SC416 datasheet(18/34 Pages) SEMTECH

18 / 34 page

SC416

Applications Information (continued)

Figure 11

Note that R3/R4 must never exceed the FB resistor ra-

tio for VOUT1. If this were the case, SS2 cannot exceed

750mV, and the FB2 comparator would never switch

over to the internal 750mV reference: VOUT2 would not

complete its startup cycle and would not reach the nomi-

nal regulation point. To prevent this it is recommended

to set R3/R4 such that SS2 is greater than 850mV when

VOUT1 is at the nominal point.

MOSFET Gate Drivers

The DH and DL drivers are optimized to drive moderate

high-side and larger low-side power MOSFETs. An adap-

tive dead-time circuit monitors the DL output and pre-

vents the high-side MOSFET from turning on until DL is

fully off; another circuit monitors the DH output and pre-

vents the low-side MOSFET from turning on until DH is

fully off.

Note: be sure there is low resistance and low inductance

between the DH and DL outputs to the gate of each

MOSFET.

SmartDriveTM

Each side uses Semtech’s proprietary SmartDrive to re-

duce switching noise. The DH drivers will turn on the

high-side MOSFET at a lower rate initially, allowing a soft-

er, smooth turn-off of the low-side diode. Once the diode

is off, the SmartDrive circuit automatically drives the high-

side MOSFET on at a rapid rate. This technique reduces

switching less while maintaining high efficiency, and also

avoids the need for snubbers or series resistors in the gate

drive.

Figure 12

Design Procedure

Prior to designing a switch mode supply, the input volt-

age, load current, and switching frequency must be

specified. The maximum input voltage (VINMAX) is de-

termined by the highest AC adaptor voltage, and the

minimum input voltage (VINMIN) is determined by the

lowest supply voltage after accounting for voltage drops

due to connectors, fuses, and switches.

In general, four parameters are needed to define the de-

sign:

Nominal output voltage (VOUT)

Static or DC output tolerance

Transient response

Maximum load current (IOUT)

VOUT1

TSTART

300mV

EN1

EN2

SS1

750mV

Rapid

discharge of

SS capacitor

+5V

VOUT2

R3/R4 > R1/R2

R3/R4 < R1/R2

TSD_RAMP

EN1

EN2

SS1

SC416

CSS

SS2

VOUT1

VOUT2

FB2

SC416 Datasheet(PDF) 18 Page - Semtech Corporation

SC416 Datasheet(HTML) 18 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Datasheet Download

Link URL