DEMO CIRCUIT 1521A LT3956 QUICK START GUIDE LT3956 High Voltage Monolithic LED Driver DESCRIPTION Demonstration circuit 1521A is a high voltage mono- PWM dimming terminal. For the highest pwm dim- lithic LED driver with an integrated 3.3A, 84V power ming ratio, it is recommended to use 100Hz as a pwm switch. The demonstration circuit is assembled as a dimming frequency. Information regarding PWM boost topology from 6V-80V* input and up to 67V dimming ratios and performance can be found in the LED output at 370mA (*VIN must not exceed VLED LT3956 datasheet in the applications section. for proper operation). DC1521A features PWM dim- Soft-start is provided with a simple and small capaci- ming, analog dimming, shutdown, undervoltage lock- tor to maintain control of the switch current during out, an open LED status pin, and open LED overvolt- turn on for high power designs. age protection. Efficiency at 12VIN is as high as 94% Modifications can be made to the DC1521A in order for 67VLED at 370mA output. to convert the board from a boost topology LED driver The switching frequency is set at 375kHz although the to any other topology. Buck-boost mode, buck mode, range for the IC is 100kHz to 1MHz. The VMODE and SEPIC schematics are provided in the datasheet. pin/terminal provides an output flag in case the LEDs Please consult the factory or LT3956 datasheet for are detached from the circuit. EN/UVLO provides un- details regarding how to customize the DC1521A or dervoltage lockout with hysteresis and a shutdown how to design different topologies for custom specifi- function that is used by simply tying the pin/terminal cations. to GND. CTRL provides analog dimming by simply The LT3956 datasheet gives a complete description of pulling the pin/terminal low or by reducing VIN below the part, operation and applications information. The 11V. Analog dimming with low VIN is changed or datasheet must be read in conjunction with this Quick removed by changing the values of the EN/UVLO re- Start Guide for demonstration circuit 1521A. The sistors that are used. PWM dimming changes the LT3956 is assembled in a 36-lead plastic UHE pack- brightness of the LEDs without changing the LED cur- age with two thermally enhanced pads for SW and rent. A PWM dimming pulse can be tied to the PWM GND. Proper board layout is essential for both proper pin/terminal. If PWM is not used, the pin must still be operation and maximum thermal performance. See the pulled high and it can be attached to INTVCC or an- datasheet section Layout Considerations. other source. Design files for this circuit board are available. Call DC1521A PWM dimming is simple. The boost topol- the LTC factory. ogy LED string returns to the LED- terminal and the LT is a trademark of Linear Technology Corporation PWM dimming MOSFET between LED- and PGND turns the LED string on and off given an input to the 1 L T3956 PERFORMANCE SUMMARY Specifications are at TA = 25C PARAMETER CONDITION VALUE TYPICAL) Input Voltage VIN Range Operating 6V to V (see text) LED LED String Voltage Range Operating VIN < V < 67V LED Switching Frequency R1 = 28.7k 375kHz I 11V < VIN < V 370mA LED LED Maximum Output Voltage - Open LED Voltage 78.4V Efficiency VIN=12V V =67V I=370mA 94% LED LED VMODE VIN=12V V =67V I =370mA 7.1V LED LED VIN=12V OPEN LED VOUT=78V 0V Undervoltage Lockout 5.5V UVLO w/ 6V rising turn-on Peak Switch Current Limit Minimum IC Rating 3.3A UVLO Undervoltage Lockout V Rising, 0C < T < 85C IN A V Rising IN V Falling IN QUICK START PROCEDURE Demonstration circuit 1521A is easy to set up to 4. Connect 5V or a 5V PWM waveform to the PWM evaluate the performance of the LT3956. Follow the terminal. If PWM is not used, connect PWM to a procedure below: 5V source or to IntVcc by stuffing a zero ohm re- NOTE. Make sure that the VIN DC input voltage does not exceed 80V sistor in R14 on the PCB. PWM must be pulled or the LED string voltage at 370mA. Start with all power supplies high for the LEDs to run. turned off. 5. Turn the VIN power supply on. 1. Connect a string of LEDs with forward voltage less than 68V (when running at 370mA) to the LED+ 6. Release the connection between EN/UVLO and and LED- terminals on the PCB as shown in Fig- GND. ure 1. 7. Observe the LED string running at the pro- 2. Connect the EN/UVLO terminal to GND. grammed LED current. 3. With power off, connect the VIN power supply to 8. For PWM dimming, connect a PWM 100Hz or the PVIN (or VIN) and GND terminals. Make sure higher signal to the PWM terminal and observe that the power setting on the supply is not set the reduction of brightness in the LED string re- above 80V or above the expected LED string volt- lated to the PWM dimming duty cycle. age before the supply is tied to VIN and turned on. 2