ADP1822

Rev. B | Page 19 of 24

The high-side MOSFET turn-on follows the rising edge of

the SYNC input by approximately 320 ns. To prevent erratic

switching frequency, make sure that the falling edge of the

SYNC input signal does not coincide with the falling edge of

the dc-to-dc converter switching, or

SETTING THE SOFT START PERIOD

The ADP1822 uses an adjustable soft start to limit the output

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

The voltage on C

()( )

[]

IN

OUT

SW

SYNC

V

V

f

D

+

≠

ns

320

SS

while it is charging is

(57)

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

=

SS

RC

CSS

e

V

1

1

V

8

.

0

(53)

where D

is the duty cycle of the synchronization waveform.

SYNC

Make sure that in all combinations of frequency, input, and

output voltages, the SYNC input fall time does not align with

the dc-to-dc converter fall time.

is achieved when V

= 0.6 V, or

CSS

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

=

)

(

kΩ

100

1

V

8

.

0

V

6

.

0

SS

SS

C

t

e

(54)

SETTING THE OUTPUT VOLTAGE TRACKING

or

()

386

.

1

V

8

.

0

V

6

.

0

1

ln

kΩ

100

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

−

=

SS

SS

C

t

(55)

(56)

The ADP1822 provides a tracking function that limits the

output voltage to or below an external tracking voltage. This is

useful in systems where multiple dc-to-dc converters are used to

power different sections of a circuit, such as a microcontroller or a

DSP that has separate I/O and core voltages. In similar circuits,

if the nominally lower of the two voltages exceeds the nominally

higher voltage at startup or shutdown, the circuit powered may

experience problems. To prevent this, use the tracking feature of

the ADP1822 to limit the output voltage to or below the tracking

voltage at all times.

SYNCHRONIZING THE CONVERTER

The dc-to-dc converter switching can be synchronized to an

external signal. This allows multiple ADP1822 converters to be

operated at the same frequency to prevent frequency beating or

other interactions.

To use the tracking feature, connect TRKP to the output voltage

and drive TRKN with the tracking voltage. To ensure that noise

does not cause unstable operation, connect a 1 nF capacitor

between TRKN and TRKP as close to the ADP1822 as possible.

To synchronize the ADP1822 switching to an external signal,

drive the SYNC input with the synchronizing signal. The ADP1822

can only synchronize up to 2× the nominal oscillator frequency.

If the frequency is set to 300 kHz (FREQ connected to GND), it

can synchronize up to 600 kHz. If the frequency is set to 600 kHz

(FREQ connected to VCC), it can synchronize to 1.2 MHz.

If either the ADP1822 output voltage or the tracking voltage at

any time exceeds the voltage at VCC, use equal voltage dividers

from the output voltage to TRKP and from the tracking voltage

to TRKN to prevent overstress on the TRKP and TRKN inputs.