IR3621 datasheet(9/29 Pages) IRF

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IR3621&(PbF)

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RSET

IR3621

OCSet

IOCSET

VOUT

Hiccup

Control

Figure 9 - Diagram of the over current sensing.

VOCSET = IOCSET

×RSET-RDS(ON)×iL

---(2)

VOCSET = IOCSET

×RSET - RDS(ON)×IL = 0

The critical inductor current can be calculated by set-

ting:

ISET = IL(CRITICAL)

---(3)

RSET

×IOCSET

RDS(ON)

The internal current source develops a voltage across

RSET. When the low side switch is turned on, the induc-

tor current flows through the Q2 and results a voltage

which is given by:

The value of RSET should be checked in an actual

circuit to ensure that the Over Current Protection

circuit activates as expected. The IR3621 current limit

is designed primarily as disaster preventing, "no blow

up" circuit, and is not useful as a precision current

regulator.

In two independent mode, the output of each channel

is protected independently which means if one output

is under overload or short circuit condition, the other

output will remain functional. The OCP set limit can be

programmed to different levels by using the external

resistors. This is valid for both hiccup mode and latch

up mode.

In 2-phase configuration, the OCP's output depends on

any one channel, which means as soon as one

channel goes to overload or short circuit condition the

output will enter either hiccup or latch-up, dependes on

status of Hiccup pin.

For a given start up time, the soft-start capacitor can be

calculated by:

CSS

≅ 28µA×TSTART/0.8V

Over-Current Protection

The IR3621 can provide two different schemes for Over-

Current Protection (OCP). When the Hiccup pin is pulled

high, the OCP will operate in hiccup mode. In this mode,

during overload or short circuit, the outputs enter hiccup

mode and stay in that mode until the overload or short

circuit is removed. The converter will automatically re-

cover.

When the Hiccup pin is pulled low, the OCP scheme

will be changed to the latch up type, in this mode the

converter will be turned off during Overcurrent or short

circuit. The power needs to be recycled for normal

operation.

Each phase has its own independent OCP circuitry.

The OCP is performed by sensing current through the

RDS(ON) of low side MOSFET. As shown in Figure 9, an

external resistor (RSET) is connected between OCSet pin

and the drain of low side MOSFET (Q2) which sets the

current limit set point.

The soft-start is part of the Over Current Protection

scheme, during the overload or short circuit condition

the external soft start capacitors will be charged and

discharged in certain slope rate to achieve the hiccup

mode function.

Out-of-Phase Operation

The IR3621 drives its two output stages 180

! out-of-phase.

In 2-phase configuration, the two inductor ripple currents

cancel each other and result in a reduction of the output

current ripple and yield a smaller output capacitor for the

same ripple voltage requirement.

In single input voltage applications, the input ripple current

reduces. This results in much smaller input capacitor's

RMS current and reduces the input capacitor quantity.

SS1 / SD 20

28uA

3uA

Hiccup

If using one soft start capacitor in dual configuration for a

precise power up the OCP needs to be set to latch mode.

Figure 8 - 3uA current source for discharging soft

start-capacitor during Hiccup mode

Pre-bias Startup

The IR3621 allows pre-bias startup without discharging

the output capacitors. The output starts in asynchro-

nous fashion and keeps the synchronous MOSFET off

until the first gate signal for control MOSFET is gener-

ated.

IR3621 Datasheet(PDF) 9 Page - International Rectifier

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