Datasheet

LTC3852
18
3852f
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse skipping mode up
to TRACK/SS = 0.64V. Between TRACK/SS = 0.64V and
0.72V, it will operate in forced continuous mode and revert
to the selected mode once TRACK/SS > 0.72V. The output
ripple is minimized during the 80mV forced continuous
mode window.
When the regulator is confi gured to track another supply,
the feedback voltage of the other supply is duplicated
by a resistor divider and applied to the TRACK/SS pin.
Therefore, the voltage ramp rate on this pin is determined
by the ramp rate of the other supply’s voltage. Note that
the small soft-start capacitor charging current is always
fl owing, producing a small offset error. To minimize this
error, one can select the tracking resistive divider value to
be small enough to make this error negligible.
In order to track down another supply after the soft-start
phase expires, the LTC3852 must be confi gured for forced
continuous operation by connecting MODE/PLLIN to
INTV
CC
.
Output Voltage Tracking
The LTC3852 allows the user to program how its output
ramps up and down by means of the TRACK/SS pins.
Through this pin, the output can be set up to either
coincidentally or ratiometrically track with another supply’s
output, as shown in Figure 4. In the following discussions,
V
MASTER
refers to a master supply and V
OUT
refers to the
LTC3852’s output as a slave supply. To implement the
coincident tracking in Figure 4a, connect a resistor divider
to V
MASTER
and connect its midpoint to the TRACK/SS pin
of the LTC3852. The ratio of this divider should be selected
the same as that of the LTC3852’s feedback divider as
shown in Figure 5a. In this tracking mode, V
MASTER
must
be higher than V
OUT
. To implement ratiometric tracking,
the ratio of the resistor divider connected to V
MASTER
is
determined by:
V
OUT
V
MASTER
=
R2
R4
R3 + R4
R1+ R2
⎛
⎝
⎜
⎞
⎠
⎟
APPLICATIONS INFORMATION
So which mode should be programmed? While either
mode in Figure 5 satisfi es most practical applications,
the coincident mode offers better output regulation. This
concept can be better understood with the help of Figure 6.
At the input stage of the LTC3852’s error amplifi er, two
common anode diodes are used to clamp the equivalent
reference voltage and an additional diode is used to match
the shifted common mode voltage. The top two current
sources are of the same amplitude. In the coincident
mode, the TRACK/SS voltage is substantially higher than
0.8V at steady-state and effectively turns off D1. D2 and
D3 will therefore conduct the same current and offer
tight matching between V
FB
and the internal precision
0.8V reference. In the ratiometric mode, however, TRACK/SS
equals 0.8V at steady-state. D1 will divert part of the bias
current to make V
FB
slightly lower than 0.8V.
Figure 4. Two Different Modes of Output Voltage Tracking
(
4a
)
Coincident Trackin
g
TIME
V
MASTER
V
OUT
OUTPUT VOLTAGE
3852 F04a
(4b) Ratiometric Tracking
V
MASTER
V
OUT
TIME
3852 F04b
OUTPUT VOLTAGE