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POWER MANAGEMENT

SC2621A

THEOR

THEOR

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THEORY OF OPERA

Y OF OPERA

Y OF OPERA

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Y OF OPERATION

TION

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The SC2621A integrates a high-speed, voltage mode

PWM controller with a linear controller into a single pack-

age. It is designed to control two independent output

voltages for high performance graphic card applications.

As shown in the block diagram of the SC2621A, the volt-

age-mode PWM controller consists of an error amplifier,

a 450kHz 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 linear controller is an error ampli-

fier. It provides the gate drive and output voltage control

for a linear regulator. Both PWM controller and linear con-

troller work with soft-start and fault monitoring circuitry

to meet application requirement.

UVLO, Start-up and Shutdown

UVLO, Start-up and Shutdown

UVLO, Start-up and Shutdown

UVLO, Start-up and Shutdown

UVLO, Start-up and Shutdown

To initiate the SC2621A, a supply voltage is applied to

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

low until Vcc voltage exceed 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 linear controller is enabled and the

LDO output is turned on. Meanwhile, the top and bottom

gates of PWM controller begin to switch. The switching

regulator output is slowly ramping up for a soft turn-on.

If the supply voltages at Vcc pin falls below UVLO thresh-

old during a normal operation, the SS capacitor begins

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

PWM controller controls the switching regulator output

to ramp down slowly for a soft turn-off. Meanwhile, the

linear controller is disabled and LDO output is turned 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 SC2621A 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

SC2621A will discharge the internal SS capacitor and

shutdown both outputs. After waiting for around 5 milli-

seconds, the SC2621A begins to charge 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.

SC2621A

GND

OCS

DRV

VCC

10

2

12

3.3k

Rset

12V

8

SC2621A

GND

OCS

DRV

VCC

10

2

12

3.3k

Rset

12V

8

Fig. 1. Programming load trip point

150

175

200

225

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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 SC2621A is fixed at around

Applications Information