IP1206PBF datasheet(13/30 Pages) IRF

13 / 30 page

2/26/2008

www.irf.com

iP1206PbF

Soft-Start (cont.)

During power up, the converter output starts at

zero and thus the voltage at Fb is about 0V. A

current (64µA) injects into the Fb pin and

generates a voltage about 1.6V (64µAx25K)

across the negative input of error amplifier, see

Fig. 10.

The magnitude of this current is inversely

proportional to the voltage at the soft-start pin.

The 28µA current source starts to charge up the

external capacitor. In the mean time, the soft-

start voltage ramps up, the current flowing into

Fb pin starts to decrease linearly and so does the

voltage at negative input of error amplifier.

When the soft-start capacitor is around 1V, the

voltage at the negative input of the error amplifier

is approximately 0.8V.

As the soft-start capacitor voltage charges up,

the current flowing into the Fb pin keeps

decreasing.

The feedback voltage increases linearly as the

injecting current decreases. The injecting current

drops to zero when soft-start voltage is around

1.8V and the output voltage goes into steady

state. Fig. 11 shows the theoretical operational

waveforms during soft-start.

The output start-up time is the time period when

soft-start capacitor voltage increases from 1V to

2V. The start-up time will be dependent on the

size of the external soft-start capacitor. The start-

up time can be estimated by:

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

(nF) can be estimated as:

For normal start up the Seq pin should be pulled

high (usually can be connected to Vout3).

Fig. 10: Soft-Start circuit for iP1206

Fig. 11: Theoretical operation waveforms

during soft-start

)

(

start

)

(

)

(

)

(

≅

μA

start

−

∗

Soft-Start

Voltage

Voltage at negative input

of Error Amp

Voltage at Fb pin

Current flowing

into Fb pin

64uA

0uA

0.8V

≅1.6V

0.8V

≅1.8V

≅1V

Output of POR

E/A2

VP2

Fb2

SS2/SD2

ISS2 = 28uA

Track

POR

OCP2

Ihiccup2 = 3uA

64uA

E/A1

VP1

Fb1

SS1/SD1

ISS1 = 28uA

POR

OCP1

Ihiccup1 = 3uA

64uA

Seq

IP1206PBF Datasheet(PDF) 13 Page - International Rectifier

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