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MIC24420 Datasheet(PDF) 13 Page - Micrel Semiconductor

Part No. MIC24420
Description  2.5A Dual Output PWM Synchronous Buck Regulator IC
Download  34 Pages
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Maker  MICREL [Micrel Semiconductor]
Homepage  http://www.micrel.com
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MIC24420 Datasheet(HTML) 13 Page - Micrel Semiconductor

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Micrel, Inc.
MIC24420/MIC24421
June 2012
13
M9999-062012-C
High-side Drive
The internal high-side drive circuit is designed to switch
the internal N-channel MOSFET. Figure 5 shows a
diagram of the high-side MOSFET, gate drive and
bootstrap circuit. D2 and CBST comprise the bootstrap
circuit, which supplies drive voltage to the high-side
MOSFET. Bootstrap capacitor CBST is charged through
diode D2 when the low-side MOSFET turns on and pulls
the SW pin voltage to ground. When the high-side
MOSFET driver is turned on, energy from CBST charges
the MOSFET gate, turning it on. Voltage on the SW pin
increases to approximately VIN. Diode D2 is reversed
biased and CBST flies high while maintaining gate voltage
on the high-side MOSFET.
A resistor should be added in series with the BST1 and
BST2 pins. This will slow down the turn-on time of the
high-side MOSFET while leaving the turn-off time
unaffected. Slowing down the MOSFET risetime will
reduce the turn-on overshoot at the switch node, which
is important when operating with an input voltage close
to the maximum operating voltage.
The recommended capacitor for CBST is a 0.01µF
ceramic capacitor. The recommended value for RBST is
20Ω to 60Ω.
Figure 5. High-side Drive Circuitry
Low-side Drive Output
The LSD pin is used to drive an external MOSFET. This
MOSFET is driven out of phase with the internal high-
side MOSFET to conduct inductor current during the
high-side MOSFETs off-time. Circuitry internal to the
regulator prevents short circuit “shoot-through” current
from flowing by preventing the high-side and low-side
MOSFETs conducting at the same time.
The low-side MOSFET gate voltage is supplied from
PVDD. Turn off of the MOSFET is accomplished by
discharging the gate through the LSD pin. The return
path is through the PGND pin and back to the
MOSFET’s Source pin. These circuit paths must be kept
short to minimize noise. See the layout section for
additional information.
Driving the low-side MOSFET on and off dissipates
power in the MIC24420/21 regulator. The power can be
calculated by the equation below:
S
IN
G
DRIVER
f
V
Q
P
×
×
=
Where:
PDRIVER is the power dissipated in the regulator by
switching the MOSFET on and off.
QG is the total Gate charge of the MOSFET at VGS =
PVDD.
VIN is the input voltage to the internal AVDD regulator.
fS is the switching frequency of the regulator
(1MHz/500kHz nominal).
dV/dt Induced Turn-on of the Low-side MOSFET
As the high-side MOSFET turns on, the rising dv/dt on
the switch-node forces current through CGD of the low-
side MOSFET causing a glitch on its gate. Figure 6
demonstrates the basic mechanism causing this issue. If
the glitch on the gate is greater than the MOSFET’s turn-
on threshold, it may cause an unwanted turn-on of the
low-side MOSFET while the high-side MOSFET is on. A
short
circuit
between
input
and
ground
would
momentarily
occur,
which
lowers
efficiency
and
increases
power
dissipation
in
both
MOSFETs.
Additionally, turning on the low-side MOSFET during the
off-time could interfere with overcurrent sensing.
Figure 6. dV/dt induced turn-on of the low-side MOSFET


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