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SC420 Datasheet(PDF) 7 Page - Semtech Corporation

Part # SC420
Description  High Speed, Combi-SenseTM, Synchronous Power MOSFET Driver for Mobile Applications
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC420 Datasheet(HTML) 7 Page - Semtech Corporation

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© 2003 Semtech Corp.
POWER MANAGEMENT
SC420
United States Patent No. 6,441,597
www.semtech.com
Applications Information
Combi-Sense (Lossless current sense)
The Combi-Sense is a way to sense the output current
on a combination of power devices. There is no sense
resistor and the current is sensed on: Top MOSFET, bot-
tom MOSFET and output inductor.
An internal phase node VPN sends a signal which is inte-
grated by the Combi-Sense network. This network con-
sists of a resistor and capacitor in series, connected
between VPN and the DRN pins. The resulting signal is
large, clean and not duty cycle sensitive. It can be used
directly for close loop current mode control and current
limit.
Fast Switching Drives
As the switching frequency of PWM controllers is increased
to reduce power supply volume and cost, fast rise and
fall times are necessary to minimize switching losses (TOP
MOSFET) and reduce dead-time (BOTTOM MOSFET)
losses. While low Rds_On MOSFET’s present a power
saving, the MOSFETs die area is larger and the effective
input capacitance of the MOSFET is increased. Often a
50% decrease in Rds_On doubles the effective input
gate charge, which must be supplied by the driver. The
Rds_On power savings can be offset by the switching
and dead-time losses with a suboptimum driver. While
discrete solution can achieve reasonable drive capabil-
ity, implementing shoot-through, programmable delay and
other housekeeping functions necessary for safe opera-
tion can become cumbersome and costly. The SC420
presents a total solution for the high-speed, high power
density applications. Wide input supply range of 4.5V-
25V allows use in battery powered applications, new high
voltage, distributed power supplies.
Shoot Through Protection
The control input (CO) to the SC420 is typically supplied
by a PWM controller that regulates the power supply out-
put. The timing diagram demonstrates the sequence of
events by which the top and bottom drive signals are
applied. The shoot-through protection is implemented
by holding the bottom FET off until the voltage at the
phase node (intersection of top FET source, the output
inductor and the bottom FET drain) has dropped below
1V. This assures that the top FET has turned off and
that a direct current path does not exist between the
input supply and ground, a shoot-through condition dur-
ing which both the top and bottom FET’s could be on
momentarily. The top FET is also prevented from turn-
ing on until the bottom FET is off. The top FET turn-on
delay is internally set to 20ns (typical) and may be
programmably extended by an external capacitor on the
Cdelay pin, the delay is increased by 1ns/pf.
The EN (enable) pin may be used to turn both TG and BG
drives off. This lowers power consumption by reducing
the quiescent current draw of the SC420 to IQsd.
Tri-State
If the CO pin is undriven it will float to an internally de-
fined voltage of 1.4V. This will switch the TG and BG
pins low and also tri-states the VPN node.
Over Temperature Shutdown
The SC420 will shutdown by pulling both driver’s low if
its junction temperature, T
J, exceeds 165°C.
The driv-
ers will resume operation when T
J declines below 155
o
C.
Layout Guidelines
As with any high speed , high current, switching regula-
tor circuit, proper layout is critical in achieving optimum
performance of the SC420. The Combi-SenseTM Con-
troller Evaluation board schematic shows a three-phase
synchronous design with all surface mountable compo-
nents.
Tight placement and short, wide traces must be used in
layout of The gate drives, DRN, and especially PGND pin.
The top gate driver supply voltage is provided by
bootstrapping the boost supply and adding it to the phase
node (DRN) voltage. Since the bootstrap capacitor sup-
plies the charge to the top gate, it must be less than
0.5in away from the SC420. Ceramic X7R capacitors
are a good choice for supply bypassing near the chip.
Supply Voltage
The SC420 can operate from 4.75V to 6V. The V
IN pin
bypass capacitor must also be less than 0.5in away from
the SC420. The ground node of this capacitor, the
SC420 PGND pin and the Source of the bottom FET must


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