LTC3876_15 Datasheet, PDF (28/48 Pages) Linear Technology – Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and 50mA VTT Reference

LTC3876
APPLICATIONS INFORMATION
can be used to set a V_IN to turn on a channel’s switching.
If resistor dividers are used on both RUN pins, when V_IN
is low enough and both RUN pins are pulled below the
~0.8V threshold, the part will shut down all bias of INTV_CC
and DRV_CC and be put in micropower shutdown mode.
The RUN pins’ bias currents depend on the RUN voltages.
The bias current changes should be taken into account
when designing the external voltage divider UVLO circuit.
An internal proportional-to-absolute-temperature (PTAT)
pull-up current source (~2.5μA at 25°C) is constantly con-
nected to this pin. When a RUN pin rises above 1.2V, the
corresponding channel’s TG and BG drives are turned on
and an additional 4μA temperature-independent pull-up
current is connected internally to the RUN pin. Pulling the
RUN pin to fall below 1.2V by more than an 80mV hyster-
esis turns off TG and BG of the corresponding channel,
and the additional 10μA pull-up current is disconnected.
As voltage on a RUN pin increases, typically beyond 3V,
its bias current will start to reverse direction and flow into
the RUN pin. Keep in mind that neither of the RUN pins
can sink more than 50μA; Even if a RUN pin may slightly
exceed 6V when sinking 50μA, a RUN pin should never
be forced to higher than 6V by a low impedance voltage
source to prevent faulty conditions.
Soft-Start and Tracking
The LTC3876 has the ability to either soft-start by itself
with a capacitor or track the output of another channel
or an external supply. Note that the soft-start or tracking
features are 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 TRACK/SS pin.
When channel 1 is configured to soft-start by itself, a capaci-
tor should be connected to its TRACK/SS pin. TRACK/SS
is pulled low until the RUN pin voltage exceeds 1.2V and
UVLO is released, at which point an internal current of
1μA charges the soft-start capacitor, C_SS, connected to the
TRACK/SS pin. Current-limit 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 TRACK/SS pin. The total
soft-start time can be calculated as:
t_SS(SEC)
=
0.6(V)
•
C_SS(µF)
1(µA)
When one particular channel is configured to track an
external supply, a voltage divider can be used from the
external supply to the TRACK/SS pin to scale the ramp
rate appropriately. Two common implementations are co-
incidental tracking and ratiometric tracking. For coincident
tracking, make the divider ratio from the external supply
the same as the divider ratio for the differential feedback
voltage. Ratiometric tracking could be achieved by using
a different ratio than the differential feedback.
Note that the 1μA soft-start capacitor charging current is
still flowing, producing a small offset error. To minimize
this error, select the tracking resistive divider values to be
small enough to make this offset error negligible.
The LTC3876 allows the user to program how channel 1
VDDQ tracks an external power supply. Channel 2 VTT will
always track VDDQ and be equal to 0.5 • VDDQ.
By selecting different resistors, the LTC3876 can achieve
different modes of tracking including the two in Figure 8a.
To implement the coincident tracking, connect an additional
resistive divider to the external power supply and connect
its midpoint to the TRACK/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 8b. In
this tracking mode, the external power supply must be set
higher than VDDQ. To implement the ratiometric tracking,
the master channel’s feedback divider can be also used
to provide TRACK/SS voltage for the slave channel, since
the additional divider, if used, should be of the same ratio
as the master channel’s feedback divider.
3876f
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