IP1206PBF datasheet(12/30 Pages) IRF

12 / 30 page

2/26/2008

www.irf.com

iP1206PbF

Current Share Mode

This feature allows the connection of both outputs

together to increase current handling capability of

the converter to support a common load. In current

sharing mode, error amplifier 1 becomes the master

which regulates the common output voltage and the

error amplifier 2 performs the current sharing

function, Fig. 9 shows the configuration of error

amplifiers. See Fig 25a for a typical application.

In this mode iP1206 makes sure the master channel

starts first followed by slave channel to prevent any

glitch during start up. This is done by clamping the

output of slave’s error amplifier until the master

channel generates the first PWM signal.

At no load condition the slave channel may be kept

off depending on the offset of error amplifier.

Lossless Inductor Current Sensing

The iP1206 uses a lossless current sensing

technique for current share purposes. The inductor

current is sensed by connecting a series resistor

and a capacitor network in parallel with the inductor

and measuring the voltage across the capacitor,

this voltage is proportional to the inductor current.

As shown in Fig. 9 the voltage across the inductor’s

DCR can be expressed by:

The voltage across the C

1 can expressed by:

Combining equations (1),(2) and (3) result in the

following expression for V

C1:

Usually the resistor R

1 and C1 are chosen so that

the time constant of R

1 and C1 equals the time

constant of the inductor which is the inductance L

over the inductor’s DCR (R

L1).

If the two time

constants

match,

the

voltage

across

proportional to the current through L

1, and

Fig. 9: Loss Less inductor current sensing

and current sharing

RL1

RL2

VP2

FB2

VOUT

Master Phase

Slave Phase

+ VC1(s) -

VL1 (s) -

Vin

IL1

the sense circuit can be treated as if only a sense

resistor with the value R

L1 was used.

The mismatch of the time constants does not

affect the measurements of inductor DC current,

but affects the AC component of the inductor

current.

Soft-Start

The iP1206 has a programmable soft-start to

control the output voltage rise and limit the inrush

current during start-up. It provides a separate

Soft-start function for each output. This enables

the user to sequence the outputs by controlling

the rise time of each output through the selection

of different value soft-start capacitors.

ensure

correct

start-up,

the

soft-start

sequence initiates when the Vcc and Enable rise

above their threshold and generate the Power On

Reset (POR) signal. Soft-start function operates

by sourcing an internal current to charge an

external capacitor to about 3V. Initially, the soft-

start function clamps the error amplifier’s output

of the PWM converter.

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