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 1 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 2 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 3 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 4 Issue 5 - August 2007 © Zetex Semiconductors plc 2006 DN61 Intentionally left blank Issue 5 - August 2007 © Zetex Semiconductors plc 2006 5 www.zetex.com DN61 Definitions Product change Zetex Semiconductors reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or service. Customers are solely responsible for obtaining the latest relevant information before placing orders. Applications disclaimer The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for the user’s application and meets with the user’s requirements. 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