L6995 datasheet(17/25 Pages) STMICROELECTRONICS

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L6995

Eq 20

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

So the maximum ESR should be:

Eq 21

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

Ω should be enough.

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 Eq22.

Eq 22

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

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

than switching frequency. The output capacitor value (COUT) and the output capacitor ESR (ESROUT) should be

large enough and small enough, to keep the output voltage ripple within the specification and to give to the de-

vice a minimum signal to noise ratio.

3.4 Power MOSFETS and Schottky Diodes

Since a 5V bus powers the gate drivers of the device, the use of logic-level MOSFETS is highly recommended,

especially for high current applications. The breakdown voltage VBRDSS must be greater than VINMAX with a

certain margin, so the selection will address 20V or 30V devices.

The RDSON can be selected once the allowable power dissipation has been established. By selecting identical

Power MOSFET for 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

RDSON (@ 25 °C) can be derived from:

Eq 23

α is the temperature coefficient of RDSON (typically, α = 510-3 °C-1 for these low-voltage classes) and ∆T the

admitted temperature rise. It is worth noticing, however, that generally the lower RDSON, the higher is the gate

charge QG, which leads to a higher gate drive consumption. In fact, each switching cycle, a charge QG moves

from the input source to ground, resulting in an equivalent drive current:

Eq 24

The SCHOTTY diode placed in parallel to the synchronous rectifier must have a reverse voltage VRRM greater

than VINMAX. For application with low Duty Cycle, where the input voltage is high (around 20V) it is very impor-

tant to select the high side MOSFET with low gate charge, to reduce the switching losses as STS11NF3LL. For

the low side section should be selected a low RDSON as STS25NH3LL.

3.5 Output voltage setting

To select the output divider network there isn't a specific criteria, but a low divider network value (around 100

Ω)

reduces the efficiency at low current; instead a high value divider network (500K

Ω) increase the noise effects.

A network divider values from 1K to 50K is right. From the Eq4:

∆

Vin

–

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

Vin

---------

⋅⋅

ESR

rippl e

∆

-----

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

Ω

Icout

rms

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

∆

RD S

Iou t

∆

⋅

()

⋅

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

Q g

⋅

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