Datasheet

LTC3546
19
3546fc
For more information www.linear.com/3546
applicaTions inForMaTion
The LTC3546 can also track an external voltage during
startup by using an external voltage divider to the TRACK/SS
pin, and also insuring that the ramp rate on the TRACK/SS
pin is slower than the internal 1.2ms ramp rate.
As indicated in Figure 3, a resistor divider from an external
voltage can be connected to the TRACK/SS pin to allow
the start-up of V
OUT
to ratiometrically track an external
voltage V
X
.
For V
TRACKSS
< 0.6V
V
OUT
= V
X
•
R2X
R1X+R2X
•
R1+ R2
R2
Table 2 summarizes the different states in which the
TRACK/SS can be used.
Table 2. The States of the TRACK/SS Pin
TRACK/SS PIN RESULT
Capacitor to Ground External Soft-Start
Floating Internal Soft-Start
Resistor Divider V
OUT
Tracking an External Voltage V
X
Regardless of the mode implemented, the TRACK/SS pin
should never be pulled high externally as this will result
in excessive current during shutdown.
The LTC3546 can smoothly handle starting up into a pre-
biased output. The tracking function will pick up from the
pre-biased voltage and ramp the output up from there.
Mode Selection
The SYNC/MODE pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connecting
this pin to V
IN
enables Burst Mode operation for both reg-
Figure 3. Tracking External Voltages
Figure 4. Coincident Tracking
3546 F03
LTC3546
V
TRACK/SS
R1
V
FB
R2
V
X
V
OUT
R1X
R2X
3546 F04
TIME
OUTPUT VOLTAGE
V
X
V
OUT
Figure 5. Ratiometric Tracking
3546 F05
TIME
OUTPUT VOLTAGE
V
X
V
OUT
Coincident tracking is where V
OUT
= V
X
during startup. To
implement coincident tracking R1X in Figure 3 is set to the
same value as R1 and R2X to the value of R2. Coincident
tracking is illustrated in Figure 4. The voltage at TRACK/SS
when V
X
is at its final value should be ≥0.8V (sufficient
margin above the 0.6V reference voltage).
Ratiometric tracking is where V
OUT
≠ V
X
during startup
but rather, it is set to some fractional value of V
X
. To
implement ratiometric tracking (as illustrated in Figure
5), set R1X in Figure 3 to the same value as R1 and R2X
to the value of R2 + ΔR. The ΔR added to R2 should be
sufficient so that the TRACK/SS voltage is ≥ 0.8V when
V
X
is at its final value.
The internal 1.15µA pull up current on TRACK/SS can cause
a tracking error at V
OUT
when using a resistor divider on
TRACK/SS. For example, if a 59k resistor is chosen for
R2X, the R2X current will be about 10µA (0.6V/59k). In this
case, the 1.15µA internal current source will cause about
11% (1.15µA/10µA) tracking error, which is about 66mV
referred to V
FB
. This is acceptable for most applications.
If a better tracking accuracy is required, the value of R2X
can be reduced or the 1.15µA current can be taken into
account in the equations.