IP1203 datasheet(12/23 Pages) IRF

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iP1203

Only a few external components are required to com-

plete a dual output synchronous buck power supply

using iP1203. The following procedure will guide the

designer through the design and selection process

of these external components.

A typical application for the iP1203 is:

IN = 12V, VOUT = 1.5V, IOUT = 15A, fsw = 300kHz, Vp-p =

= 50mV

Setting the Output Voltage

The output voltage of the iP1203 is set by the 0.8V

reference V

REF and external voltage dividers.

Fig. 14: Typical Scheme for Output Voltage Setting

OUT is set according to equation (1):

OUT = VREF x (1 + R2 /R5 ) (see Fig. 14)

(1)

Setting R

2 to 1K, VOUT to 1.5V and VREF to 0.8V, will

result in R

5= 1.14 Kohms. Final values can be se-

lected according to the desired accuracy of the out-

put.

To set the output voltage for Type III compensation,

refer to equation (24) in Type III compensation sec-

tion.

iP1203 Design Procedure

Setting the Overvoltage Trip

The output of the iP1203 will shut down if it experi-

ences a voltage in the range of 115% of V

OUT. The

overvoltage sense pin FBs is connected to the out-

put through voltage dividers, R26 and R27 (Fig. 14),

and the trip setpoint is programmed according to

equation (1). A separate overvoltage sense pin FBs

is provided to protect the power supply output if for

some reason the main feedback loop is lost (for in-

stance, loss of feedback resistors). An optional 100pF

capacitor (C14) is used for delay and filtering.

If this redundancy is not required and if a Type II con-

trol loop compensation scheme is utilized, FBs pin

can be connected to FB.

Selecting the Soft-Start Capacitor

The soft start capacitor C

ss is selected according to

equation (2):

ss = 40 x Css

(2)

where,

ss is the output voltage ramp time in milliseconds,

and C

ss is the soft start capacitor in µF.

A 0.1µF capacitor will provide an output voltage ramp-

up time of about 4ms.

Input Capacitor Selection

The switching currents impose RMS current require-

ments on the input capacitors. Equation (3) allows

the selection of the input capacitors.

where, I

out is the output current, and D is the duty

cycle and is expressed as:

D = V

OUT / VIN.

For the above example D= 0.13 and, using equation

(3) the capacitor rms current yields 5.0A.

For Type II compensation,

(3)

)

(

out

RMS

−

R26

R27

iP1203

C14

(Optional)

Vout

FBS

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