LTC3851A_15 Datasheet, PDF (15/30 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851A
Applications Information
0V to 0.8V on the TK/SS pin. The total soft-start time can
be calculated as:
t_SOFT-START
=
0.8
•
C_SS
1.0µA
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 configured 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 flowing, 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 LTC3851A must be configured for
forced continuous operation by connecting MODE/PLLIN
to INTV_CC.
Output Voltage Tracking
The LTC3851A 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_MASTER refers to
a master supply and V_OUT refers to the LTC3851A’s output
as a slave supply. To implement the coincident tracking
in Figure 3a, connect a resistor divider to V_MASTER and
connect its midpoint to the TK/SS pin of the LTC3851A.
The ratio of this divider should be selected the same as
that of the LTC3851A’s feedback divider as shown in Figure
4a. 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
RR31_++RR24


So which mode should be programmed? While either
mode in Figure 4 satisfies 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 amplifier, 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_MASTER
V_OUT
V_OUT
TIME
(3a) Coincident Tracking
TIME
3851A F03
(3b) Ratiometric Tracking
Figure 3. Two Different Modes of Output Voltage Tracking
3851afa
15