ADP3168 datasheet(21/24 Pages) AD

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ADP3168

Rev. B | Page 21 of 24

CIN SELECTION AND INPUT CURRENT

DI/DT REDUCTION

In continuous inductor current mode, the source current of the

high-side MOSFET is approximately a square wave with a duty

ratio equal to n × VOUT/VIN and an amplitude of one-nth of the

maximum output current. To prevent large voltage transients, a

low ESR input capacitor sized for the maximum rms current

must be used. The maximum rms capacitor current is given by

CRMS

125

−

(34)

Note that the capacitor manufacturer’s ripple current ratings are

often based on only 2,000 hours of life. This makes it advisable

to further derate the capacitor or choose a capacitor rated at a

higher temperature than required. Several capacitors may be

placed in parallel to meet size or height requirements in the

design. In this example, the input capacitor bank is formed by

three 2,200 µF, 16 V Nichicon capacitors with a ripple current

rating of 3.5 A each.

To reduce the input current di/dt to a level below the

recommended maximum of 0.1 A/µs, an additional small

inductor (L > 1 µH @ 15 A) should be inserted between the

converter and the supply bus. That inductor also acts as a filter

between the converter and the primary power source.

()

FLHOT

FLCOLD

OLD

NEW

−

(35)

TUNING PROCEDURE FOR THE ADP3168

Build circuit based on compensation values computed

from design spreadsheet.

Hook up dc load to circuit, turn on, and verify operation.

Also check for jitter at no-load and full-load.

DC Loadline Setting

Measure output voltage at no-load (VNL). Verify that it is

within tolerance.

Measure output voltage at full-load cold (VFLCOLD). Let

board set for ~10 minutes at full-load and measure output

(VFLHOT). If there is a change of more than a couple of

millivolts, adjust RCS1 and RCS2 using Equations 35 and 37.

Repeat Step 4 until cold and hot voltage measurements

remain the same.

Measure output voltage from no-load to full-load using 5 A

steps. Compute the loadline slope for each change and then

average to get overall loadline slope (ROMEAS).

If ROMEAS is off from RO by more than 0.05 mΩ, use the

following to adjust the RPH values:

()

OMEAS

OLD

NEW

(36)

Repeat Steps 6 and 7 to check loadline and repeat

adjustments if necessary.

Once complete with dc loadline adjustment, do not change

RPH, RCS1, RCS2, or RTH for rest of procedure.

10. Measure output ripple at no-load and full-load with scope

and make sure it is within specifications.

()

(

)

()

(

)

(

)

(

)

()

(

)

()

OLD

NEW

OLD

NEW

−

(37)

100

010

OUTPUT CURRENT (A)

Figure 14. Efficiency of the Circuit of Figure 11 vs. Output Current

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