AP64501

Document number: DS41980 Rev. 4 - 2

19 of 26

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AP64501

Application Information (cont.)

16

External Loop Compensation Design (cont.)

The loop gain at the crossover frequency has unity gain. Therefore, the compensator resistance, R5, is determined by:

Eq. 17

Where:



gm is 0.15mS



RT is 0.089V/A



VFB is 0.8V



fc is the desired crossover frequency

Be aware that most ceramic capacitors will degrade with voltage stress or temperature extremes. Refer to its datasheet and use its worst case

capacitance value for calculations.

The compensation capacitors C5 and C6 are then equal to:

Eq. 18

(

)

Eq. 19

Where:



IOUT is the output load current

The inclusion of C6 can increase gain margin and can decrease phase margin. In most cases, C6 is optional and may be omitted.

The zero,



z2, is optional as it can increase both the phase margin and gain bandwidth and can decrease gain margin. If used, place this zero at

around two to five times fC. Thus, C4 is in the approximate range of:

[

]

Eq. 20