MAX20004 datasheet(16/20 Pages) MAXIM

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function is the product of the product of the modulator,

compensator, and feedback transfer functions as follows:

REF

OUT

z_esr

z_comp

p_load

p1_comp

p2_comp

F(s)C(s)M(s)













ωω



















ωω







The goal of compensation design is to reduce the loop

transfer function to an approximate single-pole system

with -20dB/decade gain slope and 90 degrees phase

margin at the crossover frequency. To achieve this, the

compensator zero is used to cancel the load pole, and

the compensator high frequency pole is used to cancel

the ESR zero. Assuming these cancellations, the loop

transfer function reduces to the following:

REF

OUT

p1_comp

F(s)C(s)M(s)

××







To derive the compensation components, the magnitude

of the loop gain at the crossover frequency is set equal to

1 and solved for CC as follows (assuming the magnitude of

the compensator pole at the crossover frequency is >>1):

(

)

REF

OUT

π×

REF

OUT

π×

Setting the compensator zero frequency equal to the load

pole frequency and solving for RC yields:

OUT

π×

OUT

REF

π×

The above leads to an alternative equation for CC as

follows:

OUT

Finally, setting the high-frequency compensator pole

equal to the minimum of the ESR zero frequency or 1/2

the switching frequency and solving for CF yields:

ESR

OUT

Min

2 2R









π×





ESR

OUT

Min





π×





π×





The above equation leads to the following compensation

design procedure:

1) Select a crossover frequency equal to one tenth of

the switching frequency (fSW/10) or 100kHz, which-

ever is lower.

2) Calculate and select the compensation resistor, RC.

3) Calculate and select the compensation capacitor, CC.

4) Calculate and select compensation capacitor CF.

5) Evaluate the gain and phase of the final loop transfer

function at the crossover frequency and adjust cross-

over frequency and/or compensation as required.

6) Verify the final design with transient line/load response

testing and gain-phase measurements and adjust as

required.

MAX20004/MAX20006/

MAX20008

36V, 220kHz to 2.2MHz, 4A/6A/8A

Fully Integrated Automotive

Step-Down Converters

www.maximintegrated.com

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