SC420A
POWER MANAGEMENT
Applications Information
Combi-Sense (Lossless current sense)
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 30ns (typical) and may be
programmably extended by an external capacitor on the
Cdelay pin, the delay is increased by 1ns/pf.
Combi-Sense is a method 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.
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 SC420A to IQsd.
CO Undriven
If the CO pin is undriven it will be pulled to GND by an
internal pull down resistor. This will switch the BG pin
high and the TG pin low.
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 SC420A
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.
Over Temperature Shutdown
The SC420A will shutdown by pulling both driver’s low if
its junction temperature, TJ, exceeds 165°C. The driv-
ers will resume operation when TJ declines below 155oC.
Supply Voltage
The SC420A can operate from 4.75V to 6V. The VIN pin
bypass capacitor must also be less than 0.5in away from
the SC420A. The ground node of this capacitor, the
SC420A PGND pin and the Source of the bottom FET
must be very close to each other, preferably with com-
mon PCB copper land with multiple vias to the ground
plane (if used). The parallel Schottky (if used) must be
physically next to the Bottom FET’s drain and source pins.
Any trace or lead inductance in these connections will drive
current away from the Schottky and allow it to flow through
the FET’s Body diode, thus reducing efficiency.
Shoot Through Protection
Preventing Inadvertent Bottom Gate Turn-on
The control input (CO) to the SC420A 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
At high VIN2 input voltages, (12V and greater) a fast turn-
on of the top FET creates a positive going spike on the
Bottom FET’s gate through the Miller capacitance, Crss of
the bottom FET. The voltage appearing on the gate due to
this spike is:
Vin *Crss
VSPIKE
=
(Crss + Ciss )
8
2004 Semtech Corp.
United States Patent No. 6,441,597
www.semtech.com
SEMTECH [ SEMTECH CORPORATION ]
