Figure 1 is a block diagram of such a driver circuit, which omits the triangular wave generator and error amplifier of the conventional circuit, and uses a CMOS logic IC and a PWM modulator. When the clock CLK signal is low, the RS flip-flop is in a reset state and the output FET is turned off. When the CLK signal is high, the output FET is turned on. The current is converted into a voltage by the resistor Rs. When the voltage is higher than the reference voltage Vref, the comparator output goes from high to low, the RS flip-flop is reset, and the output FET is turned off. As a result, even if the power supply voltage changes, the energy stored in the inductor is constant. A constant power output is achieved. Here, the voltage drop on Rs is taken as the triangular wave voltage, and the PWM comparator directly compares this voltage with the reference voltage to function as an error amplifier.

The actual circuit is shown in Figure 2. It consists of discrete components, with emphasis on the PWM comparator and the double-voltage pulse output circuit.

The PWM comparator consists of a NAND gate IC2a ​​and an analog switch IC3, a so-called chopper comparator, and sets an input terminal of the non-u7yh to a high level to become an inverter. This inverter works repeatedly in the "precharge" and "comparison" states.

Precharge state

In the precharge state, capacitor C5 is charged by the reference voltage Vref supplied by IC6 (LM385), at which time the output is neither high nor low; it remains in the output state when the comparator is about to flip, and is triggered with ItS The combiner is combined into a PWM circuit.

Comparison status

The input voltage shifts the voltage level charged on C5 and inputs it to the inverter, which is compared with the threshold voltage of the inverter to output a high level or a low level.

Double voltage pulse output circuit

As the name implies, this is to generate a pulse twice the supply voltage. First, when the input low level, the capacitor C7 is charged, the output is 0V; when the input is high, the voltage on the voc and the capacitor is connected in series, and the voltage at the YlX) terminal of the inverter IC5 is 2Vcc; Voc voltage, so the output is 2Vcc.

In FIG. 2, the 555 (IC1) time base circuit generates a clock pulse, and the charging voltage of C2 is changed by the brightness adjusting switch 51 to select the two frequency of 11.4 kHz and 965 Hz to change the clock frequency to achieve the purpose of changing the brightness.

The output circuit and the PWM comparator are directly connected to the power supply, but the power consumption is only 10uA without turning on the light.

The output is connected to 35 white light diodes, which are divided into five groups in series and in parallel.