MIC28512 datasheet(21/34 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005524A-page 21

MIC28512

4.6

Power Good (PG)

The power good (PG) pin is an open-drain output that

indicates logic-high when the output is nominally 90%

of its steady state voltage.

4.7

MOSFET Gate Drive

The Functional Block Diagram shows a bootstrap

circuit, consisting of DBST, CBST, and RBST. This circuit

supplies energy to the high-side drive circuit. Capacitor

CBST is charged, while the low-side MOSFET is on,

and the voltage on the SW pin is approximately 0V.

When the high-side MOSFET driver is turned on,

energy from CBST is used to turn the MOSFET on. As

the high-side MOSFET turns on, the voltage on the SW

pin increases to approximately VIN. Diode DBST is

reverse-biased and CBST floats high while continuing to

bias the high-side gate driver. The bias current of the

high-side driver is less than 10 mA, so a 0.1 μF to 1 μF

capacitor is sufficient to hold the gate voltage with

minimal droop for the power stroke (high-side

switching) cycle, i.e. ∆BST = 10 mA x 1.25 μs/0.1 μF =

125 mV. When the low-side MOSFET is turned back

on, CBST is then recharged through the boost diode. A

30Ω resistor RBST, which is in series with the BST pin,

is required to slow down the turn-on time of the

high-side N-channel MOSFET.

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