MIC2103 Datasheet, PDF (21/36 Pages) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

Micrel, Inc.
MOSFET Gate Drive
The MIC2103/04 high-side drive circuit is designed to
switch an N-Channel MOSFET. Figure 1 shows a
bootstrap circuit, consisting of D1 (a Schottky diode is
recommended) and C_BST. This circuit supplies energy to
the high-side drive circuit. Capacitor C_BST is charged
while the low-side MOSFET is on and the voltage on the
SW pin is approximately 0V. When the high-side
MOSFET driver is turned on, energy from C_BST is used to
turn the MOSFET on. As the high-side MOSFET turns
on, the voltage on the SW pin increases to
approximately V_IN. Diode D1 is reverse biased and C_BST
floats high while continuing to keep the high-side
MOSFET on. The bias current of the high-side driver is
less than 10mA so a 0.1μF to 1μF is sufficient to hold
MIC2103/04
the gate voltage with minimal droop for the power stroke
(high-side switching) cycle, i.e., ΔBST = 10mA x
3.33μs/0.1μF = 333mV. When the low-side MOSFET is
turned back on, C_BST is recharged through D1. A small
resistor R_G, which is in series with C_BST, can be used to
slow down the turn-on time of the high-side N-channel
MOSFET.
The drive voltage is derived from the V_DD supply voltage.
The nominal low-side gate drive voltage is V_DD and the
nominal high-side gate drive voltage is approximately
V_DD – V_DIODE, where V_DIODE is the voltage drop across
D1. An approximate 30ns delay between the high-side
and low-side driver transitions is used to prevent current
from simultaneously flowing unimpeded through both
MOSFETs.
August 2012
21
M9999-080712-A