SC4524SETRT datasheet(8/21 Pages) SEMTECH

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ã 2006 Semtech Corp.

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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 V

IN.

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 (V

SS <

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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