SC4519 Просмотр технического описания (PDF) - Semtech Corporation

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SC4519

600kHz, 3A Step-Down Switching Regulator

Semtech Corporation 

SC4519

POWER MANAGEMENT

Application Information

The SC4519 is a current mode buck converter regulator.

SC4519 has an internal fixed-frequency clock. The

SC4519 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. The current sense amplifier

in the SC4519 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 quickly discharged using an internal

npn transistor. Once the COMP voltage has fallen below

250mV, an internal timer prevents any operation for

50µs, after which 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, 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:

Tretry = Ccomp⋅1220VuA+50us

Oscillator

Its internal free running oscillator sets the PWM frequency

at 600kHz for the SC4519 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 750kHz to 1.2MHz.

UVLO

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

on Pin IN determines the operation of the SC4519. 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 in the switch is internally limited.

PEAK

For a specific application, the allowed load current IOMAX

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

V = input voltage.

I

Figure 1 shows the voltage on COMP during a sustained

overcurrent condition.

2V

250mV

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 derated according to the

thermal situation of the application.

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 SC4519 which reduces the input

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

mode, the switch is turned off. The SC4519 is turned on

if the EN pin is pulled high.

 2007 Semtech Corp.

7

www.semtech.com

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