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www.semtech.com

POWER MANAGEMENT

SC1211VX

VPN and Vout. During Q1 turn-on, Qcst turns on as well.

The voltage drop across Q1 and Lo charges Ccs. During

Q2 turn-on, Qcsb turns on as well. The voltage drop across

Q2 and Lo discharges Ccs. Both voltage drops are pro-

portional to the inductor current and a resistance equal

to FET’s Rdson plus ESR of the inductor. If the time con-

stant Rcs x Ccs is close to the Lo/Ro of the inductor,

where Ro is given by

)

1

(

*

*

_

_

D

R

D

R

R

Ro

ls

dson

hs

dson

inductor

-

+

+

=

the signal developed across Ccs will be proportional to

the inductor current, where Ro is the equivalent current

sensing resistance. In the above equation, Rinductor is

ESR of the inductor, Rdson_hs and Rdson_ls are the top

and bottom FET’s Rdson, and D is the duty cycle of the

converter.

Since a perfect timing match down to the nanosecond is

impossible, the VPN totem pole is held in tri-state during

the communtations of DRN in the SC1211VX. This avoids

errors and offset on the current detection which can be

significant since the timing mismatch is multiplied by the

input voltage. An optional capacitor between VPN and

DRN allows these two nodes to be AC coupled during the

tri-state window, hence yields a perfect timing match.

Refer to Semtech SC2643VX Combi-Sense® Current

Mode Controller about the details of the Combi-Sense®

technique.

Thermal Shut Down

Thermal Shut Down

Thermal Shut Down

Thermal Shut Down

Thermal Shut Down

The SC1211VX will shut down by pulling both driver out-

COMPONENT SELECTION

COMPONENT SELECTION

COMPONENT SELECTION

COMPONENT SELECTION

COMPONENT SELECTION

Switching F

Switching F

Switching F

Switching F

Switching Freq

req

req

req

requency

uency

uency

uency

uency, Induct

, Induct

, Induct

, Induct

, Inductor and MOSFET

or and MOSFET

or and MOSFET

or and MOSFET

or and MOSFETsssss

The SC1211VX is capable of providing up to 3.5A peak

drive current, and operating up to 1.5MHz PWM frequency

without causing thermal stress on the driver. The selec-

tion of switching frequency, together with inductor and

FETs is a trade-off between the cost, size, and thermal

management of a multi-phase voltage regulator. In mod-

ern microprocessor applications, these parameters could

Applications Information (Cont.)

be in the range of:

Switching Frequency 100kHz to 500kHz per phase

Inductor Value

0.2uH to 2uH

FETs

20nC to 100nC total gate charge

Bootstrap Circuit

Bootstrap Circuit

Bootstrap Circuit

Bootstrap Circuit

Bootstrap Circuit

The SC1211VX uses an external bootstrap circuit to pro-

vide a voltage for the top FET drive. This voltage, refer-

ring to the Phase Node, is held up by a bootstrap capaci-

tor. The capacitor value can be calculated based on the

voltage ripple on the capacitor,

Typically, it is recommended to use a 1uF ceramic ca-

pacitor with 25V rating and a commonly available diode

IN4148 for the bootstrap circuit. In addition, a small re-

sistor (one ohm) has to be added in between DRN of the

SC1211VX and the Phase Node. The resistor is used to

allievate the stress of the SC1211VX from exposing to

the negative spike at the Phase node. A negative spike

could occur at the Phase Node during the top FET turn-

off due to parasitic inductance in the switching loop. The

spike could be minimized with a careful PCB layout. In

those applications with TO-220 package FETs, it is rec-

ommended to use a clamping diode on the DRN pin to

mitigate the impact of the excessive phase node nega-

tive spike.

Filters for Supply Power

Filters for Supply Power

Filters for Supply Power

Filters for Supply Power

Filters for Supply Power

For VCC pin of the SC1211VX, it is recommended to use

a 1uF to 4.7uF, 25V rating ceramic capacitor for

decoupling.

LA

LA

LA

LA

LAYYYYYOUT GUIDELINES

OUT GUIDELINES

OUT GUIDELINES

OUT GUIDELINES

OUT GUIDELINES

The switching regulator is a high di/dt power circuit. Its

Printed Circuit Board (PCB) layout is critical. A good lay-

out can achieve an optimum circuit performance while

minimized the component stress, resulting in better sys-

tem reliability. For a multi-phase voltage regulator, the

SC1211VX driver, FETs, inductor, and supply decoupling

capacitors in each phase have to be considered as a