19/26

L6996

Considering 10uF capacitors ceramic, that have ICINRMS =1.5A, 6 pzs. are needed.

3.3 Inductor selection

In order to determine the inductor value is necessary considering the maximum output current to decide the in-

ductor current saturation. Once the inductor current saturation is found automatically it is found the inductor val-

ue also. The inductor value is important also to determine the duration of the dynamic output voltage transition.

In our design it is considered a very low profile inductor.

L = 0.6

µA

The saturation current for this choke is 25A

3.4 Output capacitors

The output capacitor is chosen by the output voltage static and dynamic accuracy. The static accuracy is related

to the output voltage ripple value, while the dynamic accuracy is related to the output current load step.

If the static precision is around +/- 4% for the 1.25V output voltage, the output accuracy is ±50mV.

To determine the ESR value from the output precision is necessary before calculate the ripple current:

Eq 18

Considering a switching frequency around 270kHz from the equation above the ripple current is around 7A.

So the maximum ESR should be:

Eq 19

The dynamic specifications are sometime more relaxed than the static requirements so the ESR value around

7m

Ω should be enough.

Sometimes can be considered the output capacitor effect also:

Eq 20

From the above equation can be calculated the minimum output capacitance value. Considering

Five capacitor of 330

µF from PANASONIC correspond to the request. To allow the device control loop to prop-

erly work, output capacitor ESR zero must be at least ten times smaller than switching frequency. Low ESR tan-

talum capacitors, which ESR zero is close to 10 kHz, are suitable for output filtering. Output capacitor value

COUT and its series resistance, should be large enough and small enough, respectively, to keep output voltage

within the accuracy range during a load transient, and to give the device a minimum signal to noise ratio.

The current ripple flows through the output capacitor, so the output capacitors should be calculated also to sus-

tain this ripple: the RMS current value is given from Eq21.

Eq 21

But this is usually a negligible constrain when choosing output capacitor.

I

∆

Vin

Vo

–

L

-----------------------

Vo

Vi n

--------- T

sw

⋅⋅

=

ESR

V

rip ple

∆

I

∆

2

-----

---------------------

50m V

3.5

----------------

14m

Ω

==

=

Vo ut

∆

Io ut

2

L

⋅

2Vo ut

⋅

----------------------

1

C out

--------------

⋅

=

Icout

rms

1

23

----------- I

L

∆

=