LTC3838-2_15 Datasheet, PDF (31/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
of the INTV_CC is typically 4.2V, with 0.5V falling hysteresis
(to re-enable UVLO). The UVLO thresholds for DR_VCC1,2 are
lower than that of INTV_CC but higher than typical threshold
voltages of power MOSFETs, to prevent them from turning
on without sufficient gate drive voltages.
Generally for V_IN > 6V, a UVLO can be set through moni-
toring the V_IN supply by using external voltage dividers
at the RUN pins from V_IN to SGND. To design the volt-
age divider, note that both RUN pins have two levels
of threshold voltages. The precision gate-drive-enable
threshold voltage of 1.2V 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.
When a channel is configured to soft-start by itself, a ca-
pacitor should be connected to its TRACK/SS pin. TRACK/
SS is pulled low until the RUN pin voltage exceeds 1.2V
and UVLO is cleared (also for channel 2, a valid external
reference voltage greater than 0.1V is present at the
EXTV_REF2 pin). After the pull-down of TRACK/SS is re-
leased, 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 the
reference V_REF (internal 0.6V for channel 1, or EXTV_REF2
pin voltage for channel 2) on the TRACK/SS pin. The total
soft-start time can be calculated as:
t_SS(SEC)
=
V_REF
(V)
•
C_SS(µF)
1(µA)
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 (~1.2µA 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  5µA 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 5µA 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.
Care should be taken for channel 2 to assure that TRACK/
SS2 settles well above the EXTV_REF2 voltage after soft-
start is complete under all operating conditions. Typically
TRACK/SS will be pulled up by the internal current source
to well above 3V when the V_IN pin is greater than 6V. How-
ever, when the V_IN pin voltage is close to its low end, the
internal pull-up current may stop at a voltage less than 3V.
An external pull-up circuit to TRACK/SS2 may be required
in applications that operate with low V_IN pin voltages
and high EXTV_REF2 voltages simultaneously, especially
when both the V_IN pin is less than 6V (down to 4.5V) and
EXTV_REF2 is greater than 1.5V (up to 2.5V). The external
pull-up circuit to the TRACK/SS2 pin can be a fixed cur-
rent source, or simply a resistor from INTV_CC. Note that
the soft-start time will change according to the external
pull-up used. The external pull-up is to prevent V_FB2 from
being regulated to a low TRACK/SS2 voltage instead of
the intended EXTV_REF2.
Soft-Start and Tracking
The LTC3838-2 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 and 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 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.
38382f
For more information www.linear.com/3838-2
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