SC2677B datasheet(8/19 Pages) SEMTECH

8 / 19 page

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POWER MANAGEMENT

SC2677B

OverCurrent Protection

Current-sense amplifiers sense the inductor DCR, and

compare with an internal OCP reference. When

overcurrent is detected, the current-sense amplifier will

trip the peak current limit on cycle-by-cycle basis. If the

overcurrent condition sustains, and the output voltage

drops below 75% of its nominal voltage level, the PWM

will be disabled and the power supply will be latched off

with a short amount of delay. The latch can be reset by

power cycling.

(R13+R19) vs.Oscillator Frequency

300

400

500

600

700

800

900

1000

1012

1416

1820

(R13+R19) (kohm)

Vphasing vs Phase Shift

100

120

140

160

180

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

Vphasing (V)

Controller Power Dissipation

Controller power dissipation is generated by the

following parameter; switching frequency, total gate

charge of all selected MOSFETs and supply voltage.

P = Vin * (I

CC + QGT* FSW)

GT = QG * N

Where

Vin : Supply voltage for controller and driving

MOSFET.

Layout Guidelines

Power and signal traces must be kept separate for noise

considerations. Feedback, current-sense traces and ana-

log ground should not cross any traces or planes carrying

high-switching currents, such as in the input loop or the

phase node.

The input loop, consisting of the input capacitors and

both MOSFETs must be kept as small as possible. Since

all of the high switching currents occur in the input loop,

the enclosed loop area must be kept small to minimize

inductance and radiated and conducted noise emissions.

This example demonstrates the procedure introduced

above.

Vin =12V

Fsw =250KHz

N =4(number of MOSFET)

Then

GT = 108nC

= 27nC (per MOSFET)

It is recommended that the below figure be studied to

ensure SC2677B is used under safe operating

conditions.

: Supply current for controller.

GT : Total gate charge of all selected MOSFETs.

: Total gate charge of per selected MOSFETs.

SW : Switching frequency.

N : Number of MOSFET.

QGT limitation (with loading)

100

140

180

220

260

300

340

380

420

460

500

540

580

620

150

200

250

300

350

400

450

500

Fsw(KHz)

5Vin

8.5Vin

12Vin

SOA

Application Information(Cont.)

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