High-brightness LED flash beacons give switching regulators new uses

　　This flashing beacon circuit (Figure 1) can be used to signal for help on highways and to indicate directions in parking lots, hospitals, and hotels. The circuit uses a high-power LED, which is brighter than a normal incandescent flashlight. It is powered by a 6V or 12V SLA (sealed lead acid) lantern battery, making the circuit very lightweight.

　　The MC34063 monolithic switching regulator subsystem is the heart of the circuit (Figure 2), originally intended for use in DC-DC converters. The device integrates a voltage reference, comparator, duty-cycle controlled oscillator with active peak current limiting circuitry, driver, and high-current output switch in an 8-pin DIP package.

　　Under a 6V~12V DC power supply, this circuit drives a 1W high-power LED to flash briefly with a duty cycle of about 5%. By monitoring the voltage drop of the 1Ω detection resistor R1 placed between VCC and the output switch (pin 1), the current limiting of the LED can be achieved.

　　The maximum current capability of a 1W white LED is about 350mA. At the beginning of a cycle, C1 starts to charge, the LED current increases rapidly, and the voltage drop across R1 (which is monitored by the Ipk sense pin IC1-7) increases accordingly.

　　When this voltage is above 330mV (or 330mA) relative to pin 6, the current limiting block in the IC provides additional current to charge the timing capacitor C1. This quickly reaches the upper oscillator threshold, and then the output switch is turned off and C1 begins to discharge. The flashing frequency can be changed by changing the value of C1, and a 100μF capacitor provides a frequency of about 4Hz.

　　Since the LED only flashes very briefly, the heat dissipation problem can be almost ignored, and a star-shaped metal matrix printed circuit board (MCPCB) can meet the heat dissipation requirements of the LED.

　　Figure 1: Flashing light circuit diagram.

　　Figure 2: MC34063 is the core of this circuit.

　　The operation of IC1 is further described in the MC34063 application note.