Datasheet

LTC3822-1
14
38221f
an external voltage V
X
, according to a ratio set by a resis-
tor divider at TRACK/SS pin (Figure 4a). The ratiometric
relation between V
OUT
and V
X
is (Figure 4c):
V
V
R
R
RR
RR
OUT
X
TA
A
AB
TA TB
=
+
+
•
For coincident tracking (V
OUT
= V
X
during start-up),
R
TA
= R
A
, R
TB
= R
B
V
X
should always be greater than V
OUT
when using the
tracking function of TRACK/SS pin.
The internal current source (1µA), which is for external
soft-start, will cause a tracking error at V
OUT
. For example,
if a 59k resistor is chosen for R
TA
, the R
TA
current will be
about 10µA (600mV/59k). In this case, the 1µA internal
current source will cause about 10% (1µA/10µA • 100%)
tracking error, which is about 60mV (600mV • 10%)
referred to V
FB
. This is acceptable for most applications.
If a better tracking accuracy is required, the value of R
TA
should be reduced.
Table 1 summarizes the different states in which TRACK/SS
can be used.
Table 1. The States of the TRACK/SS Pin
TRACK/SS Pin FREQUENCY
Capacitor C
SS
External Soft-Start
V
IN
Internal Soft-Start
Resistor Divider V
OUT
Tracking an External Voltage V
X
Phase-Locked Loop and Frequency Synchronization
The LTC3822-1 has a phase-locked loop (PLL) comprised
of an internal voltage-controlled oscillator (VCO) and a
phase detector. This allows the turn-on of the external top
MOSFET to be locked to the rising edge of an external clock
signal applied to the SYNC/MODE pin. The phase detector
is an edge sensitive digital type that provides zero degrees
phase shift between the external and internal oscillators.
This type of phase detector does not exhibit false lock to
harmonics of the external clock.
The output of the phase detector is a pair of comple-
mentary current sources that charge or discharge the
external fi lter network connected to the PLLLPF pin. The
relationship between the voltage on the PLLLPF pin and
operating frequency, when there is a clock signal applied
to SYNC/MODE, is shown in Figure 5 and specifi ed in the
electrical characteristics table. Note that the LTC3822-1
can only be synchronized to an external clock whose fre-
quency is within range of the LTC3822-1’s internal VCO,
which is nominally 200kHz to 1MHz. This is guaranteed,
APPLICATIONS INFORMATION
TIME
(4b) Coincident Tracking
V
X
V
OUT
OUTPUT VOLTAGE
TIME
38221 F04b,c
(4c) Ratiometric Tracking
V
X
V
OU
T
OUTPUT VOLTAGE
LTC3822-1
V
FB
V
OUT
V
X
TRACK/SS
R
B
R
A
38221 F05a
R
TB
R
TA
Figure 4b and 4c. Two Different Modes of Output Voltage Tracking
Figure 4a. Using the TRACK/SS Pin to Track V
X