ADP1829ACPZ-R7 datasheet(22/32 Pages) AD

22 / 32 page

ADP1829

Rev. 0 | Page 22 of 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.

Because CHF << CI, combining Equation 29 and Equation 39

yields

(44)

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

RTOP, then Equation 28 is simplified to

TOP

(45)

Solving CFF in Equation 45 yields

TOP

(46)

where fZ2 is obtained from Equation 40 or Equation 41.

The feedforward resistor,

RFF, can be calculated by combining

Equation 30 and Equation 39

(47)

Check that the calculated component values are reasonable. For

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

In addition, the ADP1829 error amplifier has 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

compensation network with a different starting value of RTOP.

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

This compensation technique should yield a good working

solution.

In general, aluminum electrolytic capacitors have high ESR, and

a Type II compensation is adequate. However, if several aluminum

electrolytic capacitors are connected in parallel, producing a low

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

ceramic capacitors have very low ESR, in the order of a few

milliohms. Type III compensation is needed for ceramic output

capacitors. 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 cross-over

frequency.

For a more exact method or to optimize for other system

characteristics, a number of references and tools are available

from your Analog Devices, Inc. applications support team.

SOFT START

The ADP1829 uses an adjustable soft start to limit the output

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

The soft start is set by selecting the capacitor, CSS, from SS1 and

SS2 to GND. The ADP1829 charges CSS to 0.8 V through an

internal 90 kΩ resistor. The voltage on the soft start capacitor

while it is charging is

⎟

⎠

⎞

⎜

⎝

⎛

−

CSS

(48)

The soft start period ends when the voltage on the soft start pin

reaches 0.6 V. Substituting 0.6 V for VSS and solving for the

number of RC time constants,

()

⎟

⎠

⎞

⎜

⎝

⎛

−

kΩ

(49)

386

(50)

Because R = 90 kΩ,

sec

μF/

(51)

where tSS is the desired soft start time in seconds.

VOLTAGE TRACKING

The ADP1829 includes a tracking feature that prevents an

output voltage from exceeding a master voltage. This is

especially important when the ADP1829 is powering separate

power supply voltages on a single integrated circuit, such as the

core and I/O voltages of a DSP or microcontroller. In these

cases, improper sequencing can cause damage to the load.

The ADP1829 tracking input is an additional positive input to

the error amplifier. The feedback voltage is regulated to the lower of

the 0.6 V reference or the voltage at TRK, so a lower voltage on

TRK limits the output voltage. This feature allows implementation

of two different types of tracking: coincident tracking, where

the output voltage is the same as the master voltage until the

master voltage reaches regulation, or ratiometric tracking, where

the output voltage is limited to a fraction of the master voltage.

In all tracking configurations, the master voltage should be

higher than the slave voltage.

Note that the soft start time setting of the master voltage should

be longer than the soft start of the slave voltage. This forces the

rise time of the master voltage to be imposed on the slave voltage.

If the soft start setting of the slave voltage is longer, the slave

comes up more slowly and the tracking relationship is not seen

at the output. The slave channel should still have a soft start

capacitor to give a small but reasonable soft start time to protect

in case of restart after a current-limit event.

COMP

TRK

MASTER

VOLTAGE

RTRKT

RTRKB

0.6V

DETAIL VIEW OF

ADP1829

ERROR

AMPLIFIER

VOUT

RTOP

RBOT

Figure 30. Voltage Tracking

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