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SC4524A

28V 2A Step-Down Switching Regulator

Semtech Corporation 

SC4524A

Applications Information

Operation

The SC4524A is a constant-frequency, peak current-

mode, step-down switching regulator with an integrated

28V, 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 6.1mW 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 20mV 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 SC4524A

is summarized in Table 1.

Table 1 Table 1: SS/EN operation modes

Table 1 SS/EN

Mode

Supply Current

<0.2V

Shutdown

18uA @ 5Vin

SS/E0N.4V to 1.23V

ModNeot switching Supply Curre2nmt A

<0.21V.23V to 2.1V

0.4V to 1.23>V2.1V

NSoSSwthwisuticwttchdihtiocniwnhgginn&g&hhicicccuuppdaisrambeled1d8uA2m@LAo5aVdindependent

1.23V to 2.1V Switching & hiccup disabled Load dependent

>2P.u1Vlling theSSwSit/cEhNingp&inhicbceuploawrm0ed.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 1.6µA current source (not

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

the internal bias circuit of the SC4524A turns on and the

SC4524A draws 2mA from VIN. The 1.6µA charging current

turns off and the 2µA 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 outputV1= 2(VSS/EN -1.23V).V1 is zero and COMP

is forced low when VSS/EN is below 1.23V. During start up,

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

the soft-start capacitor is charged above 1.23V. Once VSS/EN

exceeds 1.23V, 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 SC4524A.

TableCo2n:dFitaiounlt conditionsFaaunltd protectioCynPcrsloet-ebcyt-icvyeclAeclitmioitnat

ConILd>itIiLoinmit, VFB>0.8V FaultOver current ProtepcrtoivgeraAmcmtioend frequency

Cycle-bCyy-ccylec-lebyli-mcyitcalet limit with

IL>ILimitI,LV>IFLBi>m0it.,8VFB<0.8VOver curOrevnetr current

programmefrdeqfrueeqnuceynfcoyldback

VSS/EN Falling Persistent over cuCrryecnlet -by-cSyhculetdloimwint ,wtihthen retry

IL>ILimit, VFB<0.8V

Over current

SS/EN<1.9V

or short circuit frequency fold(bHaiccckup)

VSS/EN Falling Persistent over current Shutdown, then retry

SS/EN<1.9TVj>160C or shoOrt vceirrcuteitmperature (HiccupS) hutdown

Tj>160C

Over temperature

Shutdown

As summarized in Table 1, overload shutdown is disabled

during soft-start (VSS/EN<2.1V). In Figure 3, the reset input of

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

is below 2.1V. Once the soft-start capacitor is charged

above 2.1V, the output of the Schmitt trigger B1 goes high,

the reset input of B2 goes low and hiccup becomes armed.



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