LTC3865-1_15 Datasheet, PDF (19/38 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
APPLICATIONS INFORMATION
which are highest at high input voltages. For V_IN < 20V
the high current efficiency generally improves with larger
MOSFETs, while for V_IN > 20V the transition losses rapidly
increase to the point that the use of a higher R_DS(ON) device
with lower C_MILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
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 LTC3865/LTC3865-1 have the ability to either soft-start
by themselves 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 1.22V. Its TK/SS pin
is actively pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.22V, 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 0.6V on the
TK/SS pin. The total soft-start time can be calculated as:
t_SOFTSTART
=
0.6
•
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 = 0.5V. Between TK/SS = 0.5V and 0.54V, it
will operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.54V. The output ripple
is minimized during the 40mV 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 LTC3865/LTC3865-1 are
forced into continuous mode of operation as soon as V_FB
is below the undervoltage threshold of 0.54V regardless of
the setting on the MODE/PLLIN pin. However, the LTC3865/
LTC3865-1 should always be set in forced continuous
mode tracking down when there is no load. After TK/SS
drops below 0.1V, the corresponding channel will operate
in discontinuous mode.
Output Voltage Tracking
The LTC3865/LTC3865-1 allow the user to program how its
output ramps up and down by means of the TK/SS pins.
Through these pins, the output can be set up to either co-
incidentally or ratiometrically track another supply’s output,
as shown in Figure 5. In the following discussions, V_OUT1
refers to the LTC3865/LTC3865-1’s output 1 as a master
channel and V_OUT2 refers to the LTC3865/LTC3865-1’s out-
put 2 as a slave channel. In practice, though, either 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 internal feedback divider
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