ltc3851-1 Linear Technology Corporation, ltc3851-1 Datasheet - Page 15

• Manufacturer Part Number: ltc3851-1
• Description: Synchronous Step-down Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

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 LTC3851-1 must be confi gured for
forced continuous operation by connecting MODE/PLLIN
to INTV
t
SOFT-START
CC
.
= 0 8
. •
(3a) Coincident Tracking
1 0
C
.
SS
μA
TIME
Figure 3. Two Different Modes of Output Voltage Tracking
V
V
MASTER
OUT
Output Voltage Tracking
The LTC3851-1 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
nect its midpoint to the TK/SS pin of the LTC3851-1. The
ratio of this divider should be selected the same as that of
the LTC3851-1’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 LTC3851-1’s error
amplifi er, two common anode diodes are used to clamp
the equivalent reference voltage and an additional diode
V
MASTER
V
OUT
=
(3b) Ratiometric Tracking
R
R
2
4
⎛
⎝ ⎜
R
R
OUT
3
1
TIME
MASTER
+
+
MASTER
R
R
refers to the LTC3851-1’s output
2
4
⎞
⎠ ⎟
is determined by:
must be higher than V
LTC3851-1
38511 F03
V
V
MASTER
OUT
MASTER
MASTER
and con-
refers to
15
38511f
OUT
.