ADP1828YRQZ-R7 datasheet(23/32 Pages) AD

23 / 32 page

ADP1828

Rev. 0 | Page 23 of 32

Use the larger value of CI from Equation 31 or Equation 32.

Because of the finite output current drive of the error amplifier,

CI needs to be less than 10 nF. If it is larger than 10 nF, choose a

larger RTOP and recalculate RZ and CI until CI is less than 10 nF.

Next, choose the high frequency pole, fP1, to be ½ of fSW.

(33)

Since CHF << CI, Equation 26 is simplified to

(34)

Combine Equation 33 and Equation 34, and solve for CHF,

(35)

Type III Compensator

(dB)

PHASE

–270°

–90°

CHF

CFF

RTOP

RBOT

VOUT

INTERNAL

VREF

COMP

RFF

–1 S

LOP

–1 S

LOP

Figure 39. Type III Compensation

If the output capacitor ESR zero frequency is greater than ½ of

the crossover frequency, use the Type III compensator as shown

in Figure 39. Set the poles and zeros as follows:

(36)

(37)

(38)

Use the lower zero frequency from Equation 37 or Equation 38.

Calculate the compensator resistor, RZ

RAMP

TOP

R =

(39)

Next, calculate CI,

(40)

Because of the finite output current drive of the error amplifier,

CI needs to be less than 10 nF. If it is larger than 10 nF, choose a

larger RTOP and recalculate RZ and CI until CI is less than 10 nF.

Since CHF << CI, combining Equation 26 and Equation 36 yields

(41)

Next, calculate the feedforward capacitor CFF. Assuming RFF <<

RTOP, then Equation 25 is simplified to

TOP

(42)

Solving CFF in Equation 42 yields

TOP

(43)

where fZ2 is obtained from Equation 37 or Equation 38.

The feedforward resistor, RFF, can be calculated by combining

Equation 27 and Equation 36

(44)

Check that the calculated component values are reasonable. For

instance, capacitors smaller than about 10 pF should be avoided.

In addition, the ADP1828 error amplifier has a finite output

current drive, so RZ values less than 3 kΩ and CI values greater

than 10 nF should be avoided. If necessary, recalculate the compen-

sation network with a different starting value of RTOP. If RZ is too

small or CI is too big, start with a larger value of RTOP. This com-

pensation technique should yield a good working solution.

In general, aluminum electrolytic capacitors have high ESR, and

Type II compensation is adequate. However, if several aluminum

electrolytic capacitors are connected in parallel, and produce a

low effective ESR, then Type III compensation is needed. In

addition, ceramic capacitors have very low ESR (only a few

milliohms) making Type III compensation a better choice.

Type III compensation offers better performance than Type II

in terms of more low frequency gain and more phase margin

and less high frequency gain at the crossover frequency.

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