MIC24420 Datasheet, PDF (13/34 Pages) Micrel Semiconductor – 2.5A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
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 C_BST comprise the bootstrap
circuit, which supplies drive voltage to the high-side
MOSFET. Bootstrap capacitor C_BST 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 C_BST charges
the MOSFET gate, turning it on. Voltage on the SW pin
increases to approximately V_IN. Diode D2 is reversed
biased and C_BST 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 C_BST is a 0.01µF
ceramic capacitor. The recommended value for R_BST is
20Ω to 60Ω.
MIC24420/MIC24421
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:
P_DRIVER = Q_G × V_IN × f_S
Where:
P_DRIVER is the power dissipated in the regulator by
switching the MOSFET on and off.
Q_G is the total Gate charge of the MOSFET at V_GS =
P_VDD.
V_IN is the input voltage to the internal A_VDD regulator.
f_S 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 C_GD 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 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
Figure 6. dV/dt induced turn-on of the low-side MOSFET
June 2012
13
M9999-062012-C