IP1201 datasheet(18/29 Pages) IRF

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iP1201

RMS = 6 x SQRT (0.75(1 - 0.75)) = 2.6A

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 3 x 100µF, 3.5A capacitors

will be required to support the input r.m.s. current

(see the parts list in the reference design section of

this datasheet).

The 180° out of phase operation of the iP1201 pro-

vides reduced voltage ripple at the input of the de-

vice. This reduction in ripple requires less input by-

pass capacitance. Therefore the input bypass ca-

pacitor selection criteria based on equation (3) pro-

vides a worst case solution for the selected operat-

ing conditions.

Output Capacitor CO Selection

Selection of the output capacitors depends on two

factors:

a. Low effective ESR for ripple and load transient

requirements

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 (4):

esr ≤ ∆V / ILoadmax

(4)

Where,

Loadmax is the maximum load current.

If output voltage ripple is required to be maintained

at specified levels then, the expression in equation

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

esr ≤ Vp-p / Iripple

(5)

Where,

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

ripple.

ripple is the inductor current peak-to peak ripple.

If the inductor current ripple I

ripple is 30% of IOUT1, the

40mV peak to peak output voltage ripple require-

ment will be met if the total e.s.r. of the output capaci-

tors is less than 11mohms. This will require 3 x

470µF POSCAP capacitors (See the parts list in the

reference design section of this datasheet). Addi-

tional ceramic capacitors 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

supply. If the amount of capacitance is not adequate,

then, when unloading the output, the magnitude of

the overshoot due to stored inductor energy, and

depending on the speed of the response of the con-

trol loop, can exceed the overvoltage trip threshold

of the iP1201 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

installing capacitor C26 as shown in Fig.19.

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

frequency. Also, the resulting ripple current in the

output capacitors will be smaller than the ripple cur-

rent of each channel. There is some cancellation

effect of these current, the magnitude of which de-

pends 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 (6).

esr = 1 / (2 π x Resr x CO)

(6)

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 LOSelection

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 ben-

efit the transient performance.

The inductor L

ois selected according to equation (7):

O = Vout x (1 - D) / (fsw x Iripple)

(7)

For output 1 of the above example, and for I

ripple of

30% of I

OUT1, LO1 is calculated to be 1.1µH.

The core must be selected according to the peak of

maximum output current.

A similar calculation can be applied to find an induc-

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