Micrel, Inc.

MIC4100/1

March 2006

11

M9999-031506

Vss

Vdd

LO

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.

HS

Vdd

HO

External

FET

HB

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

synchronous buck converter shown in Figure 5, the HS pin

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

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

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

discharging.

HS

HB

HO

Vdd

C

B

LO

Level

shift

HI

LI

Vss

Vin

Vout

Cout

Lout

C

VDD

Q1

Q2

Figure 5