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Electronic Design

Series LED driver operates on 3-V input

The circuit described here consists of a constant current source with a voltage compliance of 30 V (see the figure). It’s capable of driving a series string of LEDs at currents up to 100 mA. A switching regulator using a SEPIC (single-ended primary inductance converter) topology drives the LEDs with a regulated constant current at efficiencies up to 79%. U1, an LT1512 switching IC, provides an output voltage of approximately 28 V to drive the LEDs. This device operates at 500 kHz and features an internal 1.5-A switch, voltage and current feedback pins, and a low quiescent-current shutdown mode. The circuit is designed to drive 16 series-connected red LEDs from an input voltage of 3 to 10 V.

The limiting factor on the maximum input voltage and the number of series-connected LEDs that the circuit can drive is the maximum switch voltage rating of 40 V. In a SEPIC converter topology, the maximum voltage seen by the switch is the input voltage plus the output voltage. To protect the internal switch from excessive voltage, input overvoltage protection is provided by Z1 and R5. If the input voltage exceeds approximately 10 V, the output voltage (which is typically 27 to 30 V) begins decreasing, thus protecting the internal switch from exceeding the 40-V maximum switch rating. Input voltage can be as high as 25 V without damage, although the LEDs turn off as the input voltage exceeds 10 V.

The maximum number of LEDs that can be driven depends on the total voltage required by the series string of LEDs and the maximum input voltage (the forward voltage drop of different types of LEDs can range from 1.6 V to as much as 3.6 V each, depending on color, current, manufacturer, and other factors).

All surface-mount components can be used in this design, resulting in less than 0.8 in.2 of pc-board area. Good electrical and thermal pc-board layout practices are necessary because of the 500-kHz switching frequency and because of the power dissipated by the internal switch. Generous amounts of pc-board copper near U1’s leads (except for the switch lead, pin 8) assist in conducting heat away from the package.

The inductor consists of two identical windings on a toroidal core (observe the phasing of the windings). Input and output capacitors can be either ceramic or tantalum, with ripple current rating greater than 150 mA RMS. Ceramic capacitors are recommended for the coupling capacitor C2 and the compensation capacitor C4 (use X7R ceramic material for C4). D1 is a 0.5- or 1-A, 40-V Schottky or ultra-fast recovery diode.

Looking at the diagram, circuitry for flashing the LEDs is contained within the dashed line. With the values shown, the LEDs are on for approximately 120 ms out of every second. Varying R5 between 68 kΩ and 5 MΩ changes the flash rate from fast (five flashes per second) to slow (one flash every three seconds). Other flash rates and/or duty cycles are possible by selecting different resistor and/or capacitor values.

U1 includes inputs for sensing both output voltage and output current. The average current through L1B and R2 is equal to the LED current. The sense voltage developed across R2 is used to regulate the LED current. Other LED currents can be programmed by selecting a resistor value for R2 that will develop 100 mV at the desired LED current. Programming resistors R3 and R4 limit the maximum output voltage to approximately 33 V in the event of an open LED connection.

When driving 16 red LEDs at 40 mA (30 V across the LEDs), the input current required is approximately 600 mA at 5 V in, and 200 mA at 8 V in. Flashing the LEDs at a 10% duty cycle will drop the average input current by a factor of 10. Pulling the shutdown pin low shuts the circuit down, drawing approximately 15 µA of quiescent current.

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