Datasheet

LTC3839
13
3839fa
Characteristics Table), then the internal 5.3V LDO is en-
abled. If the EXTV
CC
pin is tied to an external voltage source
greater than this EXTV
CC
switchover voltage, then the LDO
is shut down and the internal EXTV
CC
switch shorts the
EXTV
CC
pin to the DRV
CC2
pin, thereby powering DRV
CC
and INTV
CC
with the external voltage source and helping
to increase overall efficiency and decrease internal self
heating from power dissipated in the LDO. This external
power source could be the output of the step-down con-
verter itself (if the output is programmed to higher than the
switchover voltage’s higher limit, 4.8V). The V
IN
pin still
needs to be powered up but now draws minimum current.
Power for most internal control circuitry other than gate
drivers is derived from the INTV
CC
pin. INTV
CC
can be
powered from the combined DRV
CC
pins (either directly,
or through an external RC filter to SGND to filter out noises
due to switching).
Shutdown and Start-Up
The RUN pin has an internal proportional-to-absolute-
temperature (PTAT) pull-up current source (around 2.5µA
at 25°C). Taking the RUN pin below a certain threshold
voltage (around 0.8V at 25°C) shuts down all bias of
INTV
CC
and DRV
CC
and places the LTC3839 into micropower
shutdown mode with a minimum I
Q
at the V
IN
pin. The
LTC3839’s DRV
CC
(through the internal 5.3V LDO regula-
tor or EXTV
CC
) and the corresponding channel’s internal
circuitry off INTV
CC
will be biased up when either or both
RUN pins are pulled up above the 0.8V threshold, either by
the internal pull-up current or driven directly by external
voltage source such as logic gate output.
Neither of the two channels will start switching until
the RUN pin is pulled up to 1.2V. When the RUN pin
rises above 1.2V, both channels’ TG and BG drivers are
enabled, and TRACK/SS released. An additional 10µA
temperature-independent pull-up current is connected
internally to the RUN pin. To turn off TG, BG and the ad-
ditional 10µA pull-up current, RUN needs to be pulled down
below 1.2V by about 100mV. These built-in current and
voltage hystereses prevent false jittery turn-on and turn-off
due to noise. Such features on the RUN pin allow input
undervoltage lockout (UVLO) to be set up using external
voltage divider from V
IN
.
The start-up of the output voltage (V
OUT
) is controlled by
the voltage on the TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the 0.6V internal reference, the
differential feedback voltage is regulated to the TRACK/SS
voltage instead of the 0.6V reference. The TRACK/SS
pin can be used to program the output voltage soft-start
ramp-up time by connecting an external capacitor from the
TRACK/SS pin to signal ground. An internal temperature-
independent 1µA pull-up current charges this capacitor,
creating a voltage ramp on the TRACK/SS pin. As the
TRACK/SS voltage rises linearly from ground to 0.6V, the
switching starts, V
OUT
ramps up smoothly to its final value
and the feedback voltage to 0.6V. TRACK/SS will keep
rising beyond 0.6V, until being clamped to around 3.7V.
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 RUN pin is
pulled below the 1.2V threshold (hysteresis applies), or
when INTV
CC
or DRV
CC
drop below their undervoltage
lockout (UVLO) threshold.
Light Load Current Operation
If the MODE/PLLIN pin is tied to INTV
CC
or an external clock
is applied to MODE/PLLIN, the LTC3839 will be forced to
operate in continuous mode. With load current less than
one-half of the full load peak-to-peak ripple, the inductor
current valley can drop to zero or become negative. This
allows constant-frequency operation but at the cost of low
efficiency at light loads.
If the MODE/PLLIN pin is left open or connected to signal
ground, the channel will transition into discontinuous mode
operation, where a current reversal comparator (I
REV
) shuts
off the bottom MOSFET (M
B
) as the inductor current ap-
proaches zero, thus preventing negative inductor current
and improving light-load efficiency. In this mode, both
switches can remain off for extended periods of time. As
the output capacitor discharges by load current and the
output voltage droops lower, EA will eventually move the
ITH voltage above the zero current level (0.8V) to initiate
another switching cycle.
OPERATION
(Refer to Functional Diagram)