White LED Array Lamp

C. V. Niras/VU3CNS

When I decided to make a battery operated LED lamp, I thought it may better to use a cluster of low power discrete LED array rather than a single high power LED. The light produced will be scattered and It considerably reduces irritation or damages to a naked eye, if accidentally looked on it. Also the light spreads nicely so it is good for room lighting and no heat sink required for the LEDs!

The circuit is designed to operate 3W, 5 x 7 white LED array powered from four NiMH cells. The design is straight forward DC to DC boost circuit using a LM3410 which includes a 170 mΩ NMOS switch. The circuit is completes with few external components. The switching frequency is internally set to either 525 kHz for LM3410-Y devices or 1.60 MHz for LM3410-X, allowing the use of extremely small inductors and capacitors. The 525 kHz switching frequency is selected due to the availability of suitable ferrite having lower core losses. The inductor value needed for the circuit depends upon the voltage and current output needed. A detailed calculation is described in the datasheet of the LM3410. A higher efficiency of approximately 88% is achieved. The IC is featured with an external shutdown and the standby current of only 80 nA.

The operating current for a single branch the LED array is ~25mA, and since there are 7 parallel paths a total output current of 190mA is drawn from the DC to DC convertors. This current is set by the 1O resistor R4 (I = 0.19/R4). An optional output over voltage protection is provided using a 22V zener diode D23 and a resistor R3. This will protect the IC if the LED load becomes an open circuit.

Figure 1: circuit diagram of the LED drive circuit.

Since the absolute maximum input voltage to the IC is 5.5 V, a voltage regulator using R1 and zener diode D31 is required if the circuit is operated from a voltage source more than 5.5 V. The DIM input pin of LM3410 can be used for either on/off or brightness control of LED array. A PWM dimming signal whose duty cycle from 0 to 100% with a frequency of 200 ~ 1 kHz is best suited for this brightness control, although a PWM with maximum of 25 KHz can be used. In this application DIM pin is tied to V_IN for maximum brightness.