SC2446 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Operation
Overview
The SC2446 is a constant frequency 2–phase current-
mode step-down PWM switching controller driving all N-
channel MOSFET’s. The two channels of the controller
operate at 180 degrees out of phase from each other.
Since input currents are interleaved in a two-phase
converter, input ripple current is lower and smaller input
capacitor can be used for filtering. Also, with lower
inductor current and smaller inductor ripple current per
phase, overall I2R losses are reduced.
The SC2446 operates in synchronous continuous-
conduction mode. It can be configured either as two
independent step-down controllers producing two
separate outputs or as a dual-phase single-output
controller by tying the IN2- pin to VCC. In single output
operation, the channel one error amplifier controls both
channels and the channel two error amplifier is disabled.
Soft-start and overload hiccup of both channels is
synchronized to channel one.
Frequency Setting and Synchronization
The internal oscillator of the SC2446 runs at twice the
phase frequency. The free-running frequency of the
oscillator can be programmed with an external resistor
from the ROSC pin to the ground. The step-down controllers
are capable of operating up to 1 MHz. It is necessary to
consider the operating duty-ratio before deciding the
switching frequency. See Applications Information section
for more details.
When synchronized externally, the applied clock frequency
should be twice the desired phase frequency. The
synchronizing clock frequency should also be between 1-
1.33 times the set free-running frequency.
Control Loop
The SC2446 uses peak current-mode control for fast
transient response, ease of compensation and current
sharing in single output operation. The low-side MOSFET
of each channel is turned off at the falling-edge of the
phase timing clock. After a brief non-overlapping time
interval of 90ns, the high-side MOSFET is turned on. The
phase inductor current ramps up. When the sensed
inductor current reaches the threshold determined by
the error amplifier output and ramp compensation, the
high-side MOSFET is turned off. After a non-overlapping
conduction time of 90ns, the low-side MOSFET is turned
on.
The supply voltages for the high-side gate drivers are
obtained from two diode-capacitor bootstrap circuits. If the
bootstrap capacitor is charged from V , the high-side gate
CC
drive voltage swing will be from approximately 2V to the
CC
ground. The power dissipated in the high-side gate driver
is not higher with higher voltage swing because the gate-
source voltage of the high-side MOSFET still swing from
zero to V .The outputs of the low-side gate drivers swing
CC
from VC to the ground.
The SC2446 has internal ramp-compensation to prevent
sub-harmonic oscillation when operating above 50% duty
cycle. There is enough ramp internally for a sensed
voltage ripple between ¼ to 1/3 of the full-scale sensed
voltage limit of 75mV. The maximum sensed voltage limit
is unaffected by the compensation ramp.
Current-Sensing
There are two ways to sense the inductor current for
current-mode control with the SC2446. Since the peak
inductor current corresponds to 75mV of sensed voltage
(CS+ - CS-), resistor current sensing can be used at the
output without resulting in excessive power dissipation.
Although accurate and far easier to lay out than high-
side resistor sensing, a pair of precision sense resistors
adds cost to the converter. The SC2446 has provision to
reconstruct a differential voltage proportional to the
inductor current at the output of the converter (U.S. patent
6,441,597). The voltage to current ratio or the equivalent
sense resistance R is a combination of high-side and low-
eq
side MOSFET R ’s and the inductor series resistance
DS(ON)
(hence the name “Combi-Sense”). The SC2446 provides
the virtual phase voltages VPN1 and VPN2 (these are
2004 Semtech Corp.
9
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