SL40307A-R19KHF datasheet(20/53 Pages) IRF

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IR3846

20 www.irf.com

August 01, 2013

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 10 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 10: Theoretical operation waveforms during

soft-start (non tracking / non sequencing)

OPERATING FREQUENCY

The switching frequency can be programmed between

300kHz – 1500kHz by connecting an external resistor

from Rt pin to LGnd. Table 1 tabulates the oscillator

frequency versus Rt.

Table 1: Switching Frequency(Fs) vs. External

Resistor(Rt)

Rt (KΩ)

Freq

(KHz)

80.6

300

60.4

400

48.7

500

39.2

600

700

29.4

800

26.1

900

23.2

1000

1100

19.1

1200

17.4

1300

16.2

1400

1500

SHUTDOWN

IR3846 can be shutdown by pulling the Enable pin

below its 1.0V threshold. During shutdown the high

side and the low side drivers are turned off.

OVER CURRENT PROTECTION

The Over Current (OC) protection is performed by

sensing the inductor current through the RDS(on) of the

Synchronous MOSFET. This method enhances the

converter’s efficiency, reduces cost by eliminating a

current sense resistor and any layout related noise

issues. The Over Current (OC) limit can be set to one

of three possible settings by floating the OCset pin, by

pulling up the OCset pin to VCC, or pulling down the

OCset pin to PGnd. The current limit scheme in the

IR3846 uses an internal temperature compensated

current source to achieve an almost constant OC limit

over temperature.

Over Current Protection circuit senses the inductor

current flowing through the Synchronous MOSFET.

To help minimize false tripping due to noise and

transients, inductor current is sampled for about 30 nS

on the downward inductor current slope approximately

12.5% of the switching period before the inductor

current valley. However, if the Synchronous MOSFET

is on for less than 12.5% of the switching period, the

current is sampled approximately 40nS after the start

of the downward slope of the inductor current. When

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