MAX8686 datasheet(21/23 Pages) MAXIM

21 / 23 page

Single/Multiphase, Step-Down,

DC-DC Converter Delivers Up to 25A Per Phase

______________________________________________________________________________________

be less than or equal to 1/5 the switching frequency.

Select a value for fC in the range:

The feedback voltage-divider gain (VREF/VOUT) should

be included for an output voltage higher than 3.3V,

where VREFIN is equal to 3.3V.

At the crossover frequency, the total loop gain must

equal 1, and is expressed as:

For the case where fzMOD is greater than fC:

Then RC can be calculated as:

where gmEA = 1.7mS.

The error-amplifier compensation zero formed by RC

and CC should be set at the modulator pole fPMOD.

Calculate the value of CC as follows:

If fPMOD is less than 5 x fC, add a second capacitor CF

from COMP to GND. The value of CF is:

As the load current decreases, the modulator pole also

decreases; however, the modulator gain increases

accordingly and the crossover frequency remains the

same.

For the case where fzMOD is less than fC:

The power modulator gain at fC is:

The error-amplifier gain at fC is:

RC is calculated as:

where gmEA = 1.7mS.

CC is calculated from:

CF is calculated from:

The current-mode control model on which the above

design procedure is based requires an additional high-

frequency term, GS(s), to account for the effect of sam-

pling the peak inductor current. The term GS(s) produces

additional phase lag at crossover and should be modeled

to estimate the phase margin obtainable by the selected

compensation components. As a final step, it is useful to

plot the dB gain and phase of the following loop-gain

transfer function and check the obtained phase margin. A

phase margin of at least 45° is recommended:

where the sampling effect quality factor is:

×× − −

[]

10 5

π ((

)

. )

cSW

()

××

()

⎛

⎝

⎜

⎞

⎠

⎟

LOOP

LOAD

()

×× −

()−

)

⎡⎣

⎤⎦

⎡

⎣

⎢

⎤

⎦

⎥

+×

zMOD

pMOD

××

+×

× +

)

) (

)

zEA

pEA

pdEA

ππ

××

gRo

mEA

REFIN

OUT

S (

CzMOD

××

pMOD

××

OUT

mEA

MOD fc

zMOD

=×

××

()

EA fc

mEA

zMOD

() =×

MOD fc

MOD dc

pMOD

zMOD

()

(

)

=×

CzMOD

××

pMOD

××

OUT

mEA

REFIN

MOD fc

××

()

EA fc

mEA

MOD fc

MOD dc

pMOD

()

(

)

=×

EA fc

MOD fc

REFIN

OUT

()

××

= 1

pMOD

≤

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