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SC4521

3A Step-Down Switching Regulator with Programmable Soft Start

Semtech Corporation 

SC4521

POWER MANAGEMENT

Application Information

The SC4521 is a current mode buck converter regulator.

The SC4521 uses two feedback loops that control the

duty cycle of the internal power switch. The error amplifier

functions like that of the voltage mode converter. The

output of the error amplifier works as a switch current

reference. This technique effectively removes one of the

double poles in the voltage mode system. With this, it is

much simpler to compensate a current mode converter

to have better performance.

Soft Start

Internally, connected to the SS pin is a 100K pull-up

resistor from an internal 1.6V regulator and the collector

of an NPN pull-down transistor from SS to GND. The NPN

transistor is “on” when the Enable pin is low or when a

fault is detected (Input UVLO, Over-Current, or Over

Temp). When the SC4521 is disabled or when a fault is

detected, the NPN transistor pulls the SS pin low. The

SS charging time is controlled by the internal 100k

resistor and external soft-start cap. This is a closed-loop

soft-start which effectively “ramps the reference”. The

SS process completes when the SS pin voltage is

exceeds the internal reference voltage, 0.8V. The SS pin

continues to charge up to 1.6V, well above the reference

voltage to ensure it does not interfere with normal

operation. The governing SS equation is:

TSS = 69000 • CSS

For example, a 22nF SS cap would give a SS time of

approximately 1.5mS.

However, when VI is higher than about 13V, the SC4521

requires a pull-up resistor from the SS pin to VI for normal

operation. The softstart time can be estimated as:

TSS

= R SS

⋅ C SS

⋅ [−Ln (1 −

0.8

VI

)]

Where:

Rss = pull-up resistance from SS pin to VI,

Css = capacitance from SS pin to GND, and

VI = input voltage.

Overcurrent Protection

The current sense amplifier in the SC4521 monitors the

switch current during each cycle. Overcurrent protection

(OCP) is triggered when the current limit exceeds the upper

limit of 3A, detected by a voltage on COMP greater than

about 2V. When an OCP fault is detected, the switch is

turned off and the external COMP capacitor is gradually

discharged at the rate of dv/dt=3µA/Ccomp. Ccomp is

the total capacitance value attached to COMP. At the

same time, the soft start cap C is quickly discharged.

SS

Once the COMP voltage has fallen below 250mV, the

part enters a normal startup cycle. In the case of

sustained overcurrent or dead-short, the part will

continually cycle through the retry sequence as described

above.

Enable

Pulling and holding the EN pin below 0.4V activates the

shut down mode of the SC4521 which reduces the input

supply current to 250µA. During the shut down mode,

the switch is turned off. The SC4521 is turned on if the

EN pin is pulled high.

Oscillator

Its internal free running oscillator sets the PWM frequency

at 600kHz for the SC4521 without any external

components to program the frequency.

UVLO

When the EN pin is pulled and held above 1.8V, the voltage

on Pin IN determines the operation of the SC4521. As

VIN increases during power up, the internal circuit senses

VIN and keeps the power transistor off until VIN reaches

4.4V.

Load Current

The peak current IPEAK in the switch is internally limited.

For a specific application, the allowed load current I

OMAX

will change if the input voltage drifts away from the original

design as given for continuous current mode:

IOMAX

=

3−

VO ⋅ (1− D)

2 ⋅ L ⋅ fs

Where:

fs = switching frequency,

Vo = output voltage and

D = duty ratio, VO/VI

VI = input voltage.

 2007 Semtech Corp.

6

www.semtech.com

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