As a new type of energy-saving and environmentally friendly green light source products, LED has broad market prospects. At present, there are thousands of LED driver ICs in the market. Among them, we have encountered a lot of single-chip line structure (Figure 1a).
According to the IC's data sheet, this type of IC is a high-efficiency LED drive control circuit that operates in PWM mode. With an external circuit, it can accommodate a wide input voltage range from 8V to 450V. The external power MOS transistor can be set by a fixed resistor (or capacitor) to reliably drive the LED string in a constant current manner. The LED current can be set by selecting the appropriate current limiting resistor. It also provides a linear dimming function that supports digital pulse (PWM) dimming with a low frequency variable duty cycle.
According to the application and according to different standards, the driving scheme can be divided into three types.
According to the PWM adjustment method, it can be divided into two types: constant frequency and constant off time (Fig. 1).
The voltage applied across the inductor in the steady state multiplied by the on-time is equal to the turn-off time inductor voltage multiplied by the turn-off time: Von*Ton=Voff*Toff, ie (Vin-Vo)*Ton=Vo*Toff.
From the calculation formula of the inductance, the difference between the two can be seen. The inductance calculation in Fig. 1a is L = (Vin–Vo) * Ton / ΔI, and the inductance calculation in Fig. 1b is L=Vo*Toff. /ΔI. Io = Ip - ΔI /2. As shown in the current waveform diagram shown in Figure 2, after the inductance is determined, the input voltage Vin changes in Figure 2a causes the ripple current ΔI to change, so that the output current changes; the ripple current ΔI in Figure 2b is independent of the input supply voltage. Therefore, in a wide voltage application environment where voltage fluctuations are large, a constant off time circuit mode is used.
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