MIC4100 datasheet(11/18 Pages) MICREL

11 / 18 page

Micrel, Inc.

MIC4100/1

March 2006

M9999-031506

Vss

Vdd

External

FET

Figure 3

High-Side Driver and Bootstrap Circuit

A block diagram of the high-side driver and bootstrap

circuit is shown in Figure 4. This driver is designed to drive

a floating N-channel MOSFET, whose source terminal is

referenced to the HS pin.

Vdd

External

FET

Figure 4

A low power, high speed, level shifting circuit isolates the

low side (VSS pin) referenced circuitry from the high-side

(HS pin) referenced driver. Power to the high-side driver

and UVLO circuit is supplied by the bootstrap circuit while

the voltage level of the HS pin is shifted high.

The bootstrap circuit consists of an internal diode and

external capacitor, CB. In a typical application, such as the

synchronous buck converter shown in Figure 5, the HS pin

is at ground potential while the low-side MOSFET is on.

The internal diode allows capacitor CB to charge up to

Vdd-Vd during this time (where Vd is the forward voltage

drop of the internal diode). After the low-side MOSFET is

turned off and the HO pin turns on, the voltage across

capacitor CB is applied to the gate of the upper external

MOSFET. As the upper MOSFET turns on, voltage on the

HS pin rises with the source of the high-side MOSFET until

it reaches Vin. As the HS and HB pin rise, the internal

diode is reverse biased preventing capacitor CB from

discharging.

Vdd

Level

shift

Vss

Vin

Vout

Cout

Lout

VDD

Figure 5

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