IR3840WM datasheet(14/34 Pages) IRF

14 / 34 page

Rev 12.0

PD-97657

IR3840WMPbF

An internal current source sources current (I

OCSet

) out of the OCSet pin. This current is a function

of the switching frequency and hence, of R

Shutdown

The IR3840W can be shutdown by pulling the

Enable pin below its 1 V threshold. This will tri-

state both, the high side driver as well as the low

side driver. Alternatively, the output

can be

shutdown by pulling the soft-start pin below 0.3V.

Normal operation is resumed by cycling the

voltage at the Soft Start pin.

Over-Current Protection

The over current protection is performed by

sensing current through the R

DS(on) of low side

MOSFET. This method enhances the converter’s

efficiency and reduces cost by eliminating a

current sense resistor. As shown in figure 7, an

external resistor (R

OCSet) is connected between

OCSet pin and the switch node (SW) which sets

the current limit set point.

Table 1. shows I

OCSet at different switching

frequencies.

The

internal

current

source

develops a voltage across R

OCSet. When the low

side MOSFET is turned on, the inductor current

flows through the Q2 and results in a voltage at

OCSet which is given by:

An over current is detected if the OCSet pin goes

below ground. Hence, at the current limit

threshold, V

OCset=0. Then, for a current limit

setting I

Limit,ROCSet is calculated as follows:

Operating Frequency

The switching frequency can be programmed

between 250kHz – 1500kHz by connecting an

external resistor from R

t pin to Gnd. Table 1

tabulates the oscillator frequency versus R

Fig. 7. Connection of over current sensing resistor

overcurrent

detection

trips

the

OCP

comparator, latches OCP signal and cycles the

soft start function in hiccup mode.

The hiccup is performed by shorting the soft-start

capacitor to ground and counting the number of

switching cycles. The Soft Start pin is held low

until 4096 cycles have been completed. The

OCP signal resets and the converter recovers.

After every soft start cycle, the converter stays in

this mode until the overload or short circuit is

removed.

)

(on

OCSet

.(3)

..........

)

(

)

(

∗

−

∗

OCSet

Limit

OCSet

(4)

....

..........

)

(

)

.....(

..........

)

μA

(

OCSet

1400

Table 1. Switching Frequency and I

OCSet vs.

External Resistor (R

The OCP circuit starts sampling current typically

160 ns after the low gate drive rises to about 3V.

This delay functions to filter out switching noise.

143.4

1400

9.76

150.3

1500

9.31

110.2

1100

12.7

121.7

1200

11.5

130.8

1300

10.7

97.9

1000

14.3

88.6

900

15.8

78.6

800

17.8

68.2

700

20.5

59.07

600

23.7

48.7

500

28.7

39.2

400

35.7

29.4

300

47.5

ocset (μA)

s (kHz)

t (kΩ)

143.4

1400

9.76

150.3

1500

9.31

110.2

1100

12.7

121.7

1200

11.5

130.8

1300

10.7

97.9

1000

14.3

88.6

900

15.8

78.6

800

17.8

68.2

700

20.5

59.07

600

23.7

48.7

500

28.7

39.2

400

35.7

29.4

300

47.5

ocset (μA)

s (kHz)

t (kΩ)

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