Datasheet

Data Sheet ADP1822
Rev. D | Page 19 of 24
or
( )
386.1
V8.0
V6.0
1ln
kΩ100
=








−−=
SS
SS
C
t
(57)
Solving for C
SS
and combining constants,
C
SS
= (7.213 × 10
−6
)t
SS
(58)
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 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.
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
( )
( )
[ ]
IN
OUT
SWSYNC
V
V
f D +≠ ns320
(59)
where D
SYNC
is the duty cycle of the synchronization waveform.
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.
SETTING THE OUTPUT VOLTAGE TRACKING
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.
To u se t he 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.
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.