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Micrel
High-Side Driver and Bootstrap Circuit
MIC4103/4104
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.
C VDD
HI
Vdd
Level
shift
HB
C B
HO
Vin
Q1
Lout
HS
Q2
LI
LO
Vss
Figure 5. High-Side Driver and Bootstrap Circuit
Power Dissipation Considerations
Power dissipation in the driver can be separated into three
areas:
?
Internal diode dissipation in the bootstrap circuit
Figure 4. High-Side Driver and Bootstrap Circuit Block
Diagram
?
?
Internal driver dissipation
Quiescent current dissipation used to supply the
internal logic and control functions.
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, C B . 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 C B to charge up to
V DD ? V D during this time (where V D 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 C B 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 V IN . As the HS and HB pin rise, the internal
diode is reverse biased preventing capacitor C B from
discharging.
Bootstrap Circuit Power Dissipation
Power dissipation of the internal bootstrap diode primarily
comes from the average charging current of the C B
capacitor times the forward voltage drop of the diode.
Secondary sources of diode power dissipation are the
reverse leakage current and reverse recovery effects of
the diode.
The average current drawn by repeated charging of the
high-side MOSFET is calculated by:
I F ( AVE ) = Q gate × f S
where : Q gate = Total Gate Charge at V HB
f S = gate drive switching frequency
The average power dissipated by the forward voltage drop
of the diode equals:
Pdiode fwd = I F ( AVE ) × V F
where : V F = Diode forward voltage drop
November 2010
11
M9999-110910-B
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