PCDC1008-R215EMO datasheet(20/51 Pages) IRF

20 / 51 page

IR3447

www.irf.com

July 17, 2013

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

IR3447 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

IR3447 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

the sampled current is higher than the OC Limit, an

OC event is detected.

When an Over Current event is detected, the

converter enters hiccup mode.

Hiccup mode is

performed by latching the OC signal and pulling the

Intl_SS signal to ground for 20.48 mS (typ.). OC

signal clears after the completion of hiccup mode and

the converter attempts to return to the nominal output

voltage using a soft start sequence. The converter will

repeat hiccup mode and attempt to recover until the

overload or short circuit condition is removed.

Because the IR3447 uses valley current sensing, the

actual DC output current limit will be greater than OC

limit. The DC output current is approximately half of

peak to peak inductor ripple current above selected

OC limit. OC Limit, inductor value, input voltage,

output voltage and switching frequency are used to

calculate the DC output current limit for the converter.

Equation (2) to determine the approximate DC output

current limit.

LIMIT

OCP







(2)

IOCP

= DC current limit hiccup point

ILIMIT

= Current Limit Valley Point

∆i

= Inductor ripple current

Figure 9: Timing Diagram for Current Limit Hiccup

THERMAL SHUTDOWN

Temperature sensing is provided inside IR3447. The

trip threshold is typically 145

oC. When trip threshold is

exceeded, thermal shutdown turns off both MOSFETs

and resets the internal soft start.

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