PCMB065T-2R2MS datasheet(17/47 Pages) IRF

17 / 47 page

Dual output, 4A/Phase, Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

IR3891

www.irf.com

May 08, 2013

Vcc

PVin=Vin

Intl_SS 1/2

EN1/2

> 1.2V

Vo 1/2

Figure 5: Recommended startup for Normal operation

Vcc

PVin=Vin

Intl_SS 2

EN2

> 1.2V

Intl_SS 1

EN1

> 1.2V

Vo1

Vo2

Figure 6: Recommended startup for sequencing

operation (ratiometric or simultaneous)

Figure 5 shows the recommended start-up sequence

for the normal (non-sequencing) operation of IR3891,

when EN pins are used as a logic input. Figure 6

shows the recommended startup sequence for

sequenced operation of IR3891.

PRE-BIAS STARTUP

IR3891 begins each start up by pre-charging the

output to prevent oscillation and disturbances to the

output voltage.

The buck converter starts in an

asynchronous fashion and keeps the synchronous

MOSFET (Sync FET) off until the first gate signal for

control MOSFET (Ctrl FET) is generated. Figure 7

shows a typical pre-bias sequence.

The sync FET

always starts with a narrow pulse width (12.5% of the

switching period). The pulse width increase after 16

pulses by 12.5% until the output reaches steady state

value. There are 16 pulses for each step. Figure 8

shows the series of 16 x 8 startup pulses.

Figure 7: Pre-bias Start Up

...

HDRv

...

End of

LDRv

12.5%

25%

87.5%

...

Figure 8: Pre-bias startup pulses

SOFT-START

IR3891 has an internal digital soft-start to control the

output voltage rise and to limit the current surge

during start-up. To ensure the correct start-up, the

soft-start sequence initiates when the EN and VCC

rise above their UVLO thresholds and generates

Power On Ready (POR) signal. The internal soft-start

rises with the typical rate of 0.2mV/µS from 0V to

1.5V. Figure 9 shows the waveforms during soft-start.

The normal Vout start-up time is fixed, and is equal to:





Tstart









(1)

During the soft-start the over-current protection (OCP)

and the over-voltage protection (OVP) is enabled to

protect the device from short circuit or over voltage

events.

PCMB065T-2R2MS Datasheet(PDF) 17 Page - International Rectifier

PCMB065T-2R2MS Datasheet(HTML) 17 Page - International Rectifier

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