17/30

L6997S

With our parameters:

Lmin

≥ 2µH

The saturation current must be higher then 5A

6.3 Output capacitor and ripple voltage

The output capacitor is selected based on both static and dynamic output voltage accuracy. The static output

voltage accuracy depends mostly on the ERS of the output capacitor, while the dynamic accuracy usually de-

pends both on the ESR and capacitance value.

If the static precision is ±1% for the 1.25V output voltage, the output ripple is ±12.5mV.

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

(21)

From the Eq. above the ripple current is around 1.25A.

So the ESR is given by:

(22)

The dynamic specifications are sometimes more relaxed than the static requirements, Anyway a minimum out-

put capacitance must be ensured to avoid output voltage variation due to the charge and discharge of Cout dur-

ing load transients.

To allow the device control loop to work properly, the zero introduced by the output capacitor ESR (

τ = ESR ·

Cout) must be at least ten times smaller than switching frequency. Low ESR tantalum capacitors, which ESR

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 capacitors, so the should be calculated also to sustain this ripple:

the RMS current value is given by Eq. 18.

(23)

But this is usually a negligible constrain.

Possible solutions:

Multilayer capacitors can not be used because their very low ESR.

6.4 MOSFET’s and Schottky Diodes

A 3.3V bus powers the gate drivers of the device, the use ultra low level MOSFET is highly recommended, es-

with a certain margin.

330

µF

EEFUE0D331R

PANASONIC

220

µF2R5TPE220M

POSCAP

I

∆

Vin

Vo

–

L

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

Vo

Vin

--------- T

sw

⋅⋅

=

ESR

V

ripple

∆

I

∆

2

-----

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

25mV

1.25

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

=

20m

Ω

==

Icout

rms

1

23

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

L

∆

=