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POWER MANAGEMENT

SC1211

Applications Information

THEORY OF OPERATION

put driver designed to drive top and bottom MOSFETs in

a synchronous Buck converter. It features adaptive de-

lay for shoot-through protection and VID-on-Fly opera-

tion; internal LDO for optimum gate drive voltage; and

drivers combined with PWM controller SC2643VX form

a multi-phase voltage regulator for advanced micropro-

cessors. A three-phase voltage regulator with 12V input

60A output is shown in the Typical Application Circuit sec-

tion.

Startup and UVLO

To startup the driver, a supply voltage is applied to VIN

pin of the SC1211. The top and bottom gates are held

low until VIN exceeds UVLO threshold of the driver, typi-

cally 4.0V. Then the top gate remains low and the bot-

tom gate is pulled high to turn on the bottom FET. Once

VIN exceeds UVLO threshold of the PWM controller, typi-

cally 7.5V, the soft-start begins and the PWM signal takes

fully control of the gate transitions.

Gate Transition and Shoot through Protection

Refer to the Timing Diagrams section, the rising edge of

the PWM input initiates the bottom FET turn-off and the

in the SC1211 monitors the bottom gate voltage to drop

expired, the top gate turns on. The delay time is set to

be 20ns typically. This prevents the top FET from turning

on until the bottom FET is off. During the transition, the

inductor current is freewheeling through the body diode

of either bottom FET or top FET, upon the direction of

the inductor current. The phase node could be low

(ground) or high (VIN).

The falling edge of the PWM input controls the top FET

turn-off and the bottom FET turn-on. After a short propa-

As the inductor current is commutated from the top FET

to the body diode of the bottom FET, the phase node

begins to fall. The adaptive circuit in the SC1211 de-

tects the phase node voltage. It holds the bottom FET

off until the phase node voltage has dropped below 1.0V.

This prevents the top and bottom FETs from conducting

simultaneously or shoot-through.

VID-on-Fly Operation

Certain new processors have required to changing the

VID dynamically during the operation, or refered as VID-

on-Fly operation. A VID-on-Fly can occur under light load

or heavy load conditions. At light load, it could force the

converter to sink current. Upon turn-off of the top FET,

the reversed inductor current has to be freewheeling

through the body diode of the top FET instead of the

bottom FET. As a result, the phase node voltage remains

high. The SC1211 incorporates the ability by pulling the

bottom gate to high internally, which over rides the adap-

tive circuit and turns the bottom FET on. The delay time

from the PWM falling egde to the bottom gate turn-on is

set at 200ns typically.

Peak-Current-Mode control is widely employed in multi-

phase voltage regulators. It features phase current bal-

ance, fast transient response, and over current protec-

tion, etc. These are essential to low-voltage high-cur-

rent regulators designed for advanced microprocessors.

Usually, a costly current sensing resistor is required to

obtain the output inductor current information for the

tured by the SC1211 is an approach to sense inductor

current without using sensing resistor.

technique. An internal totem pole (Qcst, Qcsb) gener-

ates a VPN (Virtual Phase Node) signal. This VPN follows

Inductor Current Signal

Vout

Qcst

VIN

Rcs

C

SC1211

PGND

VIN

VPN

Lo

Q2

Qcsb

DRN

Q1

+

Co

Ccs