QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 850A 3V-14.4VIN, BOOST LED DRIVER LTC3783 DESCRIPTION Demonstration circuit 850A is a 3V-14.4Vin, Boost LED Driver featuring the LTC3783 – a high PWM dimming ratio capable, LED driver controller. This circuit was designed to attain a highly efficient, output current regulating power supply from a wide input voltage range while achieving an unprecedented dimming ratio of greater than 1000:1. This circuit also incorporates an output over voltage protection feature to limit output voltage to less than 27V if the LED string is inadvertently disconnected. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. Table 1. Performance Summary (TA = 25°C) PARAMETER CONDITION VALUE Minimum Input Voltage 3V Maximum Input Voltage 14.4V Output Voltage VOUT VIN = 3V to 14.4V VIN < VOUT < 25V Typical Output Ripple VOUT VIN = 8V 65mVP–P Maximum Output Current Output Current Regulation 360mA Over line and load Nominal Switching Frequency Efficiency Run Control SYNC Control PWM Dimming Control ±2.0% 290kHz VIN = 8V, VOUT = 20V 91.5% Typical Falling Voltage-Off 1.223V Min Rising Voltage-On 1.348V Typical Logic Low Voltage-Off 0.5V MAX Logic High Voltage-On 1.2V Min Logic Low Voltage-Off 0.8V Typical Logic High Voltage-On 1.6V Typical PWM Dimming Ratio 1000:1 OPERATING PRINCIPLES The LTC3783 LED controller will operate when an input voltage of 3V to 14.4V is applied. The exception is when the RUN terminal, E6, is tied to GND, E7. This inhibits the LTC3783’s switching and puts it into shutdown. (This can be accomplished via an open collector/drain configuration with a transistor or a mechanical switch.) When the LTC3783 is taken out of shutdown, it begins a controlled soft-start increase of the output current. If the PWMIN pin, E4, is high or left floating, R9 will conduct the LED current. This current develops a voltage across R9 that is fed to the FBN pin. This voltage is compared to the voltage on the FBP pin which is developed from the resistor divider, R5 and 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 850A 3V-14.4VIN, BOOST LED DRIVER R6, from the reference voltage, VREF. When the voltages at the FBP and FBN pins are equal, the controller is in output current regulation. ming ratios are achievable due to the output current state being saved by the controller each time that the PWM FET is turned off. If for any reason the impedance at the LED terminals gets higher than design limits (i.e. disconnected LEDs), the LTC3783 will limit the output voltage to its over voltage point (OVP) of 27V. This ensures no damage to the circuit components. The LTC3783 can be synchronized to an external clock that is faster than the free-running frequency via the SYNC pin, E5. As always, see the LTC3783 datasheet for more detailed information on all of the above features. PWM dimming is carried out by applying a PWM signal to the PWMIN terminal, E4. This signal is then buffered and used to drive the PWM FET, Q3. This FET interrupts the current through the LEDs, thus effecting PWM dimming of the LED string. High dim- Depending on system ripple requirements, fewer/lower value input and output capacitors can be used. This demo board is shipped with a mere 65mVpp of output and 85mVpp of input ripple! QUICK START PROCEDURE Demonstration circuit 850A is easy to set up to evaluate the performance of the LTC3783. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: measuring the input or output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the output (or input) voltage ripple by touching the probe tip and probe ground directly across the VOUT/LED+ (E3) and GND (E9) terminals. See Figure 2 for proper scope probe measurement technique. nals in order to get an accurate input voltage measurement. 3. Connect a string of LEDs that will yield < 25V at 360mA between the output terminals VOUT/LED+ (E3) to LED- (E8). 4. Turn on the power to the input power supply. NOTE: When 1. Set an input power supply that is capable of 3V to 15V at 3A to a voltage of 8V. Then turn off the supply. 2. With power off, connect the supply to the input terminals VIN (E1) and GND (E2). a. Input voltages lower than 3V can result in too much power dissipation due to the larger currents needed to sustain regulation. b. If efficiency measurements are desired, an ammeter capable of measuring at last 3Adc can be put in series with the input supply in order to measure the DC850’s input current. c. A voltmeter with a capability of measuring at least 15V can be placed across the input termi- NOTE: Make sure that the input voltage never exceeds 15V – electrical overstresses can occur. 5. Check for the proper output current of 360mA ± 2%. If the output current is < 360mA ± 2% and the output voltage is 25V, the OVP may be activated. Check connections and LED string voltage and make adjustments as necessary. a. If efficiency measurements are desired, an ammeter that is capable of handling 500mA can be put in series with the output LED string in order to measure the DC850’s output current. b. A voltmeter with a capability of measuring at least 30V can be placed across the output terminals VOUT/LED+ (E3) and GND (E9) in order to get an accurate output voltage measurement. 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 850A 3V-14.4VIN, BOOST LED DRIVER Figure 1. Proper Measurement Equipment Setup 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 850A 3V-14.4VIN, BOOST LED DRIVER GND VIN Figure 2. Measuring Input or Output Ripple MEASURED DATA Figure 3 shows efficiency data for a typical DC850A. E ffic ien c y 92% E fficien cy (%) 90% 88% 86% 84% 82% 80% 3.0 5.0 7.0 9.0 11.0 13.0 15.0 V in (V ) Figure 3. Efficiency with VLED of 16V. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 850A 3V-14.4VIN, BOOST LED DRIVER 5