IP1203 datasheet(13/23 Pages) IRF

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iP1203

For better efficiency and low input ripple, select low

ESR ceramic capacitors. The amount of the capaci-

tors is determined based on the rms rating. In the

above example, a total of 3 x 22µF, 2A capacitors will

be required to support the input rms current.

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, ESR

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 peak to peak output ripple voltage .

ripple is the inductor peak-to peak ripple current.

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 (6) 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 ESR of the output capacitors is

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

capacitors. Additional ceramic capacitors can be

added in parallel to further reduce the ESR. Care

should be given to properly compensate the control

loop for low output capacitor ESR 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 iP1203

and can cause undesirable shutdown of the output.

The magnitude of the overshoot should be kept be-

low 1.125V

OUT . To prevent the overshoot from trip-

ping the output a delay can be added by installing

capacitor C14 as shown in Fig.14.

b. Stability

The value of the output capacitor ESR zero frequency

esr plays a major role in determining stability. fesr is

calculated by the expression in equation (7).

ESR = 1 / (2 π x RESR x CO)

(7)

Details on how to consider this parameter to design

for stability are outlined in the control loop compen-

sation 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.

The inductor L

ois selected according to equation (8):

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

(8)

For the above example, and for I

ripple of 30% of IOUT, LO

is calculated to be 1.0µH.

The core must be selected according to the peak of

maximum output current.

(6)

ESR

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