SC4518HEVB datasheet(7/15 Pages) SEMTECH

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POWER MANAGEMENT

SC4518H

Application Information

General Overview

The SC4518H is a high frequency current mode buck

PWM regulator. It has an internal clock with fixed-

frequency. The SC4518H uses two feedback loops

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

of the internal power switch. The error amplifier functions

like that 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 Over Current Protection

The current sense amplifier in the SC4518H monitors

the switch current during each cycle. Overcurrent

protection (OCP) is triggered when the current limit

exceeds the upper limit of 2A, detected by a voltage on

COMP being 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/

comp.

Once the COMP voltage has fallen below 250mV,

the part enters a normal startup cycle. C

comp 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/C

comp.

Therefore, the retry time for a

sustained overcurrent can be approximately calculated

as:

120uA

comp

retry

•

Figure 1 shows the voltage on COMP during a sustained

overcurrent condition.

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 SC4518H which reduces the input

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

mode, the switch is turned off. The SC4518H is turned

on if the EN pin is pulled high.

Oscillator

Its internal free running oscillator sets the PWM frequency

at 600kHz for the SC4518H 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 SC4518H. 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 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 continuous current mode:

OMAX

)

(

⋅

−

⋅

−

Where:

fs = switching frequency,

Vo = output voltage and

D = duty ratio, VO/VI

= 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 derated according to the

thermal situation of the application.

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