MIC25400YML TR Micrel Inc, MIC25400YML TR Datasheet - Page 11

• Manufacturer Part Number: MIC25400YML TR
• Description: IC REG PWM SYNC BUCK 2A 24MLF
• Manufacturer: Micrel Inc
• Series: -r

Specifications of MIC25400YML TR

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 1.2MHz
• Duty Cycle: 75%
• Voltage - Supply: 4.5 V ~ 13.2 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 24-VFQFN Exposed Pad, 24-MLF®
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 576-3902-2

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
circuit, which supplies drive voltage to the high-side
MOSFET. Bootstrap capacitor C
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
the MOSFET gate, turning it on. Voltage on the SW pin
increases to approximately V
biased and C
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
ceramic capacitor. The recommended value for R
20Ω.
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 from conducting at the same time.
Micrel, Inc.
January 2011
Figure 5. High-side Drive Circuitry
BST
flies high while maintaining gate voltage
BST
IN
. Diode D2 is reversed
comprise the bootstrap
BST
is charged through
BST
is a 0.01µF
BST
charges
BST
is
11
The low-side MOSFET gate voltage is supplied from
V
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 of this
datasheet for additional information.
Driving the low-side MOSFET on and off dissipates
power in the MIC25400 regulator. The power can be
calculated by the equation below:
Where:
P
switching the MOSFET on and off.
Q
V
equal to the voltage on P
f
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
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
momentarily
increases power dissipation in both FETs. Additionally,
turning on the low-side FET during the off-time could
interfere with overcurrent sensing.
S
PVDD
DRIVER
GS
Figure 6. dv/dt induced turn-on of the low-side MOSFET
G
is the switching frequency of the regulator (1MHz
is the total Gate charge of the MOSFET at V
is the MOSFET’s Gate to Source voltage which is
. Turn off of the MOSFET is accomplished by
circuit
P
is the power dissipated in the regulator by
DRIVER
occur,
between
=
Q
G
×
which
V
VDD.
GS
input
×
f
S
lowers
and
M9999-020111-C
efficiency
ground
GD
MIC25400
of the low-
GS.
would
and