LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 15

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
from V
an internal EXTV
2Ω) can short the EXTV
If the EXTV
(typically 4.6V with 200mV hysteresis, see the Electri-
cal Characteristics Table), then the internal 5.3V LDO is
enabled. If the EXTV
source greater than this EXTV
the LDO is shut down and the internal EXTV
shorts the EXTV
ing DRV
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
minimum current.
Power for most internal control circuitry other than gate
drivers is derived from the INTV
ered from the combined DRV
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
places the LTC3838 into micropower shutdown mode
with a minimum I
(through the internal 5.3V LDO regulator or EXTV
the corresponding channel’s internal circuitry off INTV
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
IN
IN
pin still needs to be powered up but now draws
CC
and can output 5.3V to DRV
CC
and INTV
pin is below the EXTV
CC
CC
Q
pin to the DRV
switch (with on-resistance of around
at the V
CC
CC
(Refer to Functional Diagram)
pin is tied to an external voltage
CC
with the external voltage source
pin to DRV
IN
CC
CC
pin. The LTC3838’s DRV
CC
pins through an external
switchover voltage, then
pin. INTV
CC2
CC
pin, thereby power-
CC2
CC
switchover voltage
CC1
and DRV
.
. Alternatively,
CC
can be pow-
CC
CC
CC
switch
) and
and
CC
CC
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
The start-up of a channel’s output voltage (V
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
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
DRV
lockout (UVLO) thresholds.
Light Load Current Operation
If the MODE/PLLIN pin is tied to INTV
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.
CC1,2
pins drop below their respective undervoltage
IN
.
OUT
ramps up smoothly to
CC
CC
or an external clock
LTC3838
or either of the
15
OUT
3838fa
) is