IP1206PBF datasheet(16/30 Pages) IRF

16 / 30 page

2/26/2008

www.irf.com

iP1206PbF

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 reduce cost by eliminating a

current sense resistor. As shown in Fig. 16, an

external resistor (ROCSET) is connected between

OCSet pin and the drain of low side MOSFET

(Q2) which sets the current limit set point.

When the overcurrent trip threshold is reached,

the

power

supply output

shuts

down and

attempts to restart, entering into the hiccup

mode. The time duration between the shutdown

of the output and the restart is determined by the

time it takes to discharge the soft start capacitor.

Fig. 16: Connection of over current sensing resistor

Fig. 17: 3uA current source for discharging

soft-start capacitor during hiccup

The duty cycle of the hiccup process is typically

5%. The hiccup is performed by charging and

discharging the soft-start capacitor at a certain

slope rate. As shown in Fig. 17 a 3µA current

source is used to discharge the soft-start

capacitor. The OCP comparator resets after

every soft start cycle, the converter stays in this

mode until the overload or short circuit is

removed. Once the condition is removed the

converter will automatically recover.

Refer to

Fig. 24 for ROCSET selection.

ROCSET

iP1206

OC1

IOCSET

VOUT

Hiccup

Control

VSW

PGND

SS1 / SD 20

28uA

3uA

OCP

22uA

Pre-Bias

iP1206 is able to start up into a pre-charged

output,

which

prevents

oscillation

and

disturbances of the output voltage.

The output starts in asynchronous fashion and

keeps the synchronous MOSFET off until the first

gate signal for control MOSFET is generated.

The figure below shows a typical Pre-Bias

condition at start up.

Depending on the system configuration, specific

amount of output capacitors may be required to

prevent discharging the output voltage.

Over Voltage Protection

Over-voltage is sensed through two dedicated

sense pins FBS1, FBS2. A separate OVP circuit

is provided for each channel.

The OVP threshold is user programmable and

can be set by two external resistors. Upon over-

voltage condition of either one of the outputs, the

OVP forces a latched shutdown on the fault

output and pulls low the low side driver.

Reset is performed by recycling the Vcc or

Enable. Overvoltage can be sensed either by

connecting

FB1s

and

FB2s

their

corresponding outputs through separate output

voltage divider resistor networks, or they can be

connected

directly

their

corresponding

feedback pins FB1 and FB2. For Type III

compensation,

FB1s

and

FB2s

should

connected through voltage dividers only.

Power Good

The iP1206 provides two separate open collector

power good signals which report the status of the

outputs. The outputs are sensed through the two

dedicated V

SEN1 and VSEN2 pins.

Once the iP1206 is enabled and the outputs

reach the set value (90% of set value) the power

good signals go open and stay open as long as

the outputs stay within the set values.These pins

are open collector and need to be externally

pulled high.

Time

Volt

Pre-Bias Voltage

(Output Voltage before startup)

IP1206PBF Datasheet(PDF) 16 Page - International Rectifier

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