AN2654

Electrical function of the design

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external PWM signal is able to control the on time of the lamp current.

Figure 14.

Digital dimming control circuit

When the oscillator is in the off state, the low side MOSFET (Q2) is always conducting due

to the IC internal logic. This creates a path for the resonance between the DC blocking

capacitor and the transformer primary leakage inductance, which introduces some

undesirable noise in the lamp off state. For this reason another transistor Q5 has been

deployed to pull down the gate junction (DDim2) of the low side MOSFET at off state, too.

The resistor R30 and R32 are used to limit the discharging current.

resumes. Normally an overshoot can be seen on the lamp current, which gives more stress

to the lamp and the MOSFET. The L6574 handles this very well by connecting the PWM

signal to the feedback pin (FB) through a resistor, and the error amplifier is compensated

during off state. In this case, when the oscillation starts again, it begins from high frequency

down, producing a lamp current waveform somewhat like a soft-start.

Figure 15.

Vgs of low-side MOSFET

Figure 16.

Vcom waveform in digital dimming