SC4517AEVB datasheet(7/15 Pages) SEMTECH

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POWER MANAGEMENT

SC4517A

Application Information

General Overview

The SC4517A is a high frequency current mode PWM

buck regulator. It has an internal clock with fixed-

frequency. The SC4517A uses two feedback loops

(voltage loop and current loop) that control the duty cycle

of the internal power switch. The error amplifier functions

like the one of the voltage mode controller. The output

of the error amplifier provides a switch current reference.

This technique effectively removes one of the double

poles in the output LC filter stage. With this, it is easier

to compensate a current mode converter for better

performance. A minimum 2.7V voltage is required to

saturate the NPN power switch when it is “ON” to reduce

its conduction loss.

Current Limit and Overcurrent Protection

The current sense amplifier in the SC4517A monitors

the switch current during each cycle. Overcurrent

protection (OCP) is triggered when the current limit

exceeds the upper limit of 1.5A, detected by a voltage

on COMP being greater than about 2V. When an OCP

fault is detected, the power switch is turned off and the

external COMP capacitor is quickly discharged using an

internal small signal NPN transistor. Once the COMP

voltage has fallen below 250mV the power switch is

turned off, control circuit is held off for 50µs determined

by a internal timer. When the 50µs time is up, an internal

timer prevents any operation for 50µs, 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, at a rate dependent on the

value of Ccomp. During start up, the voltage on COMP

rises roughly at the rate of dv/dt = 120µA/C

comp. Ccomp is

the total capacitance value attached to COMP. Therefore,

the retry time for a sustained overcurrent can be

approximately calculated as:

50us

120uA

comp

retry

⋅

Figure 1 shows the voltage on COMP during a sustained

overcurrent condition.

250mV

Figure 1. Voltage on COMP for Startup and OCP

Enable

Pulling and holding the EN pin below 0.4V activates the

shut down mode of the SC4517A which reduces the input

supply current to less than 10µA. During the shut down

mode, the switch is turned off. The SC4517A is turned

on if the EN pin is pulled high.

Oscillator

Its internal free running oscillator sets the PWM frequency

at 1.25MHz for the SC4517A without any external

components to program the frequency. An external clock

with a duty cycle from 20% to 80% connected to the

SYNC pin activates synchronous mode. The frequency

of the external clock can be from 1.6MHz to 2MHz.

UVLO

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

on Pin IN determines the operation of the SC4517A. As

VIN increases during power up, the internal circuit senses

VIN and keeps the power transistor off until VIN reaches

2.6V.

Load Current

The peak current I

PEAK 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 current continuous mode:

OMAX

)

(

⋅

−

⋅

−

Where:

fs = switching frequency,

O = output voltage;

D = duty ratio, V

o/VI;

I = input voltage

Figure 2 shows the theoretical maximum load current

for the specific cases. In a real application, however, the

allowed maximum load current also depends on the layout

and the air cooling condition. Therefore, the maximum

load current may need to be degraded according to the

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