SC4524 데이터 시트보기 (PDF) - Semtech Corporation
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SC4524 Datasheet PDF : 21 Pages
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SC4524
POWER MANAGEMENT
Operation
The SC4524 is a 30V constant-frequency peak current-
mode step-down switching regulator with an integrated
2.3A power transistor. The switching frequency 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 SC4524.
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
6.3mW sense resistor. The sensed current is summed
with slope-compensating ramp before it is compared with
the transconductance error amplifier output. The PWM
comparator trip 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.
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 (or bootstrap circuit) generates a voltage higher
than the input rail at the BST 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 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 regulator.
When the SS pin is pulled below 0.8V, the regulator is
turned off. If the SS pin is pulled below 0.2V, then the
SC4524 will undergo overall shutdown. The current drawn
from the input power supply reduces to 40µA. When the
SS pin is released, the soft-start capacitor is charged
with a 2µA current source (not shown in Figure 3). As the
SS voltage exceeds 0.4V, the internal bias circuit of the
SC4524 is enabled. The SC4524 draws 3.5mA from VIN.
An internal fast charge 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 SC4524.
Overload shutdown is disabled during soft-start (VSS <
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 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.
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