ZCC1937 White LED Step-Up Converter in SC70 and ThinSOT FEATURES DESCRIPTION Inherently Matched LED Current High Efficiency: 84% Typical Drives Up to Four LEDs from a 3.2V Supply Drives Up to Six LEDs from a 5V Supply 36V Rugged Bipolar Switch Fast 1.2MHz Switching Frequency Uses Tiny 1mm Tall Inductors Requires Only 0.22μF Output Capacitor Low Profile SC70 and ThinSOTTM Packaging The ZCC1937 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The device can drive two, three or four LEDs in series from a Li-Ion cell. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors. The ZCC1937 switches at 1.3MHz, allowing the use of tiny external components. The output capacitor can be as small as 0.22μF, saving space and cost versus alternative solu-tions. A low 95mV feedback voltage minimizes power loss in the current setting resistor for better efficiency. APPLICATIONS Cellular Phones PDAs, Handheld Computers Digital Cameras MP3 Players GPS Receivers The zcc1937 is available in low profile SC70 and ThinSOT packages. TYPICAL APPLICATION Conversion L1 22μH VIN 3V TO 5V C1 1μF C2 0.22μF LED 1 VIN SW ZCC1937 OFF ON 90 D1 SHDN LED 2 85 80 15mA VIN = 3V LED 3 VIN = 3.6V 75 FB GND 70 R1 6.34Ω 1937 F01a C1, C2: X5R OR X7R DIELECTRIC D1: CENTRAL SEMICONDUCTOR CMDSH-3 L1: MURATA LQH3C-220 OR EQUIVALENT 65 60 0 Figure 1. Li-Ion Powered Driver for Three White LEDs 5 15 10 LED CURRENT (mA) 20 1937 TA01b 1937f 1 ZCC1937 ABSOLUTE MAXIMUM RATINGS Input Voltage (VIN) ................................................. SW Voltage ............................................................. FB Voltage .............................................................. SHDN Voltage ......................................................... (Note 1) 10V 36V 10V 10V Extended Commercial Operating Temperature Range (Note 2) ... – 40°C to 85°C Maximum Junction Temperature .......................... 125°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C 1. PACKAGE/ORDER InFORWATION ORDER PART NUMBER TOP VIEW SW 1 TOP VIEW 5 VIN GND 2 FB 3 ORDER PART NUMBER ZCC1937ES5 4 SHDN SW 1 6 VIN GND 2 5 GND FB 3 S5 PACKAGE 5-LEAD PLASTIC TSOT-23 S5 PART MARKING TJMAX = 125°C, θJA = 256°C/ W IN FREE AIR θJA = 120°C ON BOARD OVER GROUND PLANE LTYN ZCC1937ESC6 4 SHDN SC6 PART MARKING SC6 PACKAGE 6-LEAD PLASTIC SC70 LAAB TJMAX = 125°C, θJA = 256°C/ W IN FREE AIR θJA = 150°C ON BOARD OVER GROUND PLANE Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS PARAMETER TA = 25°C, VIN = 3V, VS HDN = 3V, unless otherwise noted. CONDITIONS MIN Minimum Operating Voltage TYP UNITS 2.5 V Maximum Operating Voltage Feedback Voltage MAX ISW = 100mA, Duty Cycle = 66% FB Pin Bias Current 95 104 mV 10 45 100 nA 1.9 0.1 2.5 1.0 mA μA 1.3 1.7 MHz SHDN = 0V Switching Frequency 0.9 Maximum Duty Cycle 85 Switch Current Limit ISW = 250mA Switch Leakage Current VSW = 5V SHDN Voltage High 90 % 320 mA 350 mV 0.01 Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. μA 5 1.5 V SHDN Voltage Low SHDN Pin Bias Current V 86 Supply Current Switch VCESAT 10 0.4 V μA 65 Note 2: The ZCC1937E is guaranteed to meet specifications from 0°C to 70°C. Specifications over the – 40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. 1937f 2 ZCC1937 TYPICAL PERFORMANCE CHARACTERISTICS Quiescent Current 2.2 –50°C 2.0 1.8 25°C 1.6 Switching Frequency SHDN Pin Bias Current 100°C 400 1.4 350 1.2 300 1.0 SHDN = 10V 1.4 250 0.8 1.2 200 1.0 0.6 150 0.8 0.6 SHDN = 3.6V 100 SHDN = 3V 0.4 0.4 0.2 50 0.2 0 0 2 4 VIN (V) 6 8 SHDN = 2.7V 0 – 50 – 25 0 50 25 TEMPERATURE (°C) 1937 G01 75 100 0 –50 75 100 1937 G03 Current Limit vs Duty Cycle Efficiency vs Temperature 85 50 50 25 0 TEMPERATURE (°C) 1937 G02 Feedback Bias Current 60 –25 350 VIN = 3.6V 3 LEDs 300 ILED = 20mA 84 250 40 83 ILED = 15mA 82 ILED = 10mA 200 30 150 20 100 81 10 0 –50 –25 50 25 0 TEMPERATURE (°C) 75 100 80 –50 50 0 0 50 100 TEMPERATURE (°C) 1937 G04 1937 G05 0 20 40 60 DUTY CYCLE (%) 80 100 1937 G06 PINFU CTIONS SW (Pin 1): Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI. SHDN (Pin 4): Shutdown Pin. Connect to 1.5V or higher to enable device; 0.4V or less to disable device. GND (Pin 2): Ground Pin. Connect directly to local ground plane. GND (Pin 5, SC70 Package): Ground Pin. Connect to Pin 2 and to local ground plane FB (Pin 3): Feedback Pin. Reference voltage is 95mV. Connect cathode of lowest LED and resistor here. Calculate resistor value according to the formula: VIN (Pin 5/Pin 6 SC70 Package): Input Supply Pin. Must be locally bypassed. RFB = 95mV/ILED 1937f 3 ZCC1937 BLOCK DIAGRA VIN (PIN 6 FOR SC70 PACKAGE) 5 FB 3 1 SW – VREF 1.25V 95mV COMPARATOR A1 + DRIVER A2 RC + R S Q Q1 CC Σ RAMP GENERATOR SHDN 4 0.2Ω (PINS 2 AND 5 FOR SC70 PACKAGE) SHUTDOWN 2 GND 1937 BD1 1.3MHz OSCILLATOR Figure 2. ZCC1937Block Diagram OPERATION The ZCC1937 uses a constant frequency, current mode control scheme to provide excellent line and load regula- tion. Operation can be best understood by referring to the block diagram in Figure 2. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 95mV. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. Minimum Output Current The ZCC1937 can regulate three series LEDs connected at low output currents, down to approximately 4mA from a 4.2V supply, without pulse skipping, using the same external components as specified for 15mA operation. As current is further reduced, the device will begin skipping pulses. This will result in some low frequency ripple, although the LED current remains regulated on an average basis down to zero. The photo in Figure 3 details circuit operation driving three white LEDs at a 4mA load. Peak inductor current is less than 50mA and the regulator operates in discontinuous mode, meaning the inductor current reaches zero during the discharge phase. After the inductor current reaches zero, the switch pin exhibits ringing due to the LC tank circuit formed by the inductor in combination with switch and diode capacitance. This ringing is not harmful; far less spectral energy is contained in the ringing than in the switch transitions. The ringing can be damped by application of a 300Ω resistor across the inductor, although this will degrade efficiency. VSW 5V/DIV IL2 50mA/DIV VOUT 100mV/DIV 0.2μs/DIV 1937 F03 1937f 4 1937 APPLICATIONS INFORMATION Capacitor Selection Inductor Selection The small size of ceramic capacitors makes them ideal for ZCC1937 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1μF input capacitor and a 0.22μF output Table␣ 1. The efficiency comparison of different inductors is capacitor are sufficient for most ZCC1937 applications. shown in Figure 4. Table 2. Recommended Ceramic Capacitor Manufacturers A 22μH inductor is recommended for most ZCC1937 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1.3MHz and low DCR (copper wire resistance). Some inductors in this category with small size are listed in Table 1. Recommended Inductors DCR (Ω) PART NUMBER LQH3C220 0.71 CURRENT RATING (mA) 250 MANUFACTURER MANUFACTURER Murata 814-237-1431 www.murata.com PHONE URL Taiyo Yuden 408-573-4150 www.t-yuden.com AVX 843-448-9411 www.avxcorp.com Murata 814-237-1431 www.murata.com Kemet 408-986-0424 www.kemet.com 80 Diode Selection Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for ZCC1937 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while the diode capacitance (CT or CD) represents the switching losses. For diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching losses at the 1.3MHz switch- ing frequency of the ZCC1937. A Schottky diode rated at 100mA to 200mA is sufficient for most ZCC1937 applications. Some recommended Schottky diodes are listed in Table 3. 75 Table 3. Recommended Schottky Diodes ELJPC220KF 4.0 160 Panasonic 714-373-7334 www.panasonic.com CDRH3D16-220 0.53 350 Sumida 847-956-0666 www.Sumida.com LB2012B220M 1.7 75 Taiyo Yuden 408-573-4150 www.t-yuden.com LEM2520-220 5.5 125 Taiyo Yuden 408-573-4150 www.t-yuden.com 90 VIN = 3 6V 85 3LEDs 70 65 MURATA LQH3C-220 PANASONIC ELJPC220KF SUMIDA CDRH3D16-220 TAIYO YUDEN LB2012B220M TAIYO YUDEN LEM2520-220 60 55 50 0 2 4 PART NUMBER CMDSH-3 FORWARD CURRENT (mA) 100 CMDSH2-3 200 BAT54 200 6 8 10 12 14 16 18 20 LOAD CURRENT (mA) 1937 F04 Figure 4. Efficiency Comparison of Different Inductors VOLTAGE DIODE DROP CAPACITANCE (V) (pF) MANUFACTURER 0.58 at 7.0 at Central 100mA 10V 631-435-1110 www.centralsemi.com 0.49 at 15 at Central 200mA 10V 631-435-1110 www.centralsemi.com 0.53 at 10 at Zetex 100mA 25V 631-543-7100 www.zetex.com 1937f 5 ZCC1937 APPLICATIONS INFORMATION LED Current Control Dimming Control The LED current is controlled by the feedback resistor (R1 in Figure 1). The feedback reference is 95mV. The LED current is 95mV/R1. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula and table for R1 selection are shown below. There are four different types of dimming control circuits: R1 = 95mV/ILED (1) Table 4. R1 Resistor Value Selection ILED (mA) R1 (Ω) 5 19.1 10 9.53 12 7.87 15 6.34 20 4.75 1. Using a PWM Signal to SHDN Pin With the PWM signal applied to the SHDN pin, the ZCC1937 is turned on or off by the PWM signal. The LEDs operate at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM signal. A 0% duty cycle will turn off the ZCC1937 and corresponds to zero LED current. A 100% duty cycle corresponds to full current. The typical frequency range of the PWM signal is 1kHz to 10kHz. The magnitude of the PWM signal should be higher than the minimum SHDN voltage high. The switching waveforms of the SHDN pin PWM control are shown in Figures 6a and 6b. Open-Circuit Protection ZCC1937 In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The ZCC1937 will then switch ata high duty cycle resulting in a high output voltage, whichmay cause the SW pin voltage to exceed its maximum 36V rating. A zener diode can be used at the output to limit the voltage on the SW pin (Figure 5). The zener voltage should be larger than the maximum forward voltage of the LED string. The current rating of the zener should be larger than 0.1mA. SHDN PWM FB 100mV/DIV SHDN 2V/DIV 200μs/DIV 1937 F06a (6a) 1kHz L 22μH D VIN CIN 1μF COUT 0.22μF VIN FB 100mV/DIV SW R2 1k YT1937 SHDN GND SHDN 2V/DIV FB R1 6.34Ω 1937 F05 20μs/DIV 1937 F06b (6b) 10kHz Figure 5. LED Driver with Open-Circuit Protection Figure 6. PWM Dimming Control Using the SHDN Pin 1937f 6 ZCC1937 APPLICATIONS INFORMATION 2. Using a DC Voltage 4. Using a Logic Signal For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The dimming control using a DC voltage is shown in For applications that need to adjust the LED current in discrete steps, a logic signal can be used as shown in Figure 9. R1 sets the minimum LED current (when the NMOS is off). RINC sets how much the LED current increases when the NMOS is turned on. The selection of R1 and RINC follows formula (1) and Table 4. Figure␣ 7. As the DC voltage increases, the voltage drop on R2 increases and the voltage drop on R1 decreases. Thus, the LED current decreases. The selection of R2 and R3 will make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias current. For VDC range from 0V to 2V, the selectionof resistors in Figure 7 gives dimming control of LEDcurrent from 0mA to 15mA. 3. Using a Filtered PWM Signal The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the variable DC voltage source in dimming control. The circuit is shown in Figure 8. Start-up and Inrush Current To achieve minimum start-up delay, no internal soft-start circuit is included in ZCC1937. When first turned on without an external soft-start circuit, inrush current is about 200mA as shown in Figure␣ 10. If soft-start is desired, the recom- mended circuit and the waveforms are shown in Figure 11.If both soft-start and dimming are used, a 10kHz PWM signal on SHDN is not recommended. Use a lower fre- quency or implement dimming through the FB pin as shown in Figures 7, 8 or 9. ZCC1937 ZCC1937 FB FB R3 90k R2 5k RINC VDC LOGIC SIGNAL R1 6.3Ω 2N7002 Figure 7. Dimming Control Using a DC Voltage R1 1937 F09 1937 F07 Figure 9. Dimming Control Using a Logic Signal IIN 100mA/DIV ZCC1937 FB 10k R3 90k FB 100mV/DIV R2 5k PWM 0.1μF R1 6.3Ω 1937 F08 Figure 8. Dimming Control Using a Filtered PWM Signal SHDN 2V/DIV VIN = 3.6V THREE LEDs 15mA 50μs/DIV 1937 F09 Figure 10. Start-Up Waveforms Without Soft-Startup Circuit 1937f 7 ZCC1937 APPLICATIONS INFORMATION D1 IIN 100mA/DIV 2.2nF YT1937 FB FB 100mV/DIV R2 1k D2 COUT SHDN 2V/DIV R1 6.34Ω 5k VIN = 3.6V THREE LEDs 15mA D2: MMBT 1937 F11a (11a) Recommended Soft-Startup Circuit 50μs/DIV 1937 F11b (11b) Soft-Startup Waveforms Figure 11. Recommended Soft-Startup Circuit and Waveforms Board Layout Consideration As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To maximize efficiency, switch rise and fall times are made as short as possible. To prevent electromagnetic interference (EMI) problems, proper layout of the high frequency switching path is essential. The voltage signal of the SW pin has sharp rise and fall edges. Minimize the length and L L VIN D CO area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize interplane coupling. In addition, the ground connection for the feedback resistor R1 should be tied directly to the GND pin and not shared with any other component, ensuring a clean, noise-free connection. Recommended component placement is shown in Figure 12. 1 5 CO CIN 2 GND 3 4 SHDN R2 6 2 5 3 4 CIN GND SHDN R1 1937 F12a R3 1 R2 R1 DIMMING CONTROL VIN D DIMMING CONTROL 1937 F12b R3 (SOT-23 Package) (SC70 Package) Figure 12. Recommended Component Placement 1937f 8 ZCC1937 TYPICAL APPLICATIONS Li-Ion to Two White LEDs VIN 3V TO 5V Two LED Efficiency 86 L 22μH D VIN = 3.6V 84 CIN 1μF COUT 1μF VDC DIMMING VIN VIN = 3V 80 SW 78 90k ZCC1937 SHDN 82 76 5k FB 74 R1 2Ω GND CIN: TAIYO YUDEN JMK107BJ105 COUT: AVX 0603ZD105 D: CENTRAL CMDSH2-3 L: MURATA LQH3C220 72 1937 TA05a 70 0 10 20 30 LED CURRENT (mA) 40 1937 TA05a Li-Ion to Three White LEDs VIN 3V TO 5V L 22μH Three LED Efficiency 90 D CIN 1μF COUT 0.22μF VDC DIMMING VIN 80 VIN = 3V SW VIN = 3.6V 75 90k ZCC1937 SHDN 85 5k 70 FB GND CIN: TAIYO YUDEN JMK107BJ105 COUT: AVX 0603YD224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220 R1 4Ω 65 1937 TA01a 60 0 5 15 10 LED CURRENT (mA) 20 1937 TA01b 1937f 9 ZCC1937 TYPICAL APPLICATIONS Li-Ion to Five White LEDs VIN 3V TO 5V Five LED Efficiency 85 L 22μH D CIN 1μF COUT 0.22μF VDC DIMMING VIN VIN = 3V SW 75 90k ZCC1937 SHDN 80 VIN = 3.6V 5k FB 70 R1 4Ω GND CIN: TAIYO YUDEN JMK107BJ105 COUT: TAIYO YUDEN GMK212BJ224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220 1937 TA03a 65 0 12 Seven LED Efficiency D CIN 1μF COUT 0.22μF VDC DIMMING 80 75 SW 90k ZCC1937 SHDN 10 85 L 22μH VIN 4 6 8 LOAD CURRENT (mA) 1937 TA03b 5V to Seven White LEDs VIN 5V 2 5k FB GND CIN: TAIYO YUDEN JMK107BJ105 COUT: TAIYO YUDEN GMK212BJ224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220 70 R1 4Ω 1937 TA04a 65 0 5 10 LOAD CURRENT (mA) 15 1937 TA04b 1937f 10 ZCC1937 PACKAGE DESCRIPTION S5 Package 5-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1635) 0.62 MAX 0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 2.80 BSC 1.4 MIN 3.85 MAX 2.62 REF 1.50 – 1.75 (NOTE 4) PIN ONE RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 TYP 5 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193 1.90 BSC S5 TSOT-23 0302 SC6 Package 6-Lead Plastic SC70 (Reference LTC DWG # 05-08-1638) 0.47 MAX 0.65 REF 1.80 – 2.20 (NOTE 4) 1.16 REF 0.96 MIN 3.26 MAX 2.1 REF 1.80 – 2.40 1.15 – 1.35 (NOTE 4) PIN 1 RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.65 BSC 0.15 – 0.30 6 PLCS (NOTE 3) 0.10 – 0.40 0.80 – 1.00 0.00 – 0.10 1.00 MAX 0.10 – 0.30 SC6 SC70 0302 0.10 – 0.18 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. DETAILS OF THE PIN 1 INDENTIFIER ARE OPTIONAL, BUT MUST BE LOCATED WITHIN THE INDEX AREA 7. EIAJ PACKAGE REFERENCE IS EIAJ SC-70 1937f 11 ZCC1937 TYPICAL APPLICATION Li-Ion to Four White LEDs L 22μH VIN 3V TO 5V D CIN 1μF COUT 0.22μF VDC DIMMING VIN SW 90k ZCC1937 SHDN 5k FB R1 4Ω GND CIN: TAIYO YUDEN JMK107BJ105 COUT: AVX 0603YD224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220 1937 TA02a Four LED Efficiency 85 Switching Waveforms VSW 10V/DIV 80 VIN = 3V VIN = 3.6V ISW 100mA/DIV 75 VOUT 100mV/DIV 70 60 0 5 10 15 LOAD CURRENT (mA) 20 VIN = 3.6V FOUR LEDs 15mA 0.2μs/DIV 1937 TA02c 1937 TA02b 1937f 12