LTC3838-2_15 Datasheet, PDF (16/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
OPERATION (Refer to Functional Diagram)
pull-up current is connected internally to the channel’s
respective RUN pin. To turn off TG, BG and the additional
5µA pull-up current, RUN needs to be pulled down be-
low 1.2V by about 100mV. These built-in current and
voltage hystereses prevent false jittery turn-on and turn-
off due to noise. These features on the RUN pins allow
input undervoltage lockout (UVLO) to be set up using
external voltage dividers from V_IN.
The start-up of a channel’s output voltage (V_OUT) is
controlled by the voltage on its TRACK/SS pin. When the
voltage on the TRACK/SS pin is less than the 0.6V internal
reference (channel 1) or the external reference voltage on
EXTV_REF2 pin (channel 2), the feedback voltage (V_FB) is
regulated to the TRACK/SS voltage instead of the reference.
The TRACK/SS pin can be used to program the output
voltage soft-start ramp-up time by connecting an external
capacitor from a TRACK/SS pin to signal ground. An inter-
nal temperature-independent 1µA pull-up current charges
this capacitor, creating a voltage ramp on the TRACK/SS
pin. As the TRACK/SS voltage rises from ground to the
reference voltage, the switching starts, and V_OUT ramps
up smoothly to its final regulated value. TRACK/SS will
keep rising until the pull-up current stops at well above
3V when the V_IN pin is greater than 6V.
Note, when the V_IN pin voltage is close to its low end, the
pull-up current may stop at a lower voltage. In applica-
tions that require both low V_IN pin voltage (< 6V, down to
4.5V) and high EXTV_REF2 (>1.5V, up to 2.5V) at the same
time, an external pull-up (current or resistor from INTV_CC)
could be added to the TRACK/SS2 pin so that TRACK/SS2
settles well above EXTV_REF2. This prevents V_FB2 from being
regulated to an insufficient TRACK/SS2 voltage instead of
the desired EXTV_REF2 after soft-start.
Upon enabling the RUN pin, if V_OUT is prebiased at a
level above zero, the top gate (TG) will remain off and
V_OUT stays prebiased. Once TRACK/SS rises above the
prebiased feedback level, and TG starts switching, V_OUT
will be regulated according to TRACK/SS or the reference,
whichever is lower.
Alternatively, the TRACK/SS pin can be used to track an
external supply like in a master slave configuration. Typi-
cally, this requires connecting a resistor divider from the
master supply to the TRACK/SS pin (see the Applications
Information section).
TRACK/SS is pulled low internally when the correspond-
ing channel’s RUN pin is pulled below the 1.2V threshold
(hysteresis applies), or when INTV_CC or either of the
DRV_CC1,2 pins drop below their respective undervoltage
lockout (UVLO) thresholds. TRACK/SS2 is also pulled low
internally when voltage on the EXTV_REF2 pin is less than
0.1V, which prevents channel 2 from starting to switch until
a valid EXTV_REF2 is present and TRACK/SS2 is released.
When EXTV_REF2 rises from below 0.1V, a fast voltage step
up can be applied on EXTV_REF2, as TRACK/SS2 allows
V_OUT2 to soft-start. However, once the soft-start ends and
TRACK/SS2 rises above EXTV_REF2, it is recommended that
dynamic voltage change on EXTV_REF2 be rate-limited. If
EXTV_REF2 jumps up, inductor current may reach its full
limit. If EXTV_REF2 steps down, overvoltage (OV) may be
triggered, which turns on BG2 and discharges V_OUT2
without any limit on inductor current. If any external R-C
filter is connected in series to control the slew rate of
EXTV_REF2 voltage, be careful about the offset caused by
the pin’s bias current (see the Electrical Characteristics
section) through the resistor.
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