SC2620 Semtech Corporation, SC2620 Datasheet - Page 8

• Manufacturer Part Number: SC2620
• Description: 30V Step-down Regulator
• Manufacturer: Semtech Corporation

The SC2620 is a 30V 2-channel constant-frequency peak
current-mode step-down switching regulator with integrated
2.3A power transistors. Both regulators in the SC2620
operate from a common input power supply and share the
same voltage reference and the master oscillator. Turn-on
of the power transistors is phase-shifted by 180 . The two
regulator cores are otherwise completely identical,
independent and are capable of producing two separate
outputs from the same input.
The channel frequency can be programmed with an
external resistor from the ROSC pin to ground. This allows
the designer to set the switching frequency according to
the input to the output voltage conversion ratio.
Peak current-mode control is utilized for the SC2620. 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 6.3m 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 point 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.
An external charge pump (formed by the capacitor C
the diode D
than the input rail at the BOOST pin. The bootstrapped
voltage generated becomes the supply voltage for the
power transistor driver. Driving the base of the power
transistor above the input power supply rail minimizes the
power transistor turn-on voltage and maximizes efficiency.
The SS pin is a multiple-function pin. An external capacitor
connected from the SS pin to ground together with the
internal 1.8µA and 2.6µA current sources set the soft-
start and overload shutoff times of the regulator (Figure
3). The SS pin can also be used to shut off the corresponding
POWER MANAGEMENT
Operation
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3
in Figure 1(a)) generates a voltage higher
2
and
8
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 SC2620 undergoes overall shutdown. The
current drawn from the input power supply reduces to
40µA. 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 SC2620 is enabled. The
SC2620 draws 3.5mA from V
circuit 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 SC2620.
Overload shutdown is disabled during soft-start (V
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 70%
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.
The power good comparator indicates that the channel 1
regulator output has risen to within 10% of its set value.
The open collector output of the power good comparator
will be actively pulled low if its feedback voltage is below
0.9V.
IN
. An internal fast charge
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SC2620
SS
< 2V).