L5994 datasheet(16/26 Pages) STMICROELECTRONICS

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L5994 - L5994A

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Power MOSFET's and Schottky Diodes

Since the gate drivers of the device are powered by a 5V bus , the use of logic-level MOSFET's is highly rec-

ommended, especially for high current applications. Their breakdown voltage V(BR)DSS must be greater than

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

The RDS(ON) can be selected once the allowable power dissipation has been established. By selecting identical

power MOSFET's 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

RDS(ON) (@ 25 °C) can be derived from:

where Iout is either ITOT5 or IOUT3, according to the section under consideration, a 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 RDS(ON), 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:

Ig = Qg · fSW

which affects efficiency at low load currents. Besides, this current is drawn from the PREG5 line whose source

capability, ISRC (25mA min), must not be exceeded, thus a further constraint concernes the MOSFET total gate

charge (@VGS = 5V):

assuming four identical MOSFET's.

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

er than VINMAX. Since it conducts for less than 5% of the switching period, the current rating can be much lower

than Iout. The selection criterion should be:

Vt(Schottky) < Vt(body-diode)@I = ILPK

Sense Resistors

The sense resistor of each section is selected according to their respective maximum output current. The cur-

rent sense comparator limits the inductor peak current and therefore the maximum DC output current is the peak

value less half of the peak-to-peak ripple. The intervention threshold is set at 50mV for both sections, thus the

resistor values should be:

Since the comparator threshold that triggers pulse-skipping mode is 11mV, the output current at which the sys-

tem enters this kind of operation is approximately one fourth of the maximum output current. The sense resistors

values are in the low milliohms thus it is important to take correctly the current sense signals. Make sure that

the Kelvin connections between the current sense pins of the IC and the sense resistor do not carry the output

current.

DS ON

()

OUT

∆

⋅

()

⋅

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

SRC

⋅

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

≤

SENSE5

L5 PK

-------------- m

Ω

[]

SENSE 5

L5 PK

-------------- m

Ω

[]

–

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