SC415_08 datasheet(14/30 Pages) SEMTECH

14 / 30 page

SC415

The 5μA current into the capacitor creates a linear voltage

ramp. The internal FB comparator tracks this SS ramp,

which forces VOUT to also track the SS ramp. The time

in msec needed for the SS ramp to reach the 750mV

reference is:

START

= Css × 150

START

in μsec, Css in nF)

At the end of this time, the SS pin has reached 750mV

and the output voltage is at its nominal value. The FB

comparator then switches over to the internal 750mV REF,

and the SS pin is thereafter ignored.

The 5μA current source remains on, so the Css capacitor

continues to charge up to +5V.

The startup waveforms when using a capacitor for the

voltage ramp are shown in Figure 7.

Figure 7

Applications Information (continued)

Shutdown

When the EN pin is set low, the SS pin is discharged and

the output DH/DL drivers are pulled low. There is a small

delay between EN going low and DH/DL turning off. The

shutdown delay is given by the equation:

SD_DELAY

= Css × 19

SD_DELAY

in usec, Css in nF)

With DH/DL set low, there is no switching, and the output

capacitor is discharged into the VOUT pin discharge

resistance of 16 ohms. This provides a soft-discharge with

no need for a clamp diode across the output capacitor.

When DH/DL stop switching, then that side is placed into

an inactive, low-power state.

MOSFET Gate Drivers

The DH and DL drivers are optimized to drive moderate

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

dead-time circuit monitors the DL output and prevents

the high-side MOSFET from turning on until DL is fully off;

another circuit monitors the DH output and prevents 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

reduce switching noise. The DH drivers will turn on the

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

softer, 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.

VOUT

750mV

SS linear ramp - 5uA current

source into CSS

CSS

SC415

TSTART

SC415 数据表(PDF) 14 Page - Semtech Corporation

SC415 数据表(HTML) 14 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

数据表下载