LTC3834 Linear Technology, LTC3834 Datasheet - Page 10

• Manufacturer Part Number: LTC3834
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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OPERATION
Main Control Loop
The LTC3834 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
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
When the EXTV
than 4.7V, an internal 5.25V low dropout linear regulator
supplies INTV
4.7V, the 5.25V regulator is turned off and a 7.5V low
dropout linear regulator is enabled that supplies INTV
power from EXTV
greater than 4.7V), the 7.5V regulator is in dropout and
INTV
is greater than 7.5V (up to an absolute maximum rating
of 10V), INTV
pin allows the INTV
effi ciency external source such as one of the LTC3834
switching regulator outputs.
LTC3834
10
CC
CC
/EXTV
is approximately equal to EXTV
OUT
CC
, to ground) to the internal 0.800V reference
FB
CC
CC
CC
relative to the reference, which cause the
Power
power from V
is regulated to 7.5V. Using the EXTV
pin is left open or tied to a voltage less
CC
TH
TH
CC
. If EXTV
(Refer to Functional Diagram)
pin, which is the output of the error
voltage until the average inductor
power to be derived from a high
FB
pin, (which is generated with
IN
CC
. If EXTV
is less than 7.5V (but
CC
CC
. When EXTV
is taken above
CC
pin.
CC
CC
CC
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
Shutdown and Start-Up (RUN and TRACK/SS Pins)
The LTC3834 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
LTC3834 draws only 4μA 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 LTC3834 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).
B
to recharge.
OUT
B
, which normally recharges during each off
OUT
, the loop may enter dropout and attempt
to “track” that of another supply. Typi-
CC
FB
regulator, at which time the
IN
voltage to the TRACK/SS
decreases to a voltage
OUT
OUT
is controlled by
rises smoothly
3834fb