Datasheet

LTC3835
10
3835fd
OPERATION
(Refer to Functional Diagram)
Main Control Loop
The LTC3835 uses a constant-frequency, current mode
step-down architecture. During normal operation, the
external top MOSFET is turned on when the clock sets the
RS latch, and is turned off when the main current compara-
tor, 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 volt-
age 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 cause 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, an internal 5.25V low dropout linear regulator
supplies INTV
CC
power from V
IN
. If EXTV
CC
is taken above
4.7V, the 5.25V regulator is turned off and a 7.5V low
dropout linear regulator is enabled that supplies INTV
CC
power from EXTV
CC
. If EXTV
CC
is less than 7.5V (but
greater than 4.7V), the 7.5V regulator is in dropout and
INTV
CC
is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 7.5V (up to an absolute maximum rating
of 10V), INTV
CC
is regulated to 7.5V. Using the EXTV
CC
pin allows the INTV
CC
power to be derived from a high
efficiency external source such as one of the LTC3835
switching regulator outputs.
The top MOSFET driver is biased from the floating bootstrap
capacitor C
B
, which normally recharges during each off
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 (RUN and TRACK/SS Pins)
The LTC3835 can be shut down using the RUN pin. Pulling
this pin below 0.7V shuts down the main control loop of the
controller. A low disables the controller and most internal
circuits, including the INTV
CC
regulator, at which time the
LTC3835 draws only 10µA of quiescent current.
Releasing the RUN pin allows an internal 0.5µA current
to pull up the pin and enable that controller. Alternatively,
the RUN pin may be externally pulled up or driven directly
by logic. Be careful not to exceed the Absolute Maximum
rating of 7V on this pin.
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 0.8V internal reference,
the LTC3835 regulates the V
FB
voltage to the TRACK/SS
pin voltage instead of the 0.8V reference. This allows
the TRACK/SS pin to be used to program a soft start by
connecting an external capacitor from the TRACK/SS pin
to SGND. An internal 1µA pull-up current charges this
capacitor creating a voltage ramp on the TRACK/SS pin.
As the TRACK/SS voltage rises linearly from 0V to 0.8V
(and beyond), the output voltage V
OUT
rises smoothly
from zero to its final value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of V
OUT
to “track” that of another supply. Typically,
this requires connecting to the TRACK/SS pin an external
resistor divider from the other supply to ground (see
Applications Information section).
When the RUN pin is pulled low to disable the LTC3835, or
when V
IN
drops below its undervoltage lockout threshold
of 3.5V, the TRACK/SS pin is pulled low by an internal
MOSFET. When in undervoltage lockout, the controller is
disabled and the external MOSFETs are held off.