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SC1211VX

High Speed, Combi-Sense®, Synchronous MOSFET Driver for Mobile Applications

Semtech Corporation 

SC1211VX

POWER MANAGEMENT

Applications Information

THEORY OF OPERATION

VID-on-Fly Operation

The SC1211VX is a high speed, Combi-Sense®, dual out-

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; 5V gate drive voltage; and Virtual Phase Node for

Combi-Sense® solution. These drivers combined with

PWM controller SC2643VX form a multi-phase voltage

regulator for advanced microprocessors. A three-phase

voltage regulator with 19V input 60A output is shown in

the Typical Application Circuit section.

Startup and UVLO

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 SC1211VX incorporates the ability by pulling

the bottom gate to high internally, which over rides the

adaptive 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.

To startup the driver, a supply voltage applied to VCC pin

of the SC1211VX. The top and bottom gates are held

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

cally 4.2V. 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.

Virtual Phase Node for Combi-Sense®

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

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

top FET turn-on. After a short propagation delay (tPDL_BG),

the bottom gate begins to fall (tF_BG). An adaptive circuit

in the SC1211VX monitors the bottom gate voltage to

drop below 1.4V. Then after a preset delay time (tPDH_TG)

is 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

obtain the output inductor current information for the

peak current control. The Combi-Sense® technique fea-

tured by the SC1211VX is an approach to sense induc-

tor current without using sensing resistor.

VIN

VIN

Q1

Qcst

VPN

C DRN

Lo

Vout

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).

Qcsb

PGND

Q2

+ Co

The falling edge of the PWM input controls the top FET

SC1211VX

Rcs

Ccs

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

gation delay (tPDL_TG), the top gate begins to fall (tF_TG).

Inductor C urrent Signal

As the inductor current is commutated from the top FET

to the body diode of the bottom FET, the phase node The above circuit shows the concept of Combi-Sense®

begins to fall. The adaptive circuit in the SC1211VX de- technique. An internal totem pole (Qcst, Qcsb) gener-

tects the phase node voltage. It holds the bottom FET ates a VPN (Virtual Phase Node) signal. This VPN follows

off until the phase node voltage has dropped below 1.0V. the DRN (or the Power Phase Node) with the same tim-

This prevents the top and bottom FETs from conducting ing. A RC network (Rcs and Ccs) is connected between

simultaneously or shoot-through.

 2005 Semtech Corp.

8

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