SC4524D データシートの表示（PDF） - Semtech Corporation

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SC4524D

18V 2A Step-Down Switching Regulator

Semtech Corporation 

SC4524D

Applications Information

Operation

The SC4524D is a constant-frequency, peak current-

mode, step-down switching regulator with an integrated

18V, 2.6A power NPN transistor. Programmable switching

frequency makes the regulator design more flexible. With

the peak current-mode control, the double reactive poles

of the output LC filter are reduced to a single real pole by

the inner current loop. This simplifies loop compensation

and achieves fast transient response with a simple Type-2

compensation network.

As shown in Figure 2, the switch collector current is

sensed with an integrated 5.5mW sense resistor. The

sensed current is summed with a slope-compensating

ramp before it is compared with the transconductance

error amplifier (EA) output. The PWM comparator trip

point determines the switch turn-on pulse width. The

current-limit comparator ILIM turns off the power switch

when the sensed signal exceeds the 18mV current-limit

threshold.

Driving the base of the power transistor above the

input power supply rail minimizes the power transistor

saturation voltage and maximizes efficiency. An external

bootstrap circuit (formed by the capacitor C1 and the

diode D1 in Figure 1) generates such a voltage at the BST

pin for driving the power transistor.

Shutdown and Soft-Start

The SS/EN pin is a multiple-function pin. An external

capacitor (4.7nF to 22nF) connected from the SS pin to

ground sets the soft-start and overload shutoff times of

the regulator (Figure 3). The effect of VSS/EN on the SC4524D

is summarized in Table 1.

Table 1: SS/EN operation modes

SS/EN

<0.2V

0.4V to 1.2V

1.2V to 2.15V

>2.15V

Mode

Shutdown

Not switching

Switching & hiccup disabled

Switching & hiccup armed

Supply Current

18uA @ 5Vin

2mA

Load dependent

Pulling the SS/EN pin below 0.2V shuts off the regulator

and reduces the input supply current to 18µA (VIN = 5V).

When the SS/EN pin is released, the soft-start capacitor

is charged with an internal 1.9µA current source (not

shown in Figure 3). As the SS/EN voltage exceeds 0.4V,

the internal bias circuit of the SC4524D turns on and the

SC4524D draws 2mA from VIN. The 1.9µA charging current

turns off and the 2.4µA current source IC in Figure 3 slowly

charges the soft-start capacitor.

The error amplifier EA in Figure 2 has two non-inverting

inputs. The non-inverting input with the lower voltage

predominates. One of the non-inverting inputs is biased

to a precision 1V reference and the other non-inverting

input is tied to the output of the amplifier A1. Amplifier A1

produces an output V1= 2(VSS/EN -1.2V). V1 is zero and COMP

is forced low when VSS/EN is below 1.2V. During start up,

the effective non-inverting input of EA stays at zero until

the soft-start capacitor is charged above 1.2V. Once VSS/

EN exceeds 1.2V, COMP is released. The regulator starts to

switch when VCOMP rises above 0.4V. If the soft-start interval

is made sufficiently long, then the FB voltage (hence the

output voltage) will track V1 during start up. VSS/EN must be

at least 1.83V for the output to achieve regulation. Proper

soft-start prevents output overshoot. Current drawn from

the input supply is also well controlled.

Overload / Short-Circuit Protection

Table 2 lists various fault conditions and their

corresponding protection schemes in the SC4524D.

Table 2: Fault conditions and protections

Condition

IL>ILimit, VFB>0.8V

Fault

Over current

IL>ILimit, VFB<0.8V

Over current

VSS/EN Falling Persistent over current

Protective Action

Cycle-by-cycle limit at

programmed frequency

Cycle-by-cycle limit with

frequency foldback

Shutdown, then retry

SS/EN<1.9V

or short circuit

(Hiccup)

Tj>160C

Over temperature

Shutdown

As summarized in Table 1, overload shutdown is disabled

during soft-start (VSS/EN<2.15V). In Figure 3, the reset input

of the overload latch B2 will remain high if the SS/EN

voltage is below 2.15V. Once the soft-start capacitor is

charged above 2.15V, the output of the Schmitt trigger

B1 goes high, the reset input of B2 goes low and hiccup



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