Circuit secures bootstrap operation under light load
The circuit in Figure 1’s green box aims to solve the problem. It starts by tapping the input signal to the bootstrap’s high-side driver to generate an inverted and delayed short pulse to control Q2. Upon activation, Q2 forces the output signal momentarily low, which provides an opportunity for CBS to charge. R8, R9, R10, R11, and C9 set the turn-on period of Q2. This period must not exceed the dead time of the PWM (pulse-width-modulated) signal. If Q2’s turn-on time is too long, the converter’s efficiency will degrade, or the CBS might not sufficiently charge. Inadequate charging of Q2 involves multiple component values and operating parameters, such as Q2’s turn-on time, and you might have to empirically tune the delay time to accommodate for these effects. The values in Figure 1 produce a Q2-turn-on time of 1 μsec and delay time of 450 nsec in a 70-kHz switching frequency.
Resistor R7 and capacitor C8 control the delay time between the falling edge of the input signal to the bootstrap’s high-side driver and the rising edge of IC1A’s LVG (low-voltage) pulse. Figure 4 displays the waveform of the same converter after the inclusion of the additional circuit. In this case, VBUS (Trace 1) remains stable and the output signal from the buck regulator switches continuously, lacking the gaps with switching that the waveforms of Figure 2 show.