceive quickly changing light levels. Depending on layout intricacies and EMC dictates, it may be necessary to exchange the positions of the inductor and LED string - this isn't always possible mechanically but does give a lower EMI signature. www.zetex.com 58 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 DN83 Figure 3 shows the measured response of the LED current drive with respect to temperature, with the values given in Table 2. The selection of components for the thermal feedback circuit is not only dependent on the choice of R2 and R3, but also on the amount of heat sink area required to extract heat from the LEDs. To maximize the light output at high ambient or operating temperature conditions, the LEDs must have a sufficient thermal extraction path, otherwise the thermal control circuit will effect current drive reduction in non-optimal conditions. The thermal control threshold point is set by adjusting R2. For this design, three values (33k, 22k and 10k) were evaluated. These values were chosen to give break points at approximately 25°C, 40°C and 60°C. Note that the light output will not continually dim to zero - the thermal control is applying DC control to the ADJ pin and therefore has a dimming ratio from maximum current of approximately 5:1. Once the reduced DC level goes below the shutdown threshold of around 200mV, the LED drive current will fall to zero and the LEDs will be extinguished. The slope of the current reduction is determined by the beta value of the thermistor. The larger the beta value, the sharper will be the resultant current control response. The slope of the current reduction is also affected by Q1's base emitter voltage (VBE) variation with temperature. Figure 3 shows the slope starts to level off at higher temperatures due to the increasing influence of the approximately 2.2mV/°C change in the VBE of the transistor. R6 2R R7 1R L1 100 uH R8 1R R9 1R R 1 4 k7 D1 ZLLS1000 LED 1 Cree CLD7090 D2 ZLLS1000 W1 R10 0 R Vin R 2 typ 10 k See table 2 W2 D4 ZLLS1000 D9 ZLLS1000 V Sense C3 1uF Not fitted LED 3 Cree CLD 7090 Q1 ZXTP2039F 3 R4 0R D3 ZLLS1000 4 5 C1 1uF 50 V LED 2 Cree CLD7090 ADJ D5 BZX284C6V2 ZXLD1350 LX 1 R5 10 k Not fitted GND 2 R3 10 k Thermistor B 3900 W3 Thermally connected C 100nF Not fitted W4 Figure 2 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 Circuit diagram of ZXLD1350 MR16 lamp solution 59 www.zetex.com DN83 Measured results for circuit of Figure 2 using 10k thermistor with beta of 3900 400 LED current mA 350 300 250 R1 set for 25°C 200 R1 Set for 40°C R1 Set for 60°C 150 100 50 0 0 20 40 60 80 100 120 140 Temperature °C Figure 3 Measured response of thermal feedback control showing threshold point Quantity Part reference Value 1 R1 4k7 1 R2 See Table 2 0603 resistor 1 R3 10K, 0603, Beta = ~3900 thermistor 1 R4 0R, 0603 link 1 R5 10k, 0603 resistor 1 R6 2R, 0603 resistor 3 R7, R8, R9 1R, 0603 resistor R10 0R 1206 link 5 D1, D2, D3, ZLLS1000 D4, D9 1 D5 BZX284C6V2 1 C1 1uF 2 3 1 C2, C3 LED1, LED2, LED3 Q1 1 L1 100H 1 IC1 ZXLD1350 XR-E ZXTP2039F Table 1 www.zetex.com Description Resistor, 1%, 0603 Resistor, 1% 0603 Source Various Various Thermistor, 5% 0603 EPCOS 0R Link, 0603 Generic Generic Generic 0R Link, 1206 Schottky diode 40V, 1A Various Various Various Various Various Zetex 6.2V Zener diode Capacitor 50v 1206 X7R NMC1206X7R105K50F C1206C105K5RAC7800 Not fitted Cree XLamp power LED Transistor, PNP Alternative: FMMT717 MSS6132 100H NPIS53D101MTRF Zetex LED driver IC NICcomponents KEMET Cree Zetex Coilcraft NIC components Zetex Bill of materials 60 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 DN83 R2 = 33k R2 = 22k R2 = 10k Temp. R1 set for 25°C R1 set for 40°C R1 set for 60°C 0 350 350 350 20 350 350 350 25 352 350 350 40 280 345 350 60 177 235 342 80 131 148 248 100 104 120 171 120 81 90 Table 2: Thermal feedback control threshold point (resistor R2) For three series-connected LEDs, the voltage can be from 12V minimum to 30V DC maximum. For AC supplies, remember to include the 1.414 factor for RMS specified values - so for 20V AC (RMS), this will provide a DC rail after the Schottky bridge of 28.3V. The nominal current is set at 350mA with a 0.283⍀ sense resistor. The sense resistor is a combination component using 4 low cost, commonly available values and allows current set point flexibility if the circuit is used as a platform design for a series of products. For three series-connected LEDs, with a nominal supply of 24V and a 100H inductor, the ZXLD1350 runs in continuous mode at approximately 500kHz. The ZXLD1350 datasheet displays this information graphically, as shown in Figure 4 (for a sense resistor of 330m⍀ in this case), which allows a fast assessment to be made of operating conditions. The switching frequency will increase as the voltage on the ADJ pin decreases. As the ZXLD1350 (and ZXLD1360) series of LED drivers use a hysteretic switching topology, the switching frequency is dependent on several factors - input voltage, target current (including any effect by voltage on the ADJ pin to reduce the current) and number of LEDs. An Excel based calculator is available which allows "what-if" initial evaluation and is a useful tool for assessing component and condition changes. Final designs should, of course, be verified by reality. Operating Frequency vs Input Voltage L=100uH, Rs=0.33 Ohms 600 Frequency (kHz) 500 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 400 300 200 100 Note: Please see the ZXLD1350 datasheet for complete characterization including efficiency, duty cycle and current variation charts for various values of inductor 0 5 10 15 20 25 30 VIN (V) Figure 4 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 Example of operating frequency chart for the ZXLD1350 61 www.zetex.com DN83 Higher current designs The ZXLD1350 is designed for LED current drive applications of up to 350mA. The monolithic NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 400mA, which with the hysteretic mode of operation (the current waveform will ramp ±15% about the nominal current set point) provides sufficient margin. For higher current operation, the 1A rated ZXLD1360 offers similar design procedures and has the following features: • Up to 1A output current • Wide input voltage range: 7V to 30V • Internal 30V 400m NDMOS switch • Can drive up to 7 series connected 3W LEDs (with due attention to thermal path design) • High efficiency (>90% for 7 LEDs) • Brightness control using DC voltage or PWM • Internal PWM filter • Optional soft-start • Up to 1MHz switching frequency www.zetex.com 62 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 DN83 Board design k The Printed Circuit Board (PCB) design and circuit employed make it particularly suitable for use in MR16 halogen lamp replacement units. The supply voltage range is nominally 12V AC or DC, making it compatible and interchangeable with existing MR16 lamps. The printed circuit tracking has been designed using only one side of the board, to facilitate the use of an aluminum or other heat-conductive substrate where through-hole technology cannot be employed. A central hole is provided to enable connection of the supply leads from the rear and for connection to a dimming circuit, where this is required. Mounting holes are also provided. Gerber-format layout files for this PCB are available from Zetex upon request. Please quote PCB number ZDB335. 2 LE D 3 R2 4 R LED R1 D5 W3 3 R Q1 k C2 R5 D3 IC1 W1 W4 D9 R9 R8 R7 R6 C1 R10 D4 C3 W2 D1 D2 L1 k LED1 Top PCB overlay Top PCB overlay and top copper k Figure 5 Top copper 2 LE D 3 R2 4 R LED R1 D5 W3 R5 R3 k C2 Q1 D3 IC1 D9 R9 R8 R7 R6 W1 W4 R10 C1 D4 C3 W2 D1 D2 L1 k LED1 Figure 6 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 Composite view 63 www.zetex.com DN83 Appendix A - ZXLD1350 operation In normal operation, when voltage is applied at +VCC, the ZXLD1350 internal NDMOS switch is turned on. Current starts to flow through the sense resistor, inductor L1, and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +VCC and the inductor L1. This rising current produces a voltage ramp across the sense resistor. The internal circuit of the ZXLD1350 senses the voltage across the sense resistor, and applies a proportional voltage to the input of the internal comparator. When this voltage reaches an internally set upper threshold, the NDMOS switch is turned off. The inductor current continues to flow through the sense resistor, L1, the LEDs, the Schottky diode SD9, and back to the supply rail, but it decays, with the rate of decay determined by the forward voltage drop of the LEDs and the Schottky diode. This decaying current produces a falling voltage at the sense resistor, which, in turn, is sensed by the ZXLD1350. A voltage proportional to the sense voltage across the sense resistor is applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the NDMOS switch is turned on again. This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor. Both DC and PWM dimming can be achieved by driving the ADJ pin through W3. For DC dimming, the ADJ pin may be driven between 300mV and 1.25V. Driving the ADJ pin below 200mV will shutdown the output current. For PWM dimming, an external open-collector NPN transistor or open-drain N-channel MOSFET can be used to drive the ADJ pin. The PWM frequency can be low, around 100Hz to 300Hz, or high between 10kHz to 50kHz. For the latter case, an on-chip filter derives the DC content and so for high frequency PWM input, the device will operate essentially as for DC control input dimming. Generally, low frequency PWM control is preferred as in this mode, the converter is shut down during PWM low signals and drives the LEDs at the defined nominal current during PWM high signals - this ensures that the LEDs can are always driven at the nominal current and therefore color temperature (CCT) shifts are minimized. The capacitor C2 should be around 10nF to decouple high frequency noise at the ADJ pin for DC dimming. Note - C2 should not be fitted when using the PWM dimming feature. The soft-start time will be nominally 0.5ms without capacitor C2. Adding C2 will increase the soft start time by approximately 0.5ms/nF Please refer to the datasheets for the threshold limits, ZXLD1350 internal circuits, electrical characteristics and parameters. Issue 2 - August 2007 © Zetex Semiconductors plc 2007 64 www.zetex.com DN84 ZXSC400 Driving 3W high power LEDs Ray Liu, Applications Engineer, Zetex Semiconductors Description This design note shows the ZXSC400 driving a single 3W LED. The input voltage ranges from 1.8V to 3.6V with constant output current of 700mA down to 2.6V with an overall 80% of efficiency. Figure 1 shows typical constant current solution with ZXSC400 driving one 3W LED. The input voltage range allows the use of two alkaline batteries or one Lithium Ion cell (CR123A) for portable flashlight applications. Q1 and Q2 forms a pseudo Darlington pair which provide enough current gain for a switching current up to 1.5A. In order to provide better switch off performance, a Schottky diode, D2, is used to drain the base current from the base of Q1 directly. In order to achieve higher efficiency, current monitor U2 is used to provide a low voltage drop LED current sensing through the low ohmic resistor, R2. The LED current is converted to 300mV feedback voltage through R3. D1 L1 VIN = 1.8V to 3V R2 U2 VSENSE+ Q2 C1 R4 VSENSE- IOUT ZXCT1009 Q1 C2 U1 C1 VCC VDRIVE Stdn ISENSE GND VFB D2 R6 D1 R5 ZXSC400 C3 R3 R1 Figure 1 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 Schematic diagram 65 www.zetex.com DN84 Ref. Value U1 Part number Manufacturer Comments ZXSC400E6 Zetex LED driver in SOT23-6 U2 ZXCT1009 Zetex Current monitor in SOT23 Q1 ZXTN25012EFH Zetex Low sat NPN in SOT23 Q2 ZXTN25012EFL Zetex Low sat NPN in SOT23 D1 ZHCS2000 Zetex 2A Schottky in SOT23 D2 ZHCS400 Zetex 400mA Schottky L1 15H 744 561 15 Wurth Electronik ISAT = 3A DCR=60m⍀ R1 20m⍀ 1% Generic Generic 0805 size low ohmic R2 50m⍀ 1% Generic Generic 0805 size low ohmic R3 820⍀ 1% Generic Generic 0805 size R4 82⍀ 5% Generic Generic 0805 size R5 4.7⍀ 5% Generic Generic 0805 size R6 10⍀ 5% Generic Generic 0805 size C1 22F 10V 10% Generic Generic 1206 size X7R/X5R C2 4.7F 10V 10% Generic Generic 1206 size X7R/X5R C3 0.22F 16V 10% Generic Generic C4 330pF/10V Generic Generic 0805 size Table 1 www.zetex.com Bill of materials 66 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 DN84 Intentionally left blank Issue 1 - August 2007 © Zetex Semiconductors plc 2007 67 www.zetex.com DN84 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) 90 0.8 Output Current (A) 1.0 Efficiency (%) 100 80 70 60 50 0.6 0.4 0.2 0.0 3 2.8 2.6 2.4 2.2 2 1.8 3 2.8 2.6 Input Voltage (V) Input Voltage vs Efficiency 2.2 2 1.8 Input Voltage vs Output Current 1.6 4 1.2 3 Output Voltage (V) Input Current (A) 2.4 Input Voltage (V) 0.8 0.4 2 1 0 0.0 3 2.8 2.6 2.4 2.2 2 3 1.8 2.8 2.6 Input Voltage vs Input Current Figure 2 www.zetex.com 2.4 2.2 2 1.8 Input Voltage (V) Input Voltage (V) Input Voltage vs Output Voltage Performance graphs 68 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 DN85 ZXSC400 1W/3W buck LED drivers Ray Liu, Applications Engineer, Zetex Semiconductors Description In Figure 1, ZXSC400 is configured as a high efficiency buck LED driver. The target applications are either 1W (350mA) or 3W (700mA) drivers for white LED driven from a 4 cell battery, or a 2 alkaline cell input for flashlights. The supply voltage for ZXSC400 reference design is: VIN = 3.8V to 6V. Parts lists for 1W and 3W design are shown in Table 1 and Table 2 respectively. Performance data is measured based on two different LED's VF binning with 0.3V VF difference. C3 L1 VIN = 4V to 6V D1 C2 U1 VCC C1 Q1 VDRIVE Stdn VFB GND ISENSE R3 R2 ZXSC400 R1 Figure 1 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 Schematic diagram 69 www.zetex.com DN85 Ref. Value Package Part number Manufacturer Contact details Notes U1 N/A SOT23-6 ZXSC400E6 Zetex www.zetex.com LED Driver Q1 N/A SOT23 ZXTN25012EFL Zetex www.zetex.com Low sat NPN transistor D1 40V/0.75A SOT23 BAT750 Zetex www.zetex.com 40V/0.75A Schottky diode L1 47H N/A 744052470 Wurth Elektronik www.we-online.com ISAT = 520mA R1 62m⍀ 0805 Generic N/A 0805 1% R2 10⍀ 0805 Generic N/A 0805 5% R3 47⍀ 0805 Generic N/A 0805 5% C1 4.7F/10V 1206 Generic N/A X7R/X5R C2 100pF/10V 0805 Generic N/A COG/NPO C3 1uF/10V Generic N/A X7R/X5R optional 0805 Table 1 www.zetex.com Bill of materials for 1W LED 70 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 DN85 Performance 90 0.4 Output Current (A) 0.5 Efficiency (%) 100 80 70 60 0.3 0.2 0.1 VF~3.3 VF~3.6 VF~3.3 50 6.2 5.8 5.4 5 4.6 4.2 3.8 6.2 5.8 Input Voltage (V) 5.4 5 4.6 4.2 3.8 Input Voltage (V) Input Voltage vs Efficiency Input Voltage vs Output Current 0.4 4 0.3 Output Voltage (V) Input Current (A) VF~3.6 0.0 3 0.2 2 0.1 1 VF~3.3 VF~3.6 VF~3.3 0.0 VF~3.6 0 6.2 5.8 5.4 5 4.6 4.2 3.8 6.2 Input Voltage (V) Issue 1 - August 2007 © Zetex Semiconductors plc 2007 5.4 5 4.6 Input Voltage (V) 4.2 3.8 Input Voltage vs Output Voltage Input Voltage vs Input Current Figure 2 5.8 Performance graphs for 1W design 71 www.zetex.com DN85 Ref. Value Package Part number Manufacturer Contact details Notes U1 N/A SOT23-6 ZXSC400E6 Zetex www.zetex.com LED Driver Q1 N/A SOT23 ZXTN25012EFL Zetex www.zetex.com Low sat NPN transistor D1 40V/1A SOT23 ZHCS1000 Zetex www.zetex.com 40V/1A Schottky diode L1 33H N/A 722065330 Wurth Elektronik www.we-online.com Isat=1.6A R1 30m⍀ 0805 Generic N/A 0805 1% R2 10⍀ 0805 Generic N/A 0805 5% R3 47⍀ 0805 Generic N/A 0805 5% C1 10F/10V 1210 Generic N/A X7R/X5R C2 100pF/10V 0805 Generic N/A COG/NPO C3 2.2uF/10V 1206 Generic N/A X7R/X5R optional Table 2 www.zetex.com Bill of materials for 3W LED 72 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 DN85 Intentionally left blank Issue 1 - August 2007 © Zetex Semiconductors plc 2007 73 www.zetex.com DN85 Performance 0.8 100 0.7 Output Current (A) Efficiency (%) 90 80 70 60 VF~3.2 0.6 0.5 0.4 0.3 0.2 0.1 VF~3.5 50 VF~3.2 VF~3.5 0.0 6.4 6 5.6 5.2 4.8 4.4 4 6.4 6 Input Voltage (V) 5.6 5.2 4.8 4.4 4 Input Voltage (V) Input Voltage vs Efficiency Input Voltage vs Output Current 0.8 4 0.6 Output Voltage (V) Input Current (A) 0.7 3 0.5 0.4 2 0.3 0.2 1 VF~3.2 0.1 VF~3.5 VF~3.2 0.0 VF~3.5 0 6.4 6 5.6 5.2 4.8 4.4 4 6.4 Input Voltage (V) www.zetex.com 5.6 5.2 4.8 Input Voltage (V) 4.4 4 Input Voltage vs Output Voltage Input Voltage vs Input Current Figure 3 6 Performance graphs for 3W design 74 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 DN86 Reduced component count and compact reference design for MR16 replacement lamps using multiple 1W LEDs Silvestro Russo, October 2007 Introduction MR16 lamps are one variety of Multifaceted Reflector (MR) lamps that usually employ a halogen filament capsule as the light source. They are used in many retail and consumer lighting applications where their size, configurability, spot-lighting capability and aesthetics provide utility and creativity. Low efficiency, heat generation and halogen capsule handling issues are among the disadvantages of the technology. They typically operate from 12V DC or 12V AC, using conventional electromagnetic transformers. LEDs offer a more energy efficient and no radiated heat solution to replace some halogen lamp applications. This reference design is intended to fit into the base connector space of an MR16 style LED lamp. The design has been optimized for part count and thermal performance. The design can be used with up to 3 1W LEDs in the Lens section. This can be arranged to suit the luminary designer's requirements. Figure 1 MR16 application with ZXLD1350 Data sheet It is recommended that this design note is used with the data sheet for the ZXLD1350 see http://www.zetex.com/3.0/pdf/ZXLD1350.pdf Issue 1 - October 2007 © Zetex Semiconductors plc 2007 75 www.zetex.com DN86 Description The system diagram of the MR16 lamp solution with ZXLD1350 and ZXSBMR16PT8 is shown in Figure 2, and Table 1 provides the bill of materials. Figure 2 System diagram of ZXLD1350 MR16 Lamp Solution The ZXLD1350 is designed for LED current drive applications of up to 350mA. The monolithic NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 400mA, which with the hysteretic mode of operation (the inductor current waveform will ramp +/-15% about the nominal current set point) provides sufficient margin. The main features of the ZXLD1350 are: • Up to 380mA output current • Wide input voltage range: 7V to 30V • Internal 30V 400mA NDMOS switch • High efficiency (>90% possible) • Up to 1MHz switching frequency The ZXSBMR16PT8 is a new space saving and thermally efficient device specifically designed for the critical requirements of MR16 applications. The device encompasses a full bridge and a freewheeling diode realized using extremely low leakage 1A, 40V Schottky diodes to allow a nominal 12V AC input operations. The Schottky bridge together with the embedded freewheeling diode enhance the system efficiency compared to the standard silicon diodes in a compact format. The reference design has solder tag pins to bypass the bridge rectifier should the final lamp design be used for purely DC operation. As the ZXLD1350 has a hysteretic switching topology, the switching frequency is dependent on several factors - input voltage, target current and number of LEDs. An Excel based calculator is available for system initial evaluation and component choice. See http://www.zetex.com/3.0/otherdocs/zxld1350calc.xls System efficiency and LED current have been measured keeping the ADJ pin floating and the current in the device at its rated value. The input impedance of the ADJ pin is high (200K) and is susceptible to leakage currents from other sources. Anything that sinks current from this pin will reduce the output current. In order to avoid any kind of electromagnetic coupling a guard track around this pin is used. www.zetex.com 76 Issue 1 - October 2007 © Zetex Semiconductors plc 2007 DN86 Quantity Part reference Value Description Source 1 R1 0.33⍀ Resistor, 1%, 0805 Various 2 C1, C2 150µF/20V Type D SMD Tantalum Cap Kemet 1 C3 0.1µF/25V SMD 0805 X7R NIC Componenents 1 C4 1µF/25V SMD 1210 X7R NIC Componenents 1 L1 100µH MSS6132-104 Coilcraft 1 U1 ZXLD1350 LED driver IC ZETEX 1 U2 ZXSBMR16PT8 Schottky bridge rectifier and freewheeling diode ZETEX Table 1 Bill of Material Referring to circuit schematic in Figure 2; the jumper connection could be used utilizing a zero ohm resistor, in order to enable the pure DC operations. Care has to be taken in this case, since the system is not reverse polarity protected. In Figure 3 the circuit layout is shown, highlighting its space saving features and compactness. Both bottom layer and top layer are shown to display effective devices arrangement. TOP COPPER AND SILKSCREEN BOTTOM COPPER AND SILK SCREEN Figure 3 Circuit layout The main layout design suggestions are: • All thin devices on one side • Employ a star connection for ground tracks • Use a ground ring protecting ADJ pin • Check that: • Tracks connecting R1 to ZXLD1350 are as short as possible (being sense tracks) • The filter capacitor C3 is connected as close as possible to the Vin pin • The freewheeling current path is as short as possible to ensure system precision and efficiency Issue 1 - October 2007 © Zetex Semiconductors plc 2007 77 www.zetex.com DN86 Circuit board views Top Layer Bottom Layer MR16 Pins - Anode LED connections + Cathode Figure 4 Circuit board views Choice of Inductor and switching circuit layout A 100 µH screened inductor was chosen to set the nominal frequency around 250kHz. A screened inductor is chosen to minimize radiated EMI. The layout with any switching regulator is crucial to minimize radiated EMI. This reference design keeps the critical track lengths to a minimum. Ground areas have been maximized around critical areas. Circuit performances Circuit performances have been evaluated taking into account two main parameters, the system efficiency and the current precision. The reference current is set to a nominal 300mA but can be adjusted to any value up to 350mA by changing the sense resistor Rsense according to the formula: Iref = 0.1/R1 For R1 = 0.33⍀ [A] Iref = 300mA Æ In Table 2 the data related to the system supplied with a DC voltage ranges from 12V to 15V. For these tests the Schottky bridge was included. The most important parameters are the system efficiency and the error between the rated LED current (300mA) and the actual LED current. In the DC case the frequency ranges between 150kHz and 300kHz, depending on the input voltage. Whatever the input voltage, the efficiency is higher than 87% and the error lower than 2%. Vin [V] Iin[A] Vout[V] Iout[A] Efficiency Current Accuracy 12.000 0.275 9.80 0.296 87,9% 1.3% 13.000 0.252 9.78 0.294 87.7% 2.0% 14.000 0.232 9.76 0.294 87.6% 2.0% 15.000 0.220 9.75 0.294 87.4% 2.0% Table 2 DC input voltage Table 3 shows the data related to the system supplied with an AC electromagnetic transformer. Using a SMD tantalum capacitor will save space and avoid using a larger aluminum electrolytic capacitor. This will improve the reliability of the system and stabilize performance during its lifetime. There is a trade off between physical size, reliability, cost and average LED current. Typical output voltages from a nominal 12V AC transformer can be ±10%. With 3 LEDs the voltage across these will be around 10V. If the input capacitor value is lower then 200µF, the AC input waveform is distorted (as can be seen in figure 8). When the rectified AC is not sufficiently www.zetex.com 78 Issue 1 - October 2007 © Zetex Semiconductors plc 2007 DN86 smoothed the ripple may drop below the combined LED forward voltage which stops the switching regulator and so reduces the average current in the LEDs. This will also reduce the average lumens output. C1 [µF] Vin [V] Iin[A] Vout[V] Iout[A] Efficiency Current Accuracy 100 12.70 0.303 9.28 0.225 54% 25% 150 12.60 0.394 9.50 0.271 52% 10% 200 12.53 0.432 9.55 0.293 52% 2% 300 12.50 0.386 9.70 0.295 60% 2% Table 3 AC input voltage Figures 5 to 7 show the input voltage ripple and LX voltage varying the input capacitance value Cin = C1 + C2 + C3. The higher the input capacitance the higher to output current precision and the average lumens outputs. The case with Cin=300µF has the best performance both as efficiency and current precision. Reducing the input capacitance the output current precision will decrease up to 25% with system efficiency always above 50%. Figure 5 Cin=300µF Issue 1 - October 2007 © Zetex Semiconductors plc 2007 Figure 6 Cin=200µF 79 www.zetex.com DN86 Figure 7 Cin=150µF Figure 8 Cin=100µF Figure 5 to 8: input ripple and LX voltage (Ch3 is the LX pin voltage and Ch4 is the input voltage) Gerber plots and further assistance are available from your local Zetex contact or Distributor. You can contact your local sales office by email. [email protected] [email protected] [email protected] Conclusion A compact, reliable, efficient and minimum part count solution can be realized using the ZXLD1350, ZXSBMR16PT8, and associated passive components. The compact design in the connector housing keeps the temperature sensitive semiconductors as far from the heat generating LEDs as possible. A compromise between LED current and size of capacitance is necessary for the final solution which accounts for efficiency, accuracy, size, and component count. This is the first design note in a series of reference designs MR16 variants solutions and options. www.zetex.com 80 Issue 1 - October 2007 © Zetex Semiconductors plc 2007 AN44 A high power LED driver for low voltage halogen replacement Introduction LED lighting is becoming more popular as a replacement technology for Halogen low voltage lighting, primarily because of the low efficiency, reliability and lifetime issues associated with Halogen bulbs. Discussed below is a novel approach for driving high power LED's as a replacement for low voltage halogen lighting systems. A typical schematic diagram is shown in Figure 1. Figure 1 Schematic diagram Operation Please refer to the typical schematic diagram in Figure 1. On period, TON The ZXSC300 turns on Q1 until it senses 19mV (nominal) on the ISENSE pin. The current in Q1 to reach this threshold is therefore 19mV/R1, called IPEAK. With Q1 on, the current is drawn from the battery and passes through C1 and LED in parallel. Assume the LED drops a forward voltage VF. The rest of the battery voltage will be dropped across L1 and this voltage, called V(L1) will ramp up the current in L1 at a rate di/dt = V(L1)/L1, di/dt in Amps/sec, V(L1) in volts and L1 in Henries. The voltage drop in Q1 and R1 should be negligible, since Q1 should have a low RDS(on) and R1 always drops less than 19mV, as this is the turn-off threshold for Q1. VIN = VF + V(L1) TON = IPEAK x L1/ V(L1) Issue 2 - July 2006 © Zetex Semiconductors plc 2006 81 www.zetex.com AN44 So TON can be calculated, as the voltage across L1 is obtained by subtracting the forward LED voltage drop from VIN. Therefore, if L1 is smaller, TON will be smaller for the same peak current IPEAK and the same battery voltage VIN. Note that, while the inductor current is ramping up to IPEAK, the current is flowing through the LED and so the average current in the LED is the sum of the ramps during the TON ramping up period and the TOFF ramping down period. Off period, TOFF The TOFF of ZXSC300 and ZXSC310 is fixed internally at nominally 1.7µs. Note that, if relying on this for current ramp calculations, the limits are 1.2µs min., 3.2µs max. In order to minimize the conductive loss and switching loss, TON should not be much smaller than TOFF. Very high switching frequencies cause high dv/dt and it is recommended that the ZXSC300 and 310 are operated only up to 200 kHz. Given the fixed TOFF of 1.7µs, this gives a TON of (5µs 1.7µs) = 3.3µs minimum. However, this is not an absolute limitation and these devices have been operated at 2 or 3 times this frequency, but conversion efficiency can suffer under these conditions. During TOFF, the energy stored in the inductor will be transferred to the LED, with some loss in the Schottky diode. The energy stored in the inductor is: ½ x L x IPEAK2 [Joules] Continuous and discontinuous modes (and average LED current) If TOFF is exactly the time required for the current to reach zero, the average current in the LED will be IPEAK/2. In practice, the current might reach zero before TOFF is complete and the average current will be less because part of the cycle is spent with zero LED current. This is called the ‘discontinuous’ operation mode and is shown in Figure 2. Figure 2 www.zetex.com 82 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 AN44 For continuous mode If the current does not reach zero after 1.7µs, but instead falls to a value of IMIN, then the device is said to be in ‘continuous’ mode. The LED current will ramp up and down between IMIN and IPEAK (probably at different di/dt rates) and the average LED current will therefore be the average of IPEAK and IMIN, as shown in Figure 3. Figure 3 Design example (Refer to Figure 1 and Table 1) Input = VIN = 12V LED forward drop = VLED = 9.6V VIN = VLED+VL Therefore VL = (12 - 9.6) = 2.4 The peak current = VSENSE / R1 (R1 is RSENSE) = 24mV/50mR = 480mA TON = IPEAK x L1/V(L1) 680mAx22μH T ON --------------------------------------- = 6.2μs 2.4 These equations make the approximation that the LED forward drop is constant throughout the current ramp. In fact it will increase with current, but they still enable design calculations to be made within the tolerances of the components used in a practical circuit. Also, the difference between VIN and VLED is small compared to either of them, so the 6.2µs ramp time will be fairly dependent on these voltages. Note that, for an LED drop of 9.6V and a Schottky drop of 300mV, the time to ramp down from 680mA to zero would be: 680mAx22μH TDIS --------------------------------------- = 1.5μs ( 9.6 + 0.3 ) Issue 2 - July 2006 © Zetex Semiconductors plc 2006 83 www.zetex.com AN44 As the TOFF period is nominally 1.7µs, the current should have time to reach zero. However, 1.5µs is rather close to 1.7µs and it is possible that, over component tolerances, the coil current will not reach zero, but this is not a big issue as the remaining current will be small. Note that, because of the peak current measurement and switch-off, it is not possible to get the dangerous ‘inductor staircasing’ which occurs in converters with fixed TON times. The current can never exceed IPEAK, so even if it starts from a finite value (i.e. continuous mode) it will not exceed the IPEAK. The LED current will therefore be approximately the average of 680mA and zero = 340mA (it will not be exactly the average, because there is a 200ns period at zero current, but this is small compared with the IPEAK and component tolerances). Ref Value Part number Manufacturer Contact details Comments U1 ZXSC310E5 Zetex www.zetex.com LED Driver in SOT23-5 Q1 ZXMN6A07F Zetex www.zetex.com N-channel MOSFET in SOT23 www.zetex.com 1A Schottky diode in SOT23 D1 1A / 40V ZHCS1000 Zetex D2 6V8 Generic Generic L1 22H DO3316P-223 Coilcraft R1 50m⍀ Generic Generic 0805 size R2 1k2⍀ Generic Generic 0805 size C1 100F/25V Generic Generic C2 1F/10V Generic Generic C3 2.2F/25V Generic Generic Table 1 www.zetex.com 6V8 Zener diode www.coilcraft.com Bill of materials 84 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 AN44 Typical performance graphs for 12V system Figure 4 Performance graphs for 12V system By changing the value of R2 from 1k2⍀ to 2k2⍀ the operating input voltage range can be adjusted from 30V to 20V, therefore the solution is able to operate from the typical operating voltage supplies of 12V and 24V for low voltage lighting. Typical performance graphs for 24V system Figure 5 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 Performance graphs for 24V system 85 www.zetex.com AN44 Useful formulae for calculations The input power from the battery during TON (assuming discontinuous operation mode) is VIN * IPEAK/2. The average input current from the battery is therefore this current multiplied by the ratio of TON to the total cycle time: T ON I PEAK ---------------- × -------------------------------2 T ON × T OFF It can be seen from this how the average battery current will increase at lower VIN as TON becomes larger compared to the fixed 1.7µs TOFF. This is logical, as the fixed (approximately) LED power will require more battery current at lower battery voltage to draw the same power. The energy which is stored in the inductor equals the energy which is transferred from the inductor to the LED (assuming discontinuous operation) is: ½ * L1 * IPEAK2 [Joules] I PEAK × L1 T ON = -------------------------------------------( V BATT – V LED ) Therefore, when the input and the output voltage difference are greater, the LED will have more energy which will be transferred from the inductor to the LED rather than be directly obtained from the battery. If the inductor size L1 and peak current IPEAK can be calculated such that the current just reaches zero in 1.7µs, then the power in the LED will not be too dependent on battery volts, since the average current in the LED will always be approximately IPEAK/2. As the battery voltage increases, the TON necessary to reach IPEAK will decrease, but the LED power will be substantially constant and it will just draw a battery current ramping from zero to IPEAK during TON. At higher battery voltages, TON will have a lower proportional of the total cycle time, so that the average battery current at higher battery voltage will be less, such that power (and efficiency) is conserved. The forward voltage which is across the Schottky diode detracts from the efficiency. For example, assuming VF of the LED is 6V and VF of the Schottky is 0.3V, the efficiency loss of energy which is transferred from the inductor is 5%, i.e. the ratio of the Schottky forward drop to the LED forward drop. The Schottky is not in circuit during the TON period and therefore does not cause a loss, so the overall percentage loss will depend on the ratio of the TON and TOFF periods. For low battery voltages where TON is a large proportion of the cycle, the Schottky loss will not be significant. The Schottky loss will also be less significant at higher LED voltages (more LED's in series) as Schottky drop becomes a lower percentage of the total voltage. www.zetex.com 86 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 AN47 Getting more out of the ZXLD1350 - dimming techniques Ray Liu, Systems Engineer, Zetex Semiconductors Introduction The ZXLD1350 has a versatile adjust pin that can be used in many ways to adjust the brightness of the LED by controlling the current in the LED. This application note deals with some the ways in which dimming the LED can be achieved and discusses the merits of the techniques. These dimming methods discussed include PWM dimming both with a low and high frequency signals, DC voltage control and resistive dimming. Low frequency dimming Low frequency dimming is preferred for LED dimming since the LED instantaneous driving current is constant. The color temperature of the LED is preserved at all dimming levels. Another advantage of low frequency dimming is that the dimming level can down to 1%. Hence result in dimming range of 100:1. Choice of frequency To avoid visible flicker the PWM signal must be greater than 100Hz. If you choose too high a frequency the internal low pass filter will start to integrate the PWM signal and produce a non linear response. Also the soft start function of the ADJ pin will cause a delay on the rising a falling edge of the PWM signal. This can give a non-linearity in the LED current which will have a greater affect as frequency increases. An upper limit of 1kHz is suggested. The effect of audible noise in the inductor may need to be considered. This may happen in some inductors with loose windings and will be more noticeable at PWM frequencies of 1kHz than 100Hz. If the PWM frequency is less than approximately 500Hz, the device will be gated 'on' and 'off' and the output will be discontinuous, with an average value of output current given by: 0.1 D PWM I OUT ≈ -------------------------RS [for 0<DWPM<1] ADJ PWM ZXLD1350 GND GND Issue 2 - August 2007 © Zetex Semiconductors plc 2007 87 www.zetex.com AN47 High frequency dimming High frequency dimming is preferred if system required low radiated emission and in/output ripple. But dimming range is reduced to 5:1. The ZXLD1350 has an internal low pass filter which integrates the high frequency PWM signal to produce a DC dimming control. If the PWM frequency is higher than approximately 10kHz and the duty cycle above the specified minimum value, the device will remain active and the output will be continuous, with a nominal output current given by: 0.1 D PWM I OUT ≈ -------------------------RS [for 0.16< DPWM <1] ADJ PWM ZXLD1350 GND GND Input buffer transistor For PWM dimming an input bipolar transistor with open collector output is recommended. This will ensure the 200mV input shutdown threshold is achieved. It is possible to PWM directly without a buffer transistor. This must be done with caution. Doing this will overdrive the internal 1.25V reference. If a 2.5V input level is used at 100% PWM (DC) the output current into the LED will be 2X the normal current which may destroy the ZXLD1350. Overdriving with a 5V logic signal is very likely to damage the device as it exceeds the ADJ pin voltage rating. Soft start and decoupling capacitors Any extra capacitor on the ADJ pin will affect the leading and falling edge of the PWM signal. Take this into account as the rise time will be increased by approximately 0.5ms/nF. Compare this with a 100Hz PWM. 50% duty cycle Ton and Toff are 5ms at 1% duty cycle Ton is 0.1ms. 1nF on the ADJ pin will cause 0.5ms rise time which result in an error and limitation in dimming at low duty cycles. www.zetex.com 88 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 AN47 DC voltage dimming The ADJ pin can be overdriven by an external DC voltage (VADJ), as shown, in order to override the internal voltage reference and adjust the output current to a value above or below the nominal value. ZXLD1350 ADJ + GND DC GND The nominal output current is then given by: 0.08 × V ADJ I OUT ≈ -------------------------------RS [for 0.3< VADJ <2.5V] Figure 1 shown the relationship of LED current against VADJ with VADJMAX =1.25V (VIN = 12V). Note that 100% brightness setting corresponds to VADJ = VREF = 1.25V with RS = 300m⍀. The minimum dimming ratio is governed by the VADJON which is 250mV typically. In this case, the minimum dimmable current is 20% of full LED current. This gives dimming ratio of 1:5. IOUT vs VADJ (RS = 300m⍀) 350 300 250 mA 200 150 100 50 0 0 0.25 0.5 0.75 1 1.25 V Figure 1 Typical output current versus ADJ voltage with RS = 300m⍀ Issue 2 - August 2007 © Zetex Semiconductors plc 2007 89 www.zetex.com AN47 Switching frequency is another factor to consider for DC voltage dimming. Figure2 shows the relationship of switching frequency current against VADJ with L=100H. As VADJ decreases, switching frequency increases. Care had to be taken for choosing the right inductor to achieve the desirable operating frequency range with the aid of the ZXLD1350 calculator. Figure 2 Typical switching frequency versus ADJ voltage with RS = 300m⍀, L=100H In order to maximize the dimming ratio, we could increase the maximum value of VADJ to 2.5V. In this case, the minimum dimmable current is 10% of full LED current. This gives dimming ratio of 1:10. RSENSE should then be increased by 2X RS. This will slightly decrease the efficiency by 1 to 2%. Figure 3 shows the relationship of LED current against VADJ with VADJMAX = 2.5V (VIN = 12V). IOUT vs VADJ (RS = 600m⍀) 350 300 mA 250 200 150 100 50 0 0 0.5 1 1.5 2 2.5 V Figure 3 www.zetex.com Typical output current versus ADJ voltage with RS = 600m⍀ 90 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 AN47 Figure 4 Typical switching frequency versus ADJ voltage with RS = 600m⍀, L=100H The input impedance of the ADJ pin is 200k⍀ ±20%. This may be the factor to consider if the DC dimming voltage is from a relatively high output resistance. Figure 5 shows a typical circuit that would provide 1.25V dimming voltage. VIN R1 4.7k VR1 10k ADJ GND ZTLV431 Figure 5 ZXLD1350 Typical circuit of DC voltage dimming The ZTLV431 acts as a shunt regulator to generate an external 1.25V reference voltage. The reference voltage is applied to pot VR1 to provide dimming voltage of 0-1.25V. Using an external regulator affects the accuracy of the current setting. If a 1% reference is used the LED current will be more accurate than using the internal reference. Issue 2 - August 2007 © Zetex Semiconductors plc 2007 91 www.zetex.com AN47 Resistor dimming By connecting a variable resistor between ADJ and GND, simple dimming can be achieved. Capacitor CADJ is optional for better AC mains interference and HF noise rejection. Recommend value of CADJ is 0.22F. RS ZXLD1350 ADJ *CADJ RADJ GND GND The current output can be determined using the equation: ( 0.08 ⁄ R S ) × R ADJ I OUT = -----------------------------------------------( R ADJ + 200k ) Note that continuous dimming is not possible with a resistor. At some point the shutdown threshold will be reached and the output current reduced to zero. This can occur below 300mV. Note that a 1M⍀ resistor will load the VREF on the ADJ pin. The VREF will now be divided down by the nominal 200k VREF resistance and the 1M RADJ. The nominal voltage will now be approximately 1V. RS will need to be adjusted to set the maximum current. The +/20% tolerance of the input resistance should also be understood. See table below: RADJ k⍀ Rint nom. k⍀ Rint min. k⍀ Rint max. k⍀ VADJ nominal % error from nominal due to Rint min. % error from nominal due to Rint max. 1000 200 160 240 1.041 3.4% -3.3% 500 200 160 240 0.892 6.1% -5.7% 200 200 160 240 0.625 11.1% -10.0% 100 200 160 240 0.416 15.4% -13.3% Table 1 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 92 www.zetex.com AN47 IOUT 0.35 0.3 0.25 0.2 IOUT 0.15 0.1 0.05 0 -0.05 0 200 Figure 6 400 600 800 1000 Typical output current against pot resistance If linear pot is used, the output current change is not linear against shaft rotation. In order to make the output current more linear, a log type pot is used. IOUT vs shaft rotation 350 300 IOUT (mA) 250 200 150 100 50 0 0 10 20 30 40 50 60 70 80 90 100 Shaft rotation (%) Figure 7 Issue 2 - August 2007 © Zetex Semiconductors plc 2007 Output current against shaft rotation of log type pot 93 www.zetex.com Intentionally left blank Issue 2 - August 2007 © Zetex Semiconductors plc 2007 94 www.zetex.com AN48 Getting more out of the ZXLD1350 - high output current Ray Liu, Systems Engineer, Zetex Semiconductors Introduction The ZXLD1350 is a continuous mode inductive step-down converter, designed for driving single or multiple series connected LEDs efficiently from a voltage source higher than the LED voltage. The device operates from an input supply between 7V and 30V and provides an externally adjustable output current of up to 350mA. In order to obtain higher output current to drive LEDs with higher power, a high current externally connected output stage is required. 700mA driver for multiple 3W LEDs in series This driver is designed to drive up to six 3W LEDs in series which could deliver total output power of 15W with an overall efficiency of around 90%. RS2 LED1 LEDN L1 RS1 D1 R3 Q3 V6 6 VIN + 4 ISENSE U1 C1 R1 LX GND ADJ 1 2 3 R2 D2 C2 Q1 Q2 0 Figure 1 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 Schematic of 700mA driver 95 www.zetex.com AN48 Part list Table 1 Part ref. Part no. Remark U1 ZXLD1350 Q1 FCX619 Q2 FMMT619 Q3 FMMT619 D1 ZLLS1000 D 25.6V Zener diode L1 68H 1A RS1 150m⍀ RS2 2.2⍀ R1 2.2K⍀ R2 470⍀ R3 15K⍀ C1 3.3F 50V X5/7R or other low ESR cap C2 0.1F Optional Circuit description The output driver consists of two NPN transistors (Q1 and Q2). Transistor Q2 acts as a small signal inverter which inverts the original LX switch signal. The collector of Q2 is connected to the base of transistor Q1 which acts as the power output switch. Transistor Q3 and Zener diode D2 form a simple regulator to supply a constant voltage to the driver stage. The voltage at emitter of Q3 is around 5V. This helps to provide a stable driving current to both Q1 and Q2. The driving currents are around 2mA and 9mA respectively. Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time of the output switch are less than 70ns when input supply voltage is 30V. www.zetex.com 96 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 AN48 Typical performance graphs 5 LEDs in series with total VF = 17.9V 100 800 Output Current (mA) Efficiency (%) 90 80 70 700 600 500 60 400 50 21 22 23 24 25 26 27 28 29 21 30 22 23 24 25 26 27 28 29 30 Input Voltage (V) Input Voltage (V) Efficiency vs Input Voltage (5 LEDs) Output Current vs Input Voltage (5 LEDs) 4 LEDs in series with total VF = 14.9V 100 800 Output Current (mA) Efficiency (%) 90 80 70 700 600 500 60 400 50 18 20 22 24 26 28 18 30 20 22 24 26 28 30 Input Voltage (V) Input Voltage (V) Efficiency vs Input Voltage (4 LEDs) Output Current vs Input Voltage (4 LEDs) 3 LEDs in series with total VF = 11.1V 100 800 Output Current (mA) Efficiency (%) 90 80 70 60 700 600 500 400 50 15 20 25 15 30 Efficiency vs Input Voltage (3 LEDs) Issue 3 - May 2007 © Zetex Semiconductors plc 2007 20 25 30 Input Voltage (V) Input Voltage (V) Output Current vs Input Voltage (3 LEDs) 97 www.zetex.com AN48 Typical performance graphs (cont.) 2 LEDs in series with total VF = 7.7V 100 800 Output Current (mA) Efficiency (%) 90 80 70 700 600 500 60 400 50 12 15 18 21 24 27 12 30 15 18 21 24 27 30 Input Voltage (V) Input Voltage (V) Efficiency vs Input Voltage (2 LEDs) Output Current vs Input Voltage (2 LEDs) 1 LED in series with VF = 3.8V 100 800 Efficiency (%) Output Current (mA) 90 80 70 60 700 600 500 400 50 10 15 20 25 10 30 Efficiency vs Input Voltage (1 LED) www.zetex.com 15 20 25 30 Input Voltage (V) Input Voltage (V) Output Current vs Input Voltage (1 LED) 98 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 AN48 A driver for supply voltage up to 16V This driver is a simplified version to the 700mA driver described above. The driver is designed to drive up to 3 Luxeon® K2 LEDs in series which could deliver a total output power of 10W with a maximum input supply voltage of 16V. LED1 LEDN RS1 L1 D1 V6 + 6 4 VIN ISENSE R1 R2 U1 C1 Q1 LX GND ADJ 1 2 3 C2 Q2 0 Figure 2 Schematic of 1A driver Part List Table 2 Part ref. Part no. U1 ZXLD1350 Q1 ZXTN25020DFH Q2 ZXTN25020DFH D1 ZLLS2000 L1 47H 1.5A RS 100m⍀ R1 4.7K⍀ R2 1.5K⍀ C1 4.7F 25V X5/7R or other low ESR cap C2 0.1F Optional Issue 3 - May 2007 © Zetex Semiconductors plc 2007 Remark 99 www.zetex.com AN48 Circuit description This circuit is similar to the 700mA driver described above. The output driver consists of two NPN transistors (Q1 and Q2). Transistor Q2 acts as a small signal inverter which inverts the original LX switch signal. The collector of Q2 is connected to the base of transistor Q1 which act as the power output switch. Unlike the 700mA driver, the driving current to both Q1 and Q2 varies with the input supply voltage. Hence, the maximum input supply voltage is limited to 16V. The driving current to Q1 is between 5mA and 10mA with input supply voltage between 8V and 16V. Lowering the maximum supply voltage to 16V enables us to use a lower voltage BJT with better switching performance. Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time of the output switch are less than 60ns when input supply voltage is 16V. www.zetex.com 100 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 AN48 Typical performance graphs 2 LEDs in series with total VF = 7.1V 1100 Output Current (mA) 100 Efficiency (%) 90 80 70 60 50 900 700 500 10 11 12 13 14 15 16 10 11 12 13 14 15 16 Input Voltage (V) Output Current vs Input Voltage (2 LEDs) Input Voltage (V) Efficiency vs Input Voltage (2 LEDs) 1 LED in series with total VF = 3.5V 1100 Output Current (mA) 100 Efficiency (%) 90 80 70 60 50 900 700 500 8 10 12 14 16 8 Input Voltage (V) Efficiency vs Input Voltage (1 LED) Issue 3 - May 2007 © Zetex Semiconductors plc 2007 10 12 14 16 Input Voltage (V) Output Current vs Input Voltage (1 LED) 101 www.zetex.com AN48 Intentionally left blank Issue 3 - May 2007 © Zetex Semiconductors plc 2007 102 www.zetex.com AN50 Feed forward compensation for ZXSC300 LED driver Yong Ang, Application Engineer, Zetex Semiconductors Input voltage feed forward compensation for ZXSC300 to improve control of the LED current Introduction The ZXSC300 LED drivers do not directly control the LED current. As a consequence the LED current is dependent of the input voltage. This application note describes a way of reducing the supply voltage dependency by a method of supply voltage feed forward compensation. The method can also be used to provide temperature compensation of the LED. The ZXSC300 works on the PFM control scheme where the LED current is simply regulated by controlling the peak current through transistor Q1. The internal voltage threshold of current sense pin is around 19mV and transistor Q1 is switched off when its current reaches the preset threshold, thereby necessitating fewer external components required. However, this threshold value is invariant to the supply voltage level. In the event where input voltage increases, peak Q1 current will stay the same and current delivered to the LED creeps up which could potentially damage the LED if it exceeds the maximum rated current of the device. The circuit diagram in Figure 1 shows how to apply input voltage and thermal correction to a typical LED. A simple design guide for a single LED driver has also been put forward. The equations can generate a design capable of sourcing up to 200mA LED current, when used with the Zetex high current gain NPN transistor-ZXTN25012EFH. Input voltage feed forward compensation Normally, IPK is set by the output current threshold voltage VISENSE divided by RSENSE. As the input voltage increases, the inductor ripple current level ⌬I decreases because the transistor off time, TOFF is fixed by the ZXSC300 ⌬I = (VOUT - VIN) • TOFF ÷ L L discharges at a flatter slope to a higher minimum choke current IMIN = IPK - ⌬I, before transistor Q1 is turned on again. Figure 1 Circuit diagram of ZXSC300 with feed forward and thermal compensation Issue 1 - October 2007 © Zetex Semiconductors plc 2007 103 www.zetex.com AN50 Consequently, the average current IAV flowing through L increases and a shorter transistor on time, TON is required to charge boost inductor to the preset threshold current level IPK TON = ⌬I • L ÷ VIN By making the aforementioned assumptions for turn-on period and average coil current, the output power delivered to the LED is now determined from POUT = VLED • IAV • TOFF ÷ (TON + TOFF) Therefore, a higher power and LED current is delivered to the LED at high VIN for a fixed RSENSE and this elevated current could potentially damage the LED if it exceeds the maximum rated current of the device. Ignoring the effect of thermistor RT for the moment, a 100⍀ resistor ROFF can be inserted in series with RSENSE and feed forward resistor Rfb (see Figure 1) to inject a slight voltage offset across resistor RSENSE. This enables a lower Q1's current to build up the required VISENSE to turn the driver off, which regulates the LED current. The Rfb value has to be sufficiently big to lower dissipation and to prevent circuit from stalling. The circuit could stall at high input voltage if Rfb drops 19mV or more across 100⍀ resistor forcing the driver off all the time. It must be noted that ISENSE pin threshold on ZXSC300 has a positive temperature coefficient of 0.4%/°C. If a circuit nominal operating temperature is higher than 65°C, it could give approximately 20% increase in average LED current from that in 25°C ambient. When a feed forward network is used, this injects an offset voltage to the threshold pin. For instance, if an offset voltage of 9.5mV is used, the effective VISENSE temperature coefficient becomes double. Therefore, it is essential that thermal compensation is used with a feed forward approach. Feed forward components calculation For initial estimation, the associated IAV(VMAX) that delivers the required LED current can be determined from IAV(VMAX) = POUT ÷ (F • TOFF • VOUT) Where the transistor switching frequency F is given by F = VIN(MAX) ÷ VOUT ÷ TOFF Figure 2 www.zetex.com Example of current and voltage waveforms for circuit using ZXSC300 with feed forward network 104 Issue 1 - October 2007 © Zetex Semiconductors plc 2006 AN50 IAV(VMAX) is used to establish the required DC current rating, IDC for boost inductor L. The minimum inductor current is given by, IMIN(VMAX) = IAVE – 0.5 • (VOUT – VIN(MAX)) • TOFF /L A high L value is recommended to minimize errors due to propagation delays at high input voltage, which results in increased ripple and lower efficiency. And the maximum inductor current which relates to the Q1 peak current is IPK(VMAX) = 2 • IAVE - IMIN(VMAX) In practice, a higher IPK(VMAX) value can be used to account for the VCE saturation and switching edge loss in the transistor. The value of feed forward resistor Rfb is selected to give IPK(VMIN) at worse case input voltage and IPK(VMAX) at maximum input voltage. The internal VISENSE threshold on the ZXSC300 is typically 19mV with ±25% tolerance at 25°C. RSENSE has to drop less voltage than that demanded by VISENSE as Rfb will make a contribution to satisfy the threshold, which lowers IPK value with increasing input. Allowing for the positive temperature coefficient on ISENSE pin, effective threshold voltage level at operating temperature TAMB is; VISENSE@TAMB = 19mV ± 25% • 0.4%/°C • (TAMB - 25°C). At low supply voltage VIN(MIN) VISENSE@TAMB = IPK(VMIN) • RSENSE + VIN(MIN) • 100⍀ ÷ (Rfb + 100⍀) Whilst at VIN(MAX), VISENSE@TAMB = IPK(VMAX) • RSENSE + VIN(MAX) • 100⍀ ÷ (Rfb + 100⍀) Solving the above simultaneous equations gives the required RSENSE and Rfb resistor values. These design equations are also available as a spreadsheet calculator from Zetex website at www.zetex.com/zxsc300feedforward Figure 3 shows the measured LED current against variation in the input voltage with feed forward compensation. For comparison purpose, the same measurement is repeated with feed forward network removed, in which case the LED current at low supply is 3 times lower than that at nominal input voltage level. 70 LED Current (A) 60 50 Feed forward compensation No feed forward network 40 30 20 10 0 1.6 1.8 2 2.2 2.4 2.6 2.8 3 Input Voltage (V) Figure 3 LED current discrepancy for ZXSC300 with feed forward compensation Issue 1 - October 2007 © Zetex Semiconductors plc 2007 105 www.zetex.com AN50 The improvement in LED current regulation through feed forward compensation is self-evident. Although some discrepancy in LED current persists at low supply, this is predominantly due to the dependency of internal VISENSE threshold level on the input voltage level. To incorporate thermal compensation into the design, Rfb can be made up from a series combination of normal resistor R1 and NTC RT. During start-up condition, the printed circuit board’s and LED’s temperatures are low, hence RT has high resistance. As circuit temperature rises to its design operating value, the effective feed forward resistance drops, increasing the offset voltage on ISENSE pin, which in turn matches the elevated VISENSE value and hence regulates the actual output current fed to the LED. For instance, the required effective feed forward resistor value (R1+ RT) for 25°C ambient start-up can be determined from Rfb = VIN(MAX) • 100⍀ • (19mV ± 25% - IPK(VMAX) • RSENSE) And the required normal resistor R1 is equivalent to Rfb - RT. For this design, three NTC values (3.3K⍀, 4.7K⍀ and 6.8K⍀) are recommended. These resistors with MURATA 0603 or 0805 size NTC thermistors with beta-constant value of 3950K are chosen to give good current control response at both normal operating temperature and start-up conditions. The NTC works to reduce the peak transistor current, facilitating thermal feedback control to ensure that LED current and lumen maintenance expectation are achieved. Note that it is sometimes difficult to achieve perfect LED current matching between start-up and normal operating temperature. In extreme cases of large temperature gradients, the average LED current should be lower at start-up giving less lumen output, and then ramps up to the rated current once it reaches the normal operating temperature. Furthermore, the thermistor can be thermally coupled to the LED to provide response tracking and prevent overheating. Conclusion Two or three additional external components can be used to provide input voltage feed forward for ZXSC300. This serves to ensure that the LED current is closely regulated. The LED current regulation improves significantly when feed forward compensation is employed. The LED current at the worse case input voltage increases from 33% to 64% of the nominal LED current with a feed forward network. The remaining discrepancy is predominantly due to the dependency of the VISENSE threshold level on the input voltage level. In applications where the circuit is designed to operate in elevated ambient temperature, a NTC thermistor can be incorporated to facilitate thermal feedback control and prevent over heating. www.zetex.com 106 Issue 1 - October 2007 © Zetex Semiconductors plc 2006