IP1202PBF datasheet(17/29 Pages) IRF

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iP1202PbF

For better efficiency and low input ripple, select low

ESR ceramic capacitors. The amount of the capaci-

tors is determined based on the r.m.s. rating. In the

above example, a total of 4 x 22µF, 2A capacitors will

be required to support the input r.m.s. current, includ-

ing derating (see the parts list in the reference de-

sign section of this datasheet).

The 180° out of phase operation of the iP1202PbF

provides reduced voltage ripple at the input of the

device. This reduction in ripple requires less input

bypass capacitance.

For single output configuration and a duty cycle

greater than 0.5, select the input capacitors accord-

ing to equation (4) :

D is the duty cycle and is expressed as:

D = V

OUT / VIN.

Output Capacitor CO Selection

Selection of the output capacitors depends on two

factors:

a. Low effective ESR for ripple and load transient re-

quirements

To support the load transients and to stay within a

specified voltage dip ∆V due to the transients, e.s.r.

selection should satisfy equation (5):

esr ≤ ∆V / ILoadmax

(5)

Where,

Loadmax is the maximum load current.

If output voltage ripple is required to be maintained

at specified levels then, the expression in equation

(6) should be used to select the output capacitors.

esr ≤ Vp-p / Iripple

(6)

Where,

p-p is the single phase peak to peak output voltage

ripple.

ripple is the inductor current peak-to peak ripple.

In addition, the voltage ripple caused by the output

capacitor needs to be significantly smaller than the

ripple caused by the ESR of the capacitor. Use equa-

tion (7) to satisfy this requirement.

If the inductor current ripple I

ripple is 30% of IOUT1, the

50mV peak to peak output voltage ripple requirement

will be met if the total e.s.r. of the output capacitors is

less than 11mΩ. This will require 2 x 470µF POSCAP

capacitors (See the parts list in the reference design

section of this datasheet). Additional ceramic capaci-

tors can be added in parallel to further reduce the

e.s.r. Care should be given to properly compensate

the control loop for low output capacitor e.s.r. values.

When selecting output capacitors, it is important to

consider the overshoot performance of the power sup-

ply. If the amount of capacitance is not adequate, then,

when unloading the output, the magnitude of the over-

shoot due to stored inductor energy, and depending

on the speed of the response of the control loop, can

exceed the overvoltage trip threshold of the

iP1202PbF and can cause undesirable shutdown of

the output. The magnitude of the overshoot should

be kept below 1.125V

OUT . To prevent the overshoot

from tripping the output a delay can be added by in-

stalling capacitor C26 as shown in Fig.17.

In paralleled single output configuration, due to 180°

phase shift, the peak to peak output voltage ripple

will be reduced because of doubling of the ripple fre-

quency. Also, the resulting ripple current in the out-

put capacitors will be smaller than the ripple current

of each channel. There is some cancellation effect of

these current, the magnitude of which depends on

the duty cycle.

b. Stability

The value of the output capacitor e.s.r. zero frequency

esr plays a major role in determining stability. fesr is

calculated by the expression in equation (8).

esr = 1 / (2 π x Resr x CO)

(8)

Details on how to consider this parameter to design

for stability will be outlined in the control loop com-

pensation section of this datasheet.

Inductor LO Selection

Inductor selection is based on trade-offs between size

and efficiency. Low inductor values result in smaller

sizes, but can cause large ripple currents and lower

efficiency. Low inductor values also benefit the tran-

sient performance.

esr

⋅

(7)

(4)

()

)(

(

−

LOAD

RMS

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