LTC3835-1 Linear Technology, LTC3835-1 Datasheet - Page 9

• Manufacturer Part Number: LTC3835-1
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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OPERATION
Main Control Loop
The LTC3835-1 uses a constant-frequency, current mode
step-down architecture. During normal operation, each
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
amplifi er EA. The error amplifi er compares the output volt-
age feedback signal at the V
an external resistor divider connected across the output
voltage, V
voltage. When the load current increases, it causes a slight
decrease in V
EA to increase the I
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
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
An internal 5.25V low dropout linear regulator supplies
INTV
The top MOSFET driver is biased from the fl oating bootstrap
capacitor C
cycle through an external diode when the top MOSFET
turns off. If the input voltage V
close to V
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
CC
CC
B
Power
power from V
to recharge.
OUT
OUT
B
, which normally recharges during each off
, to ground) to the internal 0.800V reference
FB
, the loop may enter dropout and attempt
relative to the reference, which cause the
TH
TH
(Refer to Functional Diagram)
IN
pin, which is the output of the error
voltage until the average inductor
.
FB
pin, (which is generated with
IN
decreases to a voltage
CC
pin.
Shutdown and Start-Up (RUN and TRACK/SS Pins)
The LTC3835-1 can be shut down using the RUN pin. Pulling
this pin below 0.7V shuts down the main control loop for the
controller. A low disables the controller and most internal
circuits, including the INTV
LTC3835-1 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
the voltage on the TRACK/SS pin. When the voltage on
the TRACK/SS pin is less than the 0.8V internal reference,
the LTC3835-1 regulates the V
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
from zero to its fi nal value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of V
cally, 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 LTC3835-1,
or when V
old 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.
IN
drops below its undervoltage lockout thresh-
OUT
to “track” that of another supply. Typi-
CC
regulator, at which time the
FB
voltage to the TRACK/SS
LTC3835-1
OUT
OUT
is controlled by
rises smoothly
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