ADP1821ARQZ-R7 datasheet(18/24 Pages) AD

18 / 24 page

ADP1821

Rev. B | Page 18 of 24

Calculate the compensator gain needed at crossover to achieve

0 dB total system gain:

AT = AMOD + AFILTER + ACOMP

(52)

0 dB = AMOD + AFILTER + ACOMP

(53)

ACOMP

= 0 dB − AMOD − AFILTER

(54)

Calculate the value of RZ to achieve the previous gain by

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

TOP

COMP

log

(55)

()

⎟

⎠

⎞

⎜

⎝

⎛

COMP

TOP

(56)

Calculate the integrator capacitor value to place the compen-

sation zero at the desired frequency:

(57)

Calculate the capacitor value for the high frequency pole by

(58)

Check that the calculated component values are reasonable.

For instance, capacitors smaller than about 10 pF should be

avoided. In addition, the ADP1821 error amplifier has finite

output current drive, so RZ values less than a few kilohm and CI

values greater than 10 nF should be avoided. If necessary, recal-

culate the compensation network with a different starting value

of RTOP. If CHF is too small, start with a smaller value RTOP. If RZ

is too small and CI is too big, start with a larger value of RTOP.

SETTING THE SOFT START PERIOD

The ADP1821 uses an adjustable soft start to limit the output

voltage ramp-up period, limiting the input inrush current. The

soft start is set by selecting the capacitor, CSS, from SS to GND.

The ADP1821 charges CSS to 0.8 V through an internal 100 kΩ

resistor. The voltage on CSS while it is charging is

⎟

⎠

⎞

⎜

⎝

⎛

−

CSS

(59)

where R is the internal 100 kΩ resistor.

The soft start period, tSS, is achieved when VCSS = 0.6 V or

⎟

⎠

⎞

⎜

⎝

⎛

−

)

(

100

(60)

()

386

kΩ

100

⎟

⎠

⎞

⎜

⎝

⎛

−

(61)

Solve for CSS by

CSS

= tSS × 7.21 μF/sec

(62)

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