LTC3853 Datasheet, PDF (18/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
APPLICATIONS INFORMATION
The optional Schottky diodes conduct during the dead time
between the conduction of the two power MOSFETs. These
prevent the body diodes of the bottom MOSFETs from turn-
ing on, storing charge during the dead time and requiring
a reverse recovery period that could cost as much as 3%
in efficiency at high V_IN. A 1A to 3A Schottky is generally
a good compromise for both regions of operation due to
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
Soft-Start and Tracking
The LTC3853 has the ability to either soft-start by itself
with a capacitor or track the output of another channel or
external supply. When one particular channel is configured
to soft-start by itself, a capacitor should be connected to
its TK/SS pin. This channel is in the shutdown state if its
RUN pin voltage is below 1.2V. Its TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.2V, the channel pow-
ers up. A soft-start current of 1.3μA then starts to charge
its soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defined to be the voltage range from 0V to 0.8V on the
TK/SS pin. The total soft-start time can be calculated as:
t_SOFTSTART
=
0.8
•
C_SS
1.3μA
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 0.74V, it
will operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.74V. The output ripple is
minimized during the 100mV forced continuous mode
window ensuring a clean PGOOD signal.
When the channel 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, select the tracking resistive divider value to be small
enough to make this error negligible.
In order to track down another channel or supply after
the soft-start phase expires, the LTC3853 is forced into
continuous mode of operation as soon as V_FB is below the
undervoltage threshold of 0.74V regardless of the setting
of the MODE/PLLIN pin. However, the LTC3853 should
always be set in force continuous mode tracking down
when there is no load. After TK/SS drops below 0.1V, its
channel will operate in discontinuous mode.
Output Voltage Tracking
The LTC3853 allows the user to program how its out-
put ramps up and down by means of the TK/SS pins.
Through these pins, the output can be set up to ei-
ther coincidentally or ratiometrically track another
supply’s output, as shown in Figure 5. In the following
discussions, V_OUT1 refers to the LTC3853’s output 1 as a
master channel and V_OUT2 refers to the LTC3853’s output 2
as a slave channel. In practice though, any phase can be
used as the master. To implement the coincident tracking
in Figure 5a, connect an additional resistive divider to
V_OUT1 and connect its midpoint to the TK/SS pin of the
slave channel. The ratio of this divider should be the same
as that of the slave channel’s feedback divider shown in
Figure 6a. In this tracking mode, V_OUT1 must be set higher
than V_OUT2. To implement the ratiometric tracking, the ratio
of the slave’s divider should be exactly the same as the
master channel’s feedback divider. By selecting different
resistors, the LTC3853 can achieve different modes of
tracking including the two in Figure 5.
So which mode should be programmed? While either
mode in Figure 6 satisfies most practical applications, there
are some tradeoffs. The ratiometric mode saves a pair of
resistors, but the coincident mode offers better output
regulation. This can be better understood with the help
of Figure 7. At the input stage of the slave channel’s error
amplifier, two common anode diodes are used to clamp
the equivalent reference voltage and an additional diode
3853f
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