DC507A Analog Devices

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 507 SINGLE CELL WHITE LED DRIVER LT1937 DESCRIPTION Demonstration circuit 507 features the LT1937 constant- solution for battery powered portable applications such current white LED driver in a small SC70 package. The as cellular phones, digital cameras, MP3 players, GPS demo board consists of two circuits: an efficiency circuit receivers, and handheld computers (PDAs). The DC507 designed to demonstrate high efficiency performance of is specifically designed for white LED backlight applica- the LT1937 using the Murata inductor and a size circuit tions using 24 LEDs in series. LT1937 offers numerous highlighting the LT1937s ability to work with a small LED-dimming circuit options, see the datasheet for more ceramic chip inductor and a small output capacitor detailed information on LED brightness control. (0.22F). The high efficiency and small circuit size are Design files for this circuit board are available. Call made possible by a 95mV feedback voltage and 1.2MHz the LTC factory. switching frequency, making the LT1937-SC70 an ideal QUICK START PROCEDURE Demonstration circuit 507 is easy to set up to evaluate TEST THE EFFICIENCY CIRCUIT (UPPER CIRCUIT) the performance of the LT1937. Refer to Figure 1 for proper measurement equipment setup and follow the 2. Place jumper JP2 in the EFFICIENCY position. procedure below. 3. Press the SW1 switch. Observe all four white LEDs NOTE: When measuring the input or output voltage rip- light up. The output voltage should measure about ple, care must be taken to avoid a long ground lead on 13V across the Vout Test terminal to GND terminal. the oscilloscope probe. Measure the input or output 4. Turn off the input power supply or disconnect the sin- voltage ripple by touching the probe tip directly across gle cell Li-Ion battery. the Vin or Vout and GND terminals. When measuring the output ripple, see Figure 2 for proper scope probe tech- nique. TEST THE SIZE CIRCUIT (LOWER CIRCUIT) 5. Place jumper JP2 in the SIZE position. SET THE INPUT VOLTAGE SOURCE 6. Turn on the input power supply or connect the single 1. The demo board can be powered from the 3V Lithium cell Li-Ion battery. battery installed on the demo board (B1) or other in- 7. Press the SW2 switch. Observe all four white LEDs put source (bench top power supply) by selecting in- light up. The output voltage should measure about put source jumper (JP1). 13V across the Vout Test terminal to GND terminal. a. For battery (B1) input operation, place the jumper (JP1) in the BATTERY position. LED BRIGHTNESS CONTROL b. For bench supply input, place the jumper (JP1) in The brightness of the LEDs can be controlled by apply- the Vin position and connect a 3.3V, 500mA bench ing a PWM signal to the SHDN terminal (use the upper supply to the Vin and GND terminals. SHDN terminal for the EFFICIENCY circuit and lower SHDN terminal for the SIZE circuit). The typical fre- quency range recommended for PWM dimming is 1KHz10KHz with the signal magnitude of the PWM sig- nal higher than 1.5V. See the LT1937 data sheet for more information on PWM dimming control. 1 GND VIN QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 507 SINGLE CELL WHITE LED DRIVER + + + Figure 1. Proper Measurement Equipment Setup Figure 2. Scope Probe Placement for Measuring Input or Output Ripple 2 +