L6997STR datasheet(18/30 Pages) STMICROELECTRONICS

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L6997S

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Power MOSFET for us and ls, the total power they dissipate does not depend 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:

(24)

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:

(25)

A SCHOTTKY diode can be added to increase the system efficiency at high switching frequency (where the

dead times could be an important part of total switching period).

This optional diode must be placed in parallel to the synchronous rectifier must have a reverse voltage VRRM

greater than VINMAX. The current size of the diode must be selected in order to keep it in safe operating condi-

tions. In order to use less space than possible, a double MOSFET in a single package is chosen: STS5DNF20V

6.5 Output voltage setting

The first step is choosing the output divider to set the output voltage. To select this value there isn't a criteria,

but a low divider network value (around 100

Ω) decries the efficiency at low current; instead a high value divider

network (100K

Ω) increase the noise effects. A network divider values from 1KΩ to 10KΩ is right. We chose:

R3 = 1K

Ω

R2 = 1.1K

Ω

The device output voltage is adjustable by connecting a voltage divider from output to VSENSE pin. Minimum

output voltage is VOUT=VREF=0.6V. Once output divider and frequency divider have been designed as to obtain

the required output voltage and switching frequency, the following equation gives the smallest input voltage,

which allows L6997S to regulate (which corresponds to TOFF=TOFFMIN):

(26)

6.6 Voltage Feedforward

From the equations 1,2 and 3, choosing the switching frequency of 270kHz the resistor divider can be selected.

For example:

R3 = 470K

Ω

R4 = 8.5K

Ω

6.7 Current limit resistor

From the equation 8 the valley current limit can be set considering the RDSON STS5DNF20V and ICIR = 5A:

R8 = 120K

Ω

6.8 Integrator capacitor

Let’s assume FU = 15kHz, VOUT = 1.25V.

Since VREF = 0.6V, from equation 2, of the device description, it follows

αOUT = 0.348 and, from equation 5 it

follows C = 250pF. The output ripple is around 22mV, so the system doesn't need the second integrator capac-

itor.

RDS

Iout

∆

⋅

()

⋅

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

Qg F

⋅

δ 1

OSC

OUT

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

OSC

OFF,MIN

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







MAX

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

⋅

–

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