MIC2159 Datasheet, PDF (8/17 Pages) Micrel Semiconductor – SYNCHRONOUS-itty™ Step-Down Converter IC

Micrel
2.9V to 5V.
MOSFET Gate Drive
The MIC2159 high-side drive circuit is designed to
switch an N-Channel MOSFET. The Functional Block
Diagram shows a bootstrap circuit, consisting of D1 and
C_BST. This circuit supplies energy to the high-side drive
circuit. Capacitor C_BST circuit is charged while the low-
side MOSFET is on and the voltage on the VSW 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 MOSFET turns on, the voltage on the VSW
pin increases to approximately VIN. Diode D1 is
reversed biased and C_BST floats high while continuing to
keep the high-side MOSFET on. When the low-side
MIC2159
switch is turned back on, C_BST is recharged through D1.
The drive voltage is derived from the internal 5V V_DD
bias supply. The nominal low-side gate drive voltage is
5V and the nominal high-side gate drive voltage is
approximately 4.5V due the voltage drop across D1. An
approximate 50ns delay between the low-side(off) to
high-side(on) driver transitions is used to prevent current
from simultaneously flowing unimpeded through both
MOSFETs (shoot-through). Adaptive gate drive is
implemented on the high-side(off) to low-side(on) driver
transition to reduce losses in the flywheel diode and to
prevent shoot-through. This is operated by detecting the
VSW pin; once this pin is detected to reach 1.5v, the
high-side MOSFET can be assumed to be off and the
low side driver is enabled.
October 2006
8
M9999-101206