L6996 datasheet(20/26 Pages) STMICROELECTRONICS

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L6996

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3.5 Power MOSFET and Schottky Diodes

Since a 5V bus powers the gate drivers of the device, the use of logic-level MOSFET 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 (depends on applications).

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

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

RDSON (@ 25 °C) can be derived from:

Eq 22

α is the temperature coefficient of RDS(ON) (typically, a = 5*10-3 °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 Q G moves

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

Eq 23

The Schottky diode to be placed in parallel to the synchronous rectifier must have a reverse voltage VRRM

greater than VINMAX.

For this application are selected: two high side MOSFET STS11NF3LL and two STS17NF3LL for the low side

section.

3.6 RSENSE selection

The droop function consists to change the output voltage changing the output current; at high output current the

output voltage is lower than the reference voltage. To implement the droop function, for the high current status,

we use the RSENSE resistor in series to the inductor. Since inductor current can be very high, so the resistor

must be capable to dissipate high power. Moreover we use the sense resistor to measure the output current for

the current limit feature, so the RSENSE value must be very accurate also for temperature variation. To ensure

higher temperature stability it could possible to split the RSENSE value. To achieve high efficiency also the

RSENSE value must be as low as possible, so the Active voltage droop implemented in L6996 is very useful. For

this application it are selected two 3mohms resistors from PANASONIC.

3.7 VP Network Design

The voltage-positioning network is selected by the load regulation needed. In this application wit is considered

4mV/A; with a RSENSE resistor around 1.5mohms it can be used a gain around 2.66 and so a rate between R1

and R2 around 1.66 from the Eq6.

It can be selected:

R1=1.66K

Ω

R2=1K

Ω

A capacitor CVP1 is required in parallel with RVP1 to correctly compensate the network response. Its value is

given by the following equation:

Eq 24

where COUT is the output capacitor value. When CVP1 is well chosen, a step decrease of output voltage should

be observed, as an effect of a step load increase. Too small or too large CVP1 produces overshoot or undershoot

instead of a step waveform.

RD S

Iou t

∆

⋅

()

⋅

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

Q g F

⋅

VP1

ESRC

OUT

VP1

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

VP2

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





⋅

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