ADP3191 datasheet(23/28 Pages) AD

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ADP3191

Rev. 0 | Page 23 of 28

AC Load Line Setting

11. Remove the dc load from the circuit and hook up the

dynamic load.

12. Hook up the scope to the output voltage and set it to dc

coupling, with the time scale at 100 μs/div.

13. Set the dynamic load for a transient step of about 40 A at

1 kHz with a 50% duty cycle.

14. Measure the output waveform (if not visible, use dc offset

on scope to view). Try to use a vertical scale of 100 mV/div

or finer. This waveform should look similar to Figure 14.

VACDRP

VDCDRP

Figure 14. AC Load Line Waveform

15. Use the horizontal cursors to measure VACDRP and VDCDRP,

as shown. Do not measure the undershoot or overshoot that

happens immediately after this step.

If VACDRP and VDCDRP are different by more than a few

millivolts, use Equation 38 to adjust CCS. It may be neces-

sary to parallel different values to get the correct one,

because there are limited standard capacitor values

available. It is a good idea to have locations for two

capacitors in the layout for this.

()

DCDRP

ACDRP

OLD

NEW

(38)

16. Repeat Step 11 to Step 13, and repeat the adjustments, if

necessary. Once complete, do not change CCS for the

remainder of the procedure.

17. Set the dynamic load step to maximum step size. Do not

use a step size larger than needed, and verify that the

output waveform is square, which means that VACDRP and

VDCDRP are equal.

Initial Transient Setting

18. With the dynamic load still set at the maximum step size,

expand the scope time scale to see 2 μs/div to 5 μs/div. The

waveform may have two overshoots and one minor under-

shoot (see Figure 15). Here, VDROOP is the final desired value.

VDROOP

VTRAN1

VTRAN2

Figure 15. Transient Setting Waveform

19. If both overshoots are larger than desired, try making the

following adjustments:

•

Make the ramp resistor larger by 25% (RRAMP).

•

For VTRAN1, increase CB, or increase the switching

frequency.

•

For VTRAN2, increase RA, and decrease CA by 25%.

If these adjustments do not change the response, the output

decoupling is the limiting factor. Check the output response

every time a change is made, or nodes are switched, to

make sure the response remains stable.

20. For load release (see Figure 16), if VTRANREL is larger than

VTRAN1 (see Figure 15), there is not enough output capaci-

tance. Either more capacitance is needed or the inductor

values need to be smaller. If inductors are changed, start

the design again using the spreadsheet and this tuning

procedure.

VDROOP

VTRANREL

Figure 16. Transient Setting Waveform

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