SC1211VX 데이터 시트보기 (PDF) - Semtech Corporation

부품명

상세내역

일치하는 목록

SC1211VX

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

Semtech Corporation 

SC1211VX

POWER MANAGEMENT

Applications Information (Cont.)

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

be in the range of:

Switching Frequency

Inductor Value

FETs

Bootstrap Circuit

100kHz to 500kHz per phase

0.2uH to 2uH

4m-ohm to 20m-ohm Rdson

20nC to 100nC total gate charge

Ro = Rinductor + Rdson_ hs * D + Rdson_ls * (1 - D)

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

The SC1211VX will shut down by pulling both driver out-

puts low if its junction temperature, Tj, exceeds 155°C.

COMPONENT SELECTION

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

total gate charge of the top FET, QTOP, and an allowed

voltage ripple on the capacitor, ∆VBST, in one PWM cycle:

CBST > QTOP/∆VBST

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

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

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

decoupling.

Switching Frequency, Inductor and MOSFETs

LAYOUT GUIDELINES

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

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

 2005 Semtech Corp.

9

www.semtech.com

Share Link: