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 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4614

Applications Information

THEOR

THEOR

THEOR

THEOR

THEORY OF OPERA

Y OF OPERA

Y OF OPERA

Y OF OPERA

Y OF OPERATION

TION

TION

TION

TION

The SC4614 is a high-speed, voltage mode PWM con-

troller that provides the control and protection features

necessary for a synchronous buck converter.

As shown in the block diagram of the SC4614, the volt-

age-mode PWM controller consists of an error amplifier,

a 500kHz ramp generator, a PWM comparator, a RS latch

circuit, and two MOSFET drivers. The buck converter out-

put voltage is fed back to the error amplifier negative

input and is regulated to a reference voltage level. The

error amplifier output is compared with the ramp to gen-

erate a PWM wave, which is amplified and used to drive

the MOSFETs in the buck converter. The PWM wave at

the phase node with the amplitude of Vin is filtered out

to get a DC output. The PWM controller works with soft-

start and fault monitoring circuitry to meet application

requirements.

UVLO, Start Up and Shut Down

UVLO, Start Up and Shut Down

UVLO, Start Up and Shut Down

UVLO, Start Up and Shut Down

UVLO, Start Up and Shut Down

To initiate the SC4614, a supply voltage is applied to the

Vcc pin. The top gate (DH) and bottom gate (DL) are held

low until Vcc voltage exceeds UVLO (Under Voltage Lock

Out) threshold, typically 4.0V. Then the internal Soft-Start

(SS) capacitor begins to charge, the top gate remains

low, and the bottom gate is pulled high to turn on the

bottom MOSFET. When the SS voltage at the capacitor

reaches 0.4V, the top and bottom gates of PWM control-

ler begin to switch. The switching regulator output is slowly

ramping up for a soft turn-on.

If the supply voltages at the Vcc pin falls below UVLO

threshold during a normal operation, the SS capacitor

begins to discharge. When the SS voltage reaches 0.4V,

the PWM controller controls the switching regulator out-

put to ramp down slowly for a soft turn-off.

Hiccup Mode Short Circuit Protection

Hiccup Mode Short Circuit Protection

Hiccup Mode Short Circuit Protection

Hiccup Mode Short Circuit Protection

Hiccup Mode Short Circuit Protection

over current protection. In every switching cycle, after

the bottom MOSFET is on for 150ns, the SC4614 de-

tects the phase node voltage and compares it with an

internal setting voltage. If the phase node is lower than

the setting voltage, an overcurrent condition occurs. The

SC4614 will discharge the internal SS capacitor and shut-

down both outputs. After waiting for around 10 millisec-

onds, the SC4614 begins to charge the SS capacitor

again and initiates a fresh startup. The startup and shut-

down cycle will repeat until the short circuit is removed.

This is called a hiccup mode short circuit protection.

To program a load trip point for short circuit protection, it

is recommended to connect a 3.3k resistor from the OCS

the DRV pin, as shown in Fig. 1.

SC4614

GND

OCS

DRV

VCC

6

2

5

3.3k

Rset

12V

7

SC4614

GND

OCS

DRV

VCC

6

2

5

3.3k

Rset

12V

7

Fig. 1. Programming load trip point

150

175

200

225

250

275

300

325

350

0

100

200

300

400

500

600

Rset (k-ohm)

150

175

200

225

250

275

300

325

350

0

100

200

300

400

500

600

Rset (k-ohm)

node voltage Vpn at the load trip point. This voltage is

the product of the inductor peak current at the load trip

on

_

ds

peak

pn

R

I

V

´

=

The soft start time of the SC4614 is fixed at around

1.5ms. Therefore, the maximum soft start current is de-