LTC3868
11
3868fe
For more information www.linear.com/LTC3868
OPERATION
(Refer to the Functional Diagram)
The LTC3868 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal op
-
eration, each external top MOSFET is turned on when the
clock for that channel sets the RS latch, and is turned off
when the main current comparator, ICMP, resets the RS
latch. The peak inductor current at which ICMP trips and
resets the latch is controlled by the voltage on the I
TH
pin,
which is the output of the error amplifier, EA. The error
amplifier compares the output voltage feedback signal at
the V
FB
pin (which is generated with an external resistor
divider connected across the output voltage, V
OUT
, to
ground) to the internal 0.800V reference voltage. When the
load current increases, it causes a slight decrease in V
FB
relative to the reference, which causes the EA to increase
the I
TH
voltage until the average inductor current matches
the new load current.
After the top MOSFET is turned off each cycle, the bottom
MOSFET is turned on until either the inductor current starts
to reverse, as indicated by the current comparator IR, or
the beginning of
the next clock cycle.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
CC
pin. When
the EXTV
CC
pin is left open or tied to a voltage less than
4.7V, the V
IN
LDO (low dropout linear regulator) supplies
5.1V from V
IN
to INTV
CC
. If EXTV
CC
is taken above 4.7V,
the V
IN
LDO is turned off and an EXTV
CC
LDO is turned
on. Once enabled, the EXTV
CC
LDO supplies 5.1V from
EXTV
CC
to INTV
CC
. Using the EXTV
CC
pin allows the INTV
CC
power to be derived from a high efficiency external source
such as one of the LTC3868 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot
-
strap capacitor,
C
B
, which normally recharges during each
cycle through an external diode when the top MOSFET
turns off. If the input voltage, V
IN
, decreases to a voltage
close to V
OUT
, the loop may enter dropout and attempt
to turn on the top MOSFET continuously. The dropout
detector detects this and forces the top MOSFET off for
about one-twelfth of the clock period every tenth cycle to
allow
C
B
to recharge.
Shutdown and Start-Up (RUN1, RUN2
and SS1, SS2 Pins)
The
two channels of the LTC3868 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.26V shuts down the main control loop
for that controller. Pulling both pins below 0.7V disables
both controllers and most internal circuits, including the
INTV
CC
LDOs. In this state, the LTC3868 draws only 8µA
of quiescent current.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low impedance
source, do not exceed the absolute maximum rating of
8V. The RUN pin has an internal 11V voltage clamp that
allows the RUN pin to be connected through a resistor to a
higher voltage (for example, V
IN
), so long as the maximum
current into the RUN pin does not exceed 100µA.
The start-up of each controller’s output voltage, V
OUT
, is
controlled by the voltage on the SS pin for that channel.
When the voltage on the SS pin is less than the 0.8V
internal reference, the LTC3868 regulates the V
FB
volt-
age to the SS pin voltage instead of the 0.8V reference.
This
allows the SS pin to be used to program a soft-start
by connecting an external capacitor from the SS pin to
SGND. An internal 1µA pull-up current charges this ca
-
pacitor creating a voltage ramp on the SS pin. As the SS
voltage
rises linearly from 0V to 0.8V (and beyond up to
the absolute maximum rating of 6V), the output voltage
V
OUT
rises smoothly from zero to its final value.
Short-Circuit Latchoff
After the controller has been started and been given
adequate time to ramp up the output voltage, the SS
capacitor is used in a short-circuit timeout circuit. Spe-
cifically, once
the voltage on the SS pin rises above 2V
(the arming threshold), the short-circuit timeout circuit is
enabled (see Figure 1). If the output voltage falls below
70% of its nominal regulated voltage, the SS capacitor
begins discharging with a net 9µA pulldown current on
the assumption that the output is in an overcurrent and/or
short-circuit condition. If the condition lasts long enough
to allow the SS pin voltage to fall below 1.5V (the latchoff
threshold), the controller will shut down (latch off) until
the RUN pin voltage or the V
IN
voltage is recycled.
