mx8830 Clare, Inc., mx8830 Datasheet - Page 9

• Manufacturer Part Number: mx8830
• Description: Synchronous Buck Mosfet Driver
• Manufacturer: Clare, Inc.

MX8830
Drawing No. 0883009
Theory of Operation
The MX8830 family are dual MOSFET drivers,
designed to drive two external N-channel power
MOSFETs. The low-side driver is designed to drive
a non-floating N-channel power MOSFET and its
output is out of phase with the PWM input. The
high-side driver is designed to drive a floating N-
channel power MOSFET and its output is in phase
with the PWM input. An external bootstrap circuit
provides the floating power supply to the high-side
driver.
The bootstrap circuit consists of a Schottky diode
and a boost capacitor. When the PWM input
transitions to a logic low, the low-side power
MOSFET turns ON, the SW node is pulled to
ground, and the bootstrap capacitor is charged to
VDD through the Schottky diode. When the PWM
transitions to a logic high, the high side power
MOSFET begins to turn on and the SW node rises
up to the input supply, VIN. In turn the boost
capacitor raises the BST node voltage to a level
equal to the input supply plus the boost capacitor
voltage, providing sufficient voltage to the BST
node to turn on the High-Side Power MOSFET. An
internal
monitors both gate driver outputs and allows each
driver output to turn ON only when the other output
driver turns OFF and falls below 1V.
Three-State PWM Input
A unique feature of these drivers and other
Micronix drivers is the addition of a shutdown
window to the PWM input. If the PWM signal
enters and remains within the shutdown window for
a set holdoff time, the driver outputs are disabled
and both MOSFET gates are pulled and held low.
The shutdown state is removed when the PWM
signal moves outside the shutdown window.
Otherwise, the PWM rising and falling thresholds
outlined in the Electrical Specifications determine
when the lower and upper gates are enabled.
This feature helps prevent a negative transient on
the output voltage when the output is shut down,
eliminating the Schottky diode that is used in some
systems for protecting the load from reversed
output voltage events.
cross-conduction
prevention
circuit
9
Detailed Circuit Description
(Refer to the Application Diagrams)
The PMW input signal controls both the High Side
and Low Side power MOSFET drivers. The Power
MOSFETs are driven so that the SW node follows
the polarity of the PWM signal.
Low-Side Gate Driver
The Low-Side Gate Driver is designed to drive a
ground referenced N-Channel Power MOSFET. In
a synchronous buck converter application, it drives
the gate of the synchronous rectifier FET, (Q2).
When
SD=LSD=VDD), the driver output is 180˚ out of
phase with the PWM input. The internal overlap
protection circuit monitors the High-Side Gate
Driver, and allows the Low-Side Gate Driver to turn
on only when the High-Side Gate Driver output
falls below 1.0 Volt. The supply rails for the Low-
Side Gate Driver are VDD and PGND.
High-Side Gate Driver
The High-Side Gate Driver is designed to drive a
floating N-Channel Power MOSFET referenced to
SW. In a synchronous buck converter application,
it drives the gate of the high side power MOSFET,
(Q1).
SD=VDD), the driver output is in phase with the
PWM input. The bootstrap supply rails for the High-
Side Gate Driver are BST and SW, and are
generated by an external bootstrap circuit. The
bootstrap circuit consists of a Schottky diode
DBST, and a bootstrap capacitor CBST. During
start up, the SW pin is at ground and the bootstrap
capacitor CBST charges up to VDD through the
Schottky diode DBST. When the PWM input
transitions high the High-Side Gate Driver begins
to turn Q1 ON by transferring charge from the
bootstrap capacitor CBST to the gate of Q1. As Q1
turns on the SW pin will rise up to VIN, forcing the
BST pin to VIN + VBOOSTCAP. This supplies the
required gate to source voltage to Q1. When PWM
transitions low the High-Side Driver and in turn Q1
switch off. When SW falls below 1 Volt the Low-
Side Gate Driver turns on and recharges the
bootstrap capacitor which completes the cycle.
MX8830B/MX8830R/MX8830X
When the driver is enabled (MX8830R/X
the
driver
is
enabled,
www.claremicronix.com
(MX8830R/X
8/9/07