SC2443MLTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2443MLTRT Datasheet PDF : 22 Pages
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SC2443
Applications Information
Description
The SC2443 is a constant frequency 2-phase current-mode
step-down PWM switching controller driving all N-channel
MOSFET. 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 supply voltages for the high-side gate drivers are
obtained from two diode-capacitor bootstrap circuits. If
the bootstrap capacitor is charged from VCC, the high-
side gate drive voltage swing will be from approximately
2VCC to the 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 VCC. The outputs of the low-side
gate drivers swing from VCC to ground.
The SC2443 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 SC2443 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 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 2
- 2.6 times the set free-running channel frequency.
Control Loop
The SC2443 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 compensation ramp, the
high-side MOSFET is turned off. After a non-overlapping
conduction time of 90ns, the low-side MOSFET is turned
on.
The SC2443 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 1/4 to 1/3 of the full-scale sensed
voltage limit of 75mV. The maximum sensed voltage limit
is unaffected by the compensating ramp.
Current-Sensing
There are two ways to sense the inductor current for
current-mode control with the SC2443. 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.
With proper RC filter, Inductor DCR sensing can also be
used for SC2443 resulting in low cost and without extra
power dissipation.
Error Amplifiers
In closed loop operation, the error amplifier output ranges
from 1.1V to 3.5V. The upper output operating range
of either error amplifier is reserved for positive current-
sense voltage (CS+ - CS-) and corresponds to positive
(sourcing) output current. If the amplifier swings to its
lower operating range, the amplifier will still modulate the
high-side gate drive duty-ratio. However the peak current-
sense voltage (hence the peak inductor current) will be
limited to a negative value. The error amplifier output
is about 2.2V when the peak sense-voltage is zero. The
built-in offset in the current sense amplifier together with
synchronous continuous-conduction mode of operation
allows the SC2443 to regulate the output irrespective of
the direction of the load current.
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