Supertex inc. HV9918DB1 High Current LED Driver Demoboard General Description The HV9918DB1 demoboard is a high current LED driver designed to drive one or two LEDs at 700mA from a 9.0 - 16VDC input. The demoboard uses Supertex’s HV9918 hysteretic buck LED driver IC. The HV9918DB1 includes digital control of PWM dimming, which allows the user to dim the LEDs using an external, TTL-compatible square wave source applied between DIM and GND. In this case, the PWM dimming frequency and duty ratio are set by the external square wave source. The demoboard is protected against short circuit and open LED conditions. It also includes thermal derating of the LED current using an external NTC resistor to prevent over-heating. The bottom of the HV9918DB1 is an exposed copper plane (connected to the input ground) which can be connected to a 1” square heatsink (eg: 374324B00035G from Aavid Thermalloy) to allow for operation in higher ambient temperatures without tripping the HV9918’s built-in over temperature protection. Connection Diagram Specifications Parameter Input voltage Value 9.0 - 16VDC Output voltage 2.0 - 7.0V Output current 700mA ± 5% Line and load regulation Output current ripple Full load efficiency (@12.0V input) < ±2% 5% (peak to peak) 87% Open LED protection Yes Output short circuit protection Yes Dimensions 25.4mm X 29.2mm Silk Screen + - NTC Connections Input Connection: Connect the input DC voltage between VIN and GND terminals. Output Connection: Connect the LED(s) between LED+ (anode of LED string) and LED- (cathode of LED string) terminals. PWM Dimming Connection: 1. If no PWM dimming is required, short PWMD and VDD terminals. Doc.# DSDB-HV9918DB1 A032913 Supertex 2. If dimming using an external PWM dimming source, connect the PWM source between the PWMD and GND terminals. NTC Thermistor Connection: 1. If no thermal derating is required, the NTC terminal can be left open. 2. If thermal derating of the LED current is required, the NTC thermistor can be connected between NTC and GND terminals as shown. Supertex inc. www.supertex.com HV9918DB1 Operation of the Board Thermal derating in the HV9918DB1 uses the analog control of PWM dimming function to limit the LED current when the LED temperature rises. During normal operating mode (constant LED current; no PWM dimming), the LED current will be PWM dimmed at 1.0kHz. During PWM dimming mode, the thermal derating function limits the maximum PWM dimming duty cycle so that the LED current does not exceed the maximum allowable current determined by the thermal derating circuit. The HV9918DB1 uses Supertex’s HV9918 hysteretic buck LED driver IC to control the LED current. Since the regulation method controls both the peak and the valley current in the inductor, the demoboard has excellent line and load regulation. The LED current can be controlled in by PWM dimming. PWM dimming can be achieved in one of two ways: 1. Analog control of PWM dimming where a 0-2V source can be applied between NTC and GND terminals (the NTC terminal can also be used for thermal derating of the LED current as explained in the next section). Direct Control of PWM Dimming In the direct control method, the PWM dimming of the LEDs is achieved by driving the PWMD terminal using an external square wave source. In this case, PWM dimming frequency and duty cycle are set by the external source. 2. Direct control of PWM dimming by applying a TTL compatible square wave source between PWMD and GND terminals. In this mode, if the thermal derating function is not desired, NTC terminal should be left open. In this case, the recommended PWM dimming frequency can be anything up to 10kHz. Analog Control of PWM Dimming / Thermal Derating Analog Control of PWM dimming can be accomplished by applying a 0 – 2.0V DC voltage between NTC and GND (the DC voltage must have a 500μA source/sink capability). In this case, PWMD is connected to VDD and the LEDs are dimmed at 1.0kHz (as set by the capacitor at the RAMP pin of the IC). The duty cycle of the LED current can be adjusted by varying the external voltage at NTC (0V = 0% LED current and 2.0V = 100% LED current). If thermal derating is desired, then the NTC thermistor should be connected between the NTC and GND terminals. In this case, the PWM dimming frequency should be greater than 1.2kHz. The same NTC terminal can instead be used to de-rate the LED current based on the LED temperature, if desired. This would reduce the LED current as the LED temperature rises and prevents over-heating of the LED. An NTC resistor can be used to sense the temperature of the LED and this resistor can be connected between the NTC and GND terminals of the HV9918DB1. The demoboard is designed to operate with a 100k NTC thermistor which has a B-constant of 4250 (eg: NCP18WF104 from Murata). With this NTC thermistor, the LED current will start dropping at 85ºC and will reach about 350mA at 125ºC. Doc.# DSDB-HV9918DB1 A032913 2 Supertex inc. www.supertex.com HV9918DB1 Typical Waveforms (All waveforms are at 12.0V input and 6.9V LED Voltage unless otherwise noted) Fig. 1. Normal Operation – Drain Voltage and LED Current Fig. 2. PWM Dimming Waveform Doc.# DSDB-HV9918DB1 A032913 3 Supertex inc. www.supertex.com HV9918DB1 Typical Waveforms (All waveforms are at 12.0V input and 6.9V LED Voltage unless otherwise noted) Fig. 3. PWM Dimming – Rising Edge Waveform Fig. 4. PWM Dimming – Falling Edge Waveform Doc.# DSDB-HV9918DB1 A032913 4 Supertex inc. www.supertex.com HV9918DB1 Typical Waveforms (All waveforms are at 12.0V input and 6.9V LED Voltage unless otherwise noted) Fig. 5. Transient Response of LED Current to a Step Change in Input Voltage from 12V to 32V Fig. 6. PWM Dimming of LED current with 2.78k between NTC and GND Doc.# DSDB-HV9918DB1 A032913 5 Supertex inc. www.supertex.com HV9918DB1 Typical Results Efficiency vs. Input Voltage 89 LED Voltage = 6.9V Efficiency (%) 87 85 83 81 79 LED Voltage = 3.7V 9 10 11 12 13 14 15 16 Input Voltage (V) Fig. 7. Efficiency vs. Input Voltage LED Current vs. Input Voltage 0.715 0.713 LED Voltage = 3.7V LED Current (A) 0.711 0.709 0.707 0.705 0.703 0.701 0.699 0.697 LED Voltage = 6.9V 9 10 11 12 13 14 15 16 Input Voltage (V) Fig. 8 Line Regulation of LED Current Doc.# DSDB-HV9918DB1 A032913 6 Supertex inc. www.supertex.com HV9918DB1 Typical Results Switching Frequency vs. Input Voltage 550 Switching Frequency (kHz) 500 450 400 LED Voltage = 3.7V 350 300 250 LED Voltage = 6.9V 200 150 100 9 10 11 12 13 14 15 16 Input Voltage (V) Fig. 9 Switching Frequency vs. Input Voltage LED Current vs NTC Temperature (NTC used: NCP18WF104) 120 LED Current (%) 100 80 60 40 20 0 25 50 75 Temperature (C) 100 125 Fig. 10. Thermal Derating of the LED Current Doc.# DSDB-HV9918DB1 A032913 7 Supertex inc. www.supertex.com HV9918DB1 HV9918DB1 Waveforms I1 Iavg = I0 + I1 2 Normal inductor current waveform I0 I1 Iavg Inductor current at high I0 + I1 duty ratios (>80% typically) 2 I0 Note: The increase in the LED current at 9.0V input and 6.7V output can be explained by the fact that when the difference between the input and output voltages is very small, the rising inductor current waveform becomes more exponential rather than linear (the falling edge of the inductor current remains linear because the output voltage is high). This causes the average inductor (and therefore LED) current to increase even though the upper and lower bounds are still the same HV9918DB1 Schematic Diagram R4 1.07Ω, 1/4W VIN LED+ R1 C2 2.2µF 25V C1 2.2µF 25V GND 0.39Ω, 1/4W D1 STPS1L60A C3 1.0µF 25V NTC C4 VDD R2 13.7k DIM R3 1.0n 4 C5 1.0nF 1.0k 2 U1 3 5 100k SW1 8 ADIM HV9918 DIM 9 TAB LED- 2 33µH 4 3 VDD 6 2 VDD 1 C6 1.0µF 10V Doc.# DSDB-HV9918DB1 A032913 1 (DRA74-330) L1 7 GND RAMP R5 VDD 1 CS VIN 4 3 NTC VDD 2 1 DIM J3 8 Supertex inc. www.supertex.com HV9918DB1 Bill of Materials Description Package Manufacturer Manufacturer’s Part Number C1,C2 2.2µF, 25V, 10% X7R ceramic capacitor SMD1206 AVX Corp 12063C225K4Z2A 1 C3 1.0µF, 25V, 10% X7R ceramic capacitor SMD1206 Kemet 3 2 C4,C5 1.0nF, 50V, 5%, C0G ceramic capacitor SMD0603 TDK Corp 4 1 C6 1.0µF, 10V, 10% X7R ceramic capacitor SMD0603 5 1 D1 60V, 1.0A schottky diode SMA ST Micro 6 4 VIN, LED-, LED+ ,GND Compact surface mount test point SMT Keystone Electronics 7 1 J3 4 position, 0.1” vertical header SMT Molex 8 1 L1 33µH, 1.6A rms, 1.3A sat inductor SMT Coiltronics 9 1 R1 0.39Ω, 1/4W, 1% chip resistor SMD1206 Rohm MCR18EZHFLR390 10 1 R2 13.7kΩ, 1/10W, 5% chip resistor SMD0603 Yageo RC0603FR-0713K7L 11 1 R3 1.00kΩ, 1/10W, 5% chip resistor SMD0603 Panasonic 12 1 R4 1.07Ω, 1/4W, 1% chip resistor SMD1206 Yageo 13 1 R5 100kΩ, 1/10W, 5% chip resistor SMD0603 Panasonic 14 1 U1 Hysteretic Buck LED Driver with integrated FET DFN-8 Supertex Item # Qty RefDes 1 2 2 C1206C105K3RACTU C1608C0G1H102J Taiyo Yuden LMK107B7105KA-T STPS1L60A 5016 68301-1015 DRA74-330-R ERJ-3GEYJ102V RC1206FR-071R07L ERJ-3GEYJ104V HV9918K7-G Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com) Supertex inc. ©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited. Doc.# DSDB-HV9918DB1 A032913 9 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com