SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 8

• Manufacturer Part Number: SC2440
• Description: 2.5 MHz Dual Switching Regulator with Integrated 2A Switches
• Manufacturer: SEMTECH [Semtech Corporation]

The SC2440 is a 2-channel constant-frequency peak
current-mode step-down switching regulator with
integrated 2A power transistors. Both regulators of the
SC2440 operate from a common input power supply and
share the same voltage reference, the master oscillator
and the synchronizing circuit. Turn-on of the power
transistors are phase-shifted by 180 . The two regulators
are otherwise completely identical, independent and are
capable of producing two separate outputs from the
same input.
The master oscillator of the SC2440 runs at twice the
channel frequency. The free-running frequency of the
master oscillator can be programmed with an external
resistor from the ROSC pin to ground. Frequency
adjustability makes switching regulator design flexible.
Peak current-mode control is utilized for the SC2440.
The double reactive poles of the output LC filter are
reduced to a single real pole by the inner current loop,
easing loop compensation. Fast transient response can
be achieved with a simple Type-2 compensation network.
Switch collector current is sensed with an integrated 7.7m
sense resistor. The sensed current is summed with slope-
compensating ramp before it is compared with the
transconductance error amplifier output. The PWM
comparator tripping instant determines the switch turn-
on pulse width (Figure 2). The current-limit comparator
ILIM turns off the power switch when the sensed-signal
exceeds the 20mV current-limit threshold. ILIM therefore
provides cycle-by-cycle limit. Current-limit does not vary
with duty-cycle.
Driving the base of the power transistor above the input
power supply rail minimizes the power transistor turn-on
voltage and maximizes efficiency. An external charge
pump (formed by the capacitor C
Figure 1) generates a voltage higher than the input rail
at the BOOST pin. The bootstrapped voltage generated
becomes the supply voltage for the power transistor
driver.
The SS pin is a multiple-function pin. An external capacitor
connected from the SS pin to the ground together with
the internal 1.8µA and 2.6µA current sources set the
POWER MANAGEMENT
Operation
2005 Semtech Corp.
2
and the diode D
3
in
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soft-start and overload shutoff times of the regulator (Figure
3). The SS pin can also be used to shut off the corresponding
regulator. When either SS pin is pulled below 0.8V, that
regulator is turned off. If both SS pins are pulled below
0.2V, then the SC2440 undergoes overall shutdown. The
current draw from the input power supply reduces to 38µA.
When either SS pin is released, the corresponding soft-
start capacitor is charged with a 2µA current source (not
shown in Figure 3). As either SS voltage exceeds 0.3V, the
internal bias circuit of the SC2440 is enabled. The SC2440
draws 3.3mA from V
quickly charges the soft-start capacitor to 1V. At this
juncture, the fast charge circuit turns off and the 1.8µA
current source slowly charges the soft-start capacitor. The
output of the error amplifier is forced to track the slow
soft-start ramp at the SS pin. When the COMP voltage
exceeds 1.1V, the switching regulator starts to switch.
During soft-start, the current limit of the converter is
gradually increased until the converter output comes into
regulation.
Hiccup overload protection is utilized in the SC2440.
Overload shutdown is disabled during soft-start (V
2V). In Figure 3 the reset input of the overload latch will
remain high if the SS voltage is below 2V. Once the soft-
start capacitor is charged above 2V, the overload
shutdown latch is enabled. As the load draws more current
from the regulator, the current-limit comparator will limit
the peak inductor current. This is cycle-by-cycle current
limiting. Further increase in load current will cause the
output voltage to decrease. If the output voltage falls
below 74% of its set point, then the overload latch will be
set and the soft-start capacitor will be discharged with a
net current of 0.8µA. The switching regulator is shut off
until the soft-start capacitor is discharged below 1V. At
this moment, the overload latch is reset. The soft-start
capacitor is recharged and the converter again undergoes
soft-start. The regulator will go through soft-start, overload
shutdown and restart until it is no longer overloaded.
Each regulator of the SC2440 has its own power good
comparator. The open collector output of the power good
comparator will be actively pulled low if the corresponding
feedback voltage is below 0.9V.
IN
. An internal fast charge circuit
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SC2440
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