Datasheet

LM5025

UVLO

PGND

AGND

COMP

OUT_A

OUT_B

VCC

SYNC

REF

TIME

RAMP

CS1

VIN

35V - 78V

OUT

3.3V

UP/DOWN

SYNC

ERROR

AMP and

ISOLATION

CS2

www.ti.com

OUTSR Drive

Figure 4. Typical Forward Converter

7 OUTSR Drive

The LM5027 has a dedicated pin (OUTSR) to drive the synchronous rectifier free wheeling MOSFET

through a drive transformer as shown in Figure 5. When the converter starts-up, the OUTSR drive is held

low and the freewheeling MOSFET is turned-off. As a result, no output current will sink into the converter.

The OUTSR is enabled after the primary side soft-start reaches approximately 4.0V. The OUTSR delay

was added to ensure that the power supply output voltage is up and in regulation prior to the freewheeling

MOSFET being turned-on, refer to Figure 6 and Figure 7. The OUTSR drive is soft-started; a capacitor on

the SSSR pin is released and is charged with a 25 µA current source, slowly increasing the duty cycle of

the freewheeling FET’s duty cycle.

SNVA400B–August 2009–Revised May 2013 AN-1976 LM5027 Evaluation Board

Submit Documentation Feedback