MIC261203 Datasheet, PDF (18/31 Pages) Micrel Semiconductor – 28V, 12A Hyper Light Load™ Synchronous DC/DC Buck Regulator

Micrel, Inc.
MOSFET Gate Drive
The Block Diagram (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 the gate voltage with minimal droop for
the power stroke (high-side switching) cycle, i.e. ΔBST =
10mA x 1.67μs/0.1μF = 167mV. When the low-side
MOSFET is turned back on, C_BST is then 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.
MIC261203
July 2011
18
M9999-071311-A