PCDC1008-R215EMO datasheet(19/51 Pages) IRF

19 / 51 page

IR3447

www.irf.com

July 17, 2013

Vcc

PVin=Vin

Intl_SS

> 1.2V

Figure 5: Recommended startup for Normal operation

Figure 5 shows the recommended startup sequence

for the typical operation of IR3447 with Enable used

as logic input.

PRE-BIAS STARTUP

IR3447 is able to start up into pre-charged output,

which prevents oscillation and disturbances of the

output voltage.

The output starts in asynchronous fashion and keeps

the synchronous MOSFET (Sync FET) off until the

first gate signal for control MOSFET (Ctrl FET) is

generated. Figure 6 shows a typical Pre-Bias condition

at start up. The sync FET always starts with a narrow

pulse width (12.5% of a switching period) and

gradually increases its duty cycle with a step of 12.5%

until it reaches the steady state value. The number of

these startup pulses for each step is 16 and it’s

internally programmed. Figure 7 shows the series of

16x8 startup pulses.

[V]

[Time]

Pre-Bias

Voltage

Figure 6: Pre-Bias startup

...

HDRv

...

End of

LDRv

12.5%

25%

87.5%

...

Figure 7: Pre-Bias startup pulses

SOFT-START

IR3447 has an internal digital soft-start to control the

output voltage rise and to limit the current surge at the

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

sequence initiates when the Enable and VCC rise

above their UVLO thresholds and generate the Power

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

(Intl_SS) signal linearly rises with the rate of 0.4mV/µs

from 0V to 1.5V.

Figure 8 shows the waveforms

during soft start. The normal Vout startup time is fixed,

and is equal to:





Tstart









(1)

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

and over-voltage protection (OVP) is enabled to

protect the device for any short circuit or over voltage

condition.

POR

Intl_SS

Vout

0.15V

0.75V

t1 t2

1.5V

3.0V

Figure 8: Theoretical operation waveforms during

soft-start

OPERATING FREQUENCY

The switching frequency can be programmed between

300kHz – 1500kHz by connecting an external resistor

PCDC1008-R215EMO Datasheet(PDF) 19 Page - International Rectifier

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