ADP1823ACPZ-R7 datasheet(22/32 Pages) AD

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ADP1823

Rev. A | Page 22 of 32

Calculate the capacitor value for the high frequency pole:

(58)

Check that the calculated component values are reasonable. For

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

In addition, the ADP1823 error amplifier has finite output

current drive, so RZ values less than a few 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 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.

This compensation technique should yield a good working

solution. 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., application

support team.

SOFT START

The ADP1823 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 ADP1823 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

(59)

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

(60)

386

(61)

Because R = 90 kΩ :

sec

μF/

(62)

where tSS is the desired soft start time in seconds.

VOLTAGE TRACKING

The ADP1823 includes a tracking feature that prevents an

output voltage from exceeding a master voltage. This is

especially important when the ADP1823 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 ADP1823 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

ADP1823

ERROR

AMPLIFIER

VOUT

RTOP

RBOT

Figure 29. Voltage Tracking

COINCIDENT TRACKING

The most common application is coincident tracking, used in

core vs. I/O voltage sequencing and similar applications.

Coincident tracking limits the slave output voltage to be the

same as the master voltage until it reaches regulation. Connect

the slave TRK input to a resistor divider from the master voltage

that is the same as the divider used on the slave FB pin. This

forces the slave voltage to be the same as the master voltage.

For coincident tracking, use RTRKT = RTOP and RTRKB = RBOT,

where RTOP and RBOT are the values chosen in the Compensating

the Voltage Mode Buck Regulator section.

TIME

SLAVE VOLTAGE

MASTER VOLTAGE

Figure 30. Coincident Tracking

As the master voltage rises, the slave voltage rises identically.

Eventually, the slave voltage reaches its regulation voltage,

where the internal reference takes over the regulation while the

TRK input continues to increase and thus removes itself from

influencing the output voltage.

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