LTC3901 datasheet(6/16 Pages) LINER

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LTC3901

3901f

BLOCK DIAGRA

APPLICATIO S I FOR ATIO

Figure 2. Push-Pull Converter Switching Waveforms

SYNC

AND

DRIVER

LOGIC

ISE

S–

S+

SYNC+

SYNC–

TMR

–1.4V

+1.4V

DISABLE

DRIVER

UVLO

ZTMR

0.5 • VCC

ZCSE

11V

SYNC

CSE+

CSE–

ISF

ZCSF

11V

CSF+

CSF–

TIMER

MTMR

180k

45k

TIMER

RESET

PGND

GND GND

PGND

VCC

PVCC

3901 BD

–

10.5mV

DRVA

DRVB

SDRA

SDRB

SYNC

3901 F02

Overview

Push-pull and full bridge converters use power transform-

ers to provide input-to-output isolation and voltage step-

up/down. Diodes are used as a simple solution for second-

ary side rectification. Unfortunately, as output currents

increase, the loss associated with diode forward voltage

drop results in low overall efficiency. The LTC3901 over-

comes this problem by providing control and drive for two

external N-channel synchronous MOSFETs. Synchroniza-

tion to the primary side controller is maintained through a

small signal transformer.

Figure 1 shows a simplified push-pull converter applica-

tion. T1 is the power transformer; MA and MB are the

primary side power transistors driven by the LTC3723

controller’s DRVA and DRVB outputs. The gate drive

transformer T2 is driven by the LTC3723’s SDRA and

SDRB outputs and provides the synchronization signal to

the LTC3901 on the secondary side. When both SDRA and

SDRB are high, there is no voltage across the transformer’s

primary and the LTC3901 SYNC input is approximately 0V.

According to the polarity of the transformer: if SDRA goes

low while SDRB is high, SYNC is positive; if SDRB goes

low while SDRA is high, SYNC is negative. ME and MF are

the secondary side synchronous switches driven by the

LTC3901’s ME and MF output. Inductor L1 and capacitor

COUT form the output filter, providing DC output voltage to

the load. The feedback path from VOUT through the opto-

coupler driver and optocoupler back to the primary side

controller is also shown in Figure 1.

Each full cycle of the push-pull converter consists of four

distinct periods. Figure 2 shows the push-pull converter

waveforms.

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