MIC2124 Datasheet, PDF (11/24 Pages) Micrel Semiconductor – Constant Frequency, Synchronous Current Mode Buck Controller

Micrel, Inc.
MOSFET Gate Drive
The MIC2124 high-side drive circuit is designed to
switch an N-Channel MOSFET. The typical application
circuit shows a bootstrap circuit, consisting of a Schottky
diode D1 and 0.1μF boostrap capacitor C_BST, as shown
in the typical application schematic on Page 1. 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 LX 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 LX pin
increases to approximately V_HSD. Diode D1 is reversed
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
MIC2124
hold 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 supply voltage V_IN.
The nominal low-side gate drive voltage is V_IN and the
nominal high-side gate drive voltage is approximately
V_IN - V_DIODE, where V_DIODE is the voltage drop across D1.
A dead-time of approximate 30ns between the high-side
and low-side driver transitions is used to prevent current
from simultaneously flowing unimpeded through both
MOSFETs.
June 2010
11
M9999-060810-D