SC4519H Просмотр технического описания (PDF) - Semtech Corporation

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SC4519H

600kHz, 3A Step-Down Switching Regulator

Semtech Corporation 

SC4519H

POWER MANAGEMENT

Application Information

The SC4519H is a current mode buck converter regulator.

SC4519H has an internal fixed-frequency clock. The

SC4519H 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 SC4519H 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 discharged at the rate of dv/dt = 3µA/

Ccomp. Once the COMP voltage has fallen below 250mV,

the part enters a normal startup cycle. Ccomp is the total

capacitance value attached to COMP. 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/Ccomp. Therefore, the retry time for a

sustained overcurrent can be approximately calculated

as:

Tretry

=

Ccomp

•

2V

120uA

+ Ccomp

•

2V

3uA

Figure 1 shows the voltage on COMP during a sustained

overcurrent condition.

Oscillator

Its internal free running oscillator sets the PWM frequency

at 600kHz for the SC4519H 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 700kHz 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 SC4519H. As

V increases during power up, the internal circuit senses

VIN and keeps the power transistor off until V reaches

4I.N4V.

IN

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 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 SC4519H which reduces the input

supply current to less than 150µA. During the shut down

mode, the switch is turned off. The SC4519H is turned

on if the EN pin is pulled high.

 2007 Semtech Corp.

7

www.semtech.com

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