MC34164-3 Datasheet, PDF (19/23 Pages) Freescale Semiconductor, Inc – 3.0 A 1.0 MHz Fully Integrated DDR Switch-Mode Power Supply

Calculate Rs by placing the Pole 1 at the ESR zero
frequency:
TYPICAL APPLICATIONS
PROTECTION AND DIAGNOSTIC FEATURES
⇒
⇒
Equating the Pole 2 at Crossover Frequency to achieve a
faster response and a proper phase margin,
BOOTSTRAP CAPACITOR
The bootstrap capacitor is needed to supply the gate
voltage for the high side MOSFET. This N-Channel MOSFET
needs a voltage difference between its gate and source to be
able to turn on. The high side MOSFET source is the SW
node, so it is not ground and it is floating and moving in
voltage, so we cannot just apply a voltage directly to the gate
of the high side that is referenced to ground, we need a
voltage referenced to the SW node. That is why the bootstrap
capacitor is needed for. This capacitor charges during the
high side off time, since the low side will be on during that
time, so the SW node and the bottom of the bootstrap
capacitor will be connected to ground and the top of the
capacitor will be connected to a voltage source, so the
capacitor will charge up to that voltage source (say 5V). Now
when the low side MOSFET switches off and the high side
MOSFET switches on, the SW nodes rises up to Vin, and the
voltage on the boot pin will be Vcap + Vin. So the gate of the
high side will have Vcap across it and it will be able to stay
enhanced. A 0.1µF capacitor is a good value for this
bootstrap element.
Analog Integrated Circuit Device Data
Freescale Semiconductor
34712
19