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IR3628MPBF

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER

International Rectifier 

Over-Current Protection

The over current protection is performed by

sensing current through the RDS(on) of low side

MOSFET. This method enhances the converter’s

efficiency and reduce cost by eliminating a

current sense resistor. As shown in figure 7, 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 internal current source develops a voltage

across RSET. When the low side MOSFET is

turned on, the inductor current flows through the

Q2 and results a voltage which is given by:

VOCSet = (IOCSet ∗ R ) OCSet − (RDS(on) ∗ IL )

--(2 )

IOCSET

IRIR33662248

Hiccup

Control

OCSet RSET

Q1

L1

Q2

VOUT

IR3628MPbF

20uA

28uA

OCP

SS1 / SD 20

3uA

Fig. 8: 3uA current source for discharging

soft-start capacitor during hiccup

The OCP circuit starts sampling current when the

low gate drive is about 3V. The OCSet pin is

internally clamped (~1.5V) during on time of high

side MOSFET including deadtime to prevent

false trigging, figure 9 shows the OCSet pin

during one switching cycle. As it is shown there

is about 150ns delay to mask the deadtime, since

this node contains switching noises, this delay

also functions as a filter.

Fig. 7: Connection of over current sensing resistor

The critical inductor current can be calculated by

setting:

VOCSet = (IOCSet ∗ R ) OCSet − (RDS(on) ∗ IL ) = 0

I = I = R R ∗I SET

L(critical)

OCSet OCSet

DS(on)

--(3 )

An over current is detected if the OCSet pin goes

below ground. This trips the OCP comparator

and cycles the soft start function in hiccup mode.

The hiccup is performed by charging and

discharging the soft-start capacitor in certain

slope rate. As shown in figure 8 a 3uA current

source is used to discharge the soft-start

capacitor.

The OCP comparator resets after every soft start

cycles, the converter stays in this mode until the

overload or short circuit is removed. The

converter will automatically recover.

Deadtime

IOCSet*ROCSet

Blanking time

Clamp voltage

Fig. 9: OCset pin during normal condition

Ch1: Inductor point, Ch2:Ldrv, Ch3:OCSet

The value of RSET should be checked in an actual

circuit to ensure that the over current protection

circuit activates as expected. The IR3628 current

limit is designed primarily as disaster preventing,

and doesn't operate as a precision current

regulator.

08/10/2007

10

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