LTC3834 Datasheet, PDF (10/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834
OPERATION (Refer to Functional Diagram)
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_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 LTC3834
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 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_CC regulator, at which time the
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_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 LTC3834 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. Typi-
cally, this requires connecting to the TRACK/SS pin an
external resistor divider from the other supply to ground
(see Applications Information section).
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