LTC3851 LINER [Linear Technology], LTC3851 Datasheet - Page 15

• Manufacturer Part Number: LTC3851
• Description: Synchronous Step-Down Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
0V to 0.8V on the TK/SS pin. The total soft-start time can
be calculated as:
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode up
to TK/SS = 0.64V. Between TK/SS = 0.64V and 0.72V, it
will operate in forced continuous mode and revert to the
selected mode once TK/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 TK/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 LTC3851 must be confi gured for forced
continuous operation by connecting MODE/PLLIN to
INTV
t
SOFT-START
CC
.
= 0 8
. •
1 0
(3a) Coincident Tracking
C
.
SS
μA
TIME
Figure 3. Two Different Modes of Output Voltage Tracking
V
V
MASTER
OUT
Output Voltage Tracking
The LTC3851 allows the user to program how its output
ramps up and down by means of the TK/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 3. In the following discussions, V
a master supply and V
as a slave supply. To implement the coincident tracking
in Figure 3a, connect a resistor divider to V
connect its midpoint to the TK/SS pin of the LTC3851. The
ratio of this divider should be selected the same as that of
the LTC3851’s feedback divider as shown in Figure 4a. In
this tracking mode, V
To implement ratiometric tracking, the ratio of the resistor
divider connected to V
So which mode should be programmed? While either
mode in Figure 4 satisfi es most practical applications,
the coincident mode offers better output regulation.
This concept can be better understood with the help of
Figure 5. At the input stage of the 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
V
MASTER
V
OUT
=
R
R
(3b) Ratiometric Tracking
2
4
⎛
⎝ ⎜
R
R
3
1
MASTER
OUT
MASTER
+
+
TIME
R
R
refers to the LTC3851’s output
2
4
⎞
⎠ ⎟
must be higher than V
is determined by:
3851 F03
LTC3851
V
V
MASTER
OUT
MASTER
MASTER
refers to
15
OUT
and
3851f
.