17/23

L6997

With our parameters:

Lmin

≥ 2µH

The saturation current is around 5A

3.3 Output capacitor and ripple voltage

The output capacitor is chosen by the output voltage static precision and also dynamic precision. The static pre-

cision regards the output voltage ripple value rated the output voltage in steady state at the end the ESR value;

while the dynamic precision regards the load step positive and negative load transient.

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

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

Eq 21

One can consider a switching frequency around 270kHz.

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

So the ESR is given from: RMS current in output capacitor is given by:

Eq 22

The dynamic specifications are sometimes more relaxed than the static requirements, so one can consider the

ESR value around 20m

Ω enough.

To allow the device control loop to properly work, output capacitor ESR zero must be at least ten times smaller

than switching frequency. Low ESR tantalum capacitors, which ESR zero is close to ten kHz, are suitable for

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.

Eq 23

But this is usually a negligible constrain.

Possible solutions:

3.4 MOSFET’s and Schottky Diodes

Since a 3.3V bus powers the gate drivers of the device, the use ultra low level MOSFET is highly recommended,

with a certain margin, so the selection will address 20V or 30V devices (depends on applications).

Power MOSFET as the main switch and the synchronous rectifier, the total power they dissipate does not de-

pend on the duty cycle. Thus, if PON is this power loss (few percent of the rated output power), the required

330

µF

EEFUE0D331R

PANASONIC

220

µF

2R5TPE220M

POSCAP

I

∆

Vi n

Vo

–

L

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

Vo

Vin

--------- T

sw

⋅⋅

=

ESR

V

ripp le

∆

I

∆

2

-----

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

25m V

1.25

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

=

20m

Ω

==

Icout

rms

1

23

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

L

∆

=