Datasheet

LTC3838
15
3838fa
from V
IN
and can output 5.3V to DRV
CC1
. Alternatively,
an internal EXTV
CC
switch (with on-resistance of around
2) can short the EXTV
CC
pin to DRV
CC2
.
If the EXTV
CC
pin is below the EXTV
CC
switchover voltage
(typically 4.6V with 200mV hysteresis, see the Electri-
cal Characteristics Table), then the internal 5.3V LDO is
enabled. 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 power-
ing 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 converter 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 pow-
ered from the combined DRV
CC
pins through an external
RC filter to SGND to filter out noises due to switching.
Shutdown and Start-Up
Each of the RUN1 and RUN2 pins has an internal proportion-
al-to-absolute-temperature (PTAT) current source (around
1.2µA at 25°C) to pull up the pins. Taking both RUN1 and
RUN2 pins below a certain threshold voltage (around 0.8V
at 25°C) shuts down all bias of INTV
CC
and DRV
CC
and
places the LTC3838 into micropower shutdown mode
with a minimum I
Q
at the V
IN
pin. The LTC3838’s DRV
CC
(through the internal 5.3V LDO regulator 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.
A channel of the LTC3838 will not start switching until the
RUN pin of the respective channel is pulled up to 1.2V.
When a RUN pin rises above 1.2V, the corresponding
channel’s TG and BG drivers are enabled, and TRACK/
SS released. An additional 5µA temperature-independent
pull-up current is connected internally to the channel’s
respective RUN pin. To turn off TG, BG and the addi-
tional 5µA 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 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, 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 capaci-
tor from a 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 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.
Light Load Current Operation
If the MODE/PLLIN pin is tied to INTV
CC
or an external clock
is applied to MODE/PLLIN, the LTC3838 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.
OPERATION
(Refer to Functional Diagram)