AN-1500| Application Note

LM3402/02HV Demo Board

National Semiconductor Application Note 1500 Chris Richardson June 2007

Introduction
The LM3402/LM3402HV is a buck regulator derived controlled current source designed to drive a series string of high power, high brightness LEDs (HBLEDs) such as the LuxeonTM I Emitter at forward currents of up to 500 mA. The board can accept an input voltage ranging from 6V to 42V when using the LM3402. When using the pin-for-pin compatible LM3402HV the upper bound of input voltage is 75V. The converter output voltage adjusts as needed to maintain a constant current through the LED array. The LM3402/02HV is a true step-down regulator with an output voltage range extending from a VO(MIN) of 200 mV (the reference voltage) to a VO(MAX) determined by the minimum off time (typically 300 ns). It can maintain regulated current through any number of LEDs as long as the combined forward voltage of the array does not exceed VO(MAX)..

20198001

FIGURE 1.

Setting the LED Current
The default forward current ILED delivered to the LED array is 350 mA, typical of many 1W LEDs. To adjust this value the current setting resistor RSNS can be changed according to the following equation:

Circuit Performance LM3402
The LM3402 circuit and BOM have been designed to provide a constant forward current of 350 mA to a single LED with a forward voltage of approximately 3.5V. (Typical of white, blue, and green LEDs using InGaN technology.) When powered from a 24V 5% input the demo board will maintain the average LED current, IF, to within 10% of 350 mA. The ripple current, iF, will not exceed 70 mA peak-to-peak. Switching frequency for the demo board is 600 kHz 10% over the input voltage range of 6V to 42V.

tSNS = 220 ns This resistor should be rated to handle the power dissipation of the LED current. For example, the closest 5% tolerance resistor to set an LED current of 350 mA is 0.56 . In steady state this resistor will dissipate (0.352 x 0.56) = 69 mW, indicating that a resistor with a 1/8W power rating is appropriate.

Circuit Performance LM3402HV
The LM3402HV circuit and BOM have been designed to provide a constant forward current of 350 mA to a single LED with a forward voltage of approximately 3.5V. When powered from a 48V 5% input the demo board will maintain IF to within 5% of 350 mA. Ripple current will not exceed 70 mA peakto-peak. Switching frequency for the demo board is 250 kHz 10% over the input voltage range of 6V to 75V.

PWM Dimming
The DIM1 terminal on the PCB provides an input for a pulse width modulation signal for dimming of the LED array. In order to fully enable and disable the LM3402/02HV the PWM signal should have a maximum logic low level of 0.8V and a minimum logic high level of 2.2V. The maximum PWM dimming frequency, minimum PWM duty cycle and maximum duty cycle are illustrated in Figure 2. PWM frequency should be at least one order of magnitude below the LM3402/02HV switching frequency. The interval tD represents the delay from a logic high at the DIM pin to the onset of the output current. The quantities tSU and tSD represent the time needed for the output current to slew up to steady state and slew down to zero, respectively. Typical response times for the standard LM3402 and LM3402HV demo boards circuits are shown in the Typical Performance Characteristics section.

Connecting to LED Array
The LM3402/02HV demo board includes a female 6-pin SIP, J1, connector as well as two standard 94mil turret connectors for the cathode and anode connections of the LED array. Figure 1 shows the pinout of J1. Solid, 18 or 20 gauge wire with about 1 cm of insulation stripped away makes a convenient, solderless connection to J1.

AN-1500

2007 National Semiconductor Corporation

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