LM2716 datasheet(10/17 Pages) NSC

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Buck Operation (Continued)

COMPENSATION COMPONENTS

In the control to output transfer function, the first pole F

can be estimated as 1/(2

πR

OUTCOUT); The ESR zero FZ1 of

the output capacitor is 1/(2

πESRC

OUT); Also, there is a high

frequency pole F

P2 in the range of 45kHz to 150kHz:

P2 =FSW/(

πn(1−D))

whereD=V

OUT/VIN, n = 1+0.348L/(VIN−VOUT)(L isinµHs

and V

IN and VOUT in volts).

The total loop gain G is approximately 500/I

OUT where IOUT

is in amperes.

A Gm amplifier is used inside the LM2716. The output resis-

tor R

o of the Gm amplifier is about 85k

Ω.C

C1 and RC1

together with R

o give a lag compensation to roll off the gain:

PC1 = 1/(2

πC

C1(Ro+RC1)), FZC1 = 1/2

πC

C1RC1.

In some applications, the ESR zero F

Z1 can not be cancelled

by F

P2. Then, CC3 is needed to introduce FPC2 to cancel the

ESR zero, F

P2 = 1/(2

πC

C3Ro\RC1).

The rule of thumb is to have more than 45˚ phase margin at

the crossover frequency (G=1).

SCHOTTKY DIODE

The breakdown voltage rating of D

1 is preferred to be 25%

higher than the maximum input voltage. Since D1 is only on

for a short period of time, the average current rating for D1

only requires being higher than 30% of the maximum output

current.

LM2716 Datasheet(PDF) 10 Page - National Semiconductor (TI)

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