General LED lighting in the driver circuit has a current limiting resistor, and resistance of the power consumption and LED light has nothing to do. In order to improve efficiency, it is necessary to use constant-current output switching power supply, and the output stage using power MOSFET.
The block diagram of this driver circuit, which omitted the traditional circuit of the triangular wave generator and error amplifier, and the use of CM0S logic IC and PWM modulator. When the clock CLK signal is low, RS flip-flop is reset, the output FET off. When the CLK signal is high, the output FET is on. The current is resistance Rs into voltage, when the voltage is higher than the reference voltage Vref, the comparator output from high to low, RS flip-flop has been reset, the output FET and then off.
As a result, the energy stored in the inductor is constant even if the supply voltage varies. Achieve constant power output. Here is the voltage drop on Rs as a triangular wave voltage, PWM comparator to compare this voltage with the reference voltage directly to play the role of error amplifier.
The PWM comparator consists of a NAND gate IC2a ​​and an analog switch IC3, the so-called chopper comparator, and sets an input terminal other than u7yh high to become an inverter. This inverter repeatedly operates in two states: "precharge" and "compare".
Precharge state
In the precharge state, the capacitor C5 is charged by the reference voltage Vref provided by IC6 (LM385), and the output is neither high nor low, while maintaining the state of the output just as the comparator is toggling, and is triggered by ItS Combined into a PWM circuit.
Compare status
The input voltage shifts the voltage level charged at C5 and is input to the inverter, which is compared with the inverter's threshold voltage to output a high level or a low level.
Double voltage pulse output circuit
As the name suggests, this is to generate twice the pulse of the power supply voltage. First, when the input low, the capacitor C7 is charged, the output is 0V; when the input high, voc and capacitor voltage in series in the inverter IC5 YlX) terminal voltage is 2Vcc; because Yss end plus voc voltage, so the output 2Vcc.
In Figure 2, 555 (IC1) time base circuit generates a clock pulse, the brightness adjustment switch 51 to change the charging voltage of C2 to select 11.4kHz and 965Hz two-block frequency to change the clock frequency, to achieve the purpose of changing the brightness.
The output circuit and the PWM comparator are connected directly to the power supply, but consume only 10uA when the lamp is on.
Output connection 35 white light diodes, divided into five groups in parallel.