LTC3851A-1
11
3851a1fa
operaTion
Main Control Loop
The LTC3851A-1 is a constant frequency, current mode
step-down controller. During normal operation, the top
MOSFET is turned on when the clock sets the RS latch,
and is turned off when the main current comparator, I
CMP
,
resets the RS latch. The peak inductor current at which
I
CMP
resets the RS latch is controlled by the voltage on
the I
TH
pin, which is the output of the error ampli fier, EA.
The V
FB
pin receives the voltage feedback signal, which is
compared to the internal reference voltage by the EA. When
the load current increases, it causes a slight decrease in
V
FB
relative to the 0.8V reference, which in turn causes the
I
TH
voltage to increase until the average inductor current
matches the new load current. After the top MOSFET has
turned off, the bottom MOSFET is turned on until either
the inductor current starts to reverse, as indicated by the
reverse current comparator, I
REV
, or the beginning of the
next cycle.
INTV
CC
Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
CC
pin. An
internal 5V low dropout linear regulator supplies INTV
CC
power from V
IN
.
The top MOSFET driver is biased from the floating boot-
strap 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
detec tor detects this and forces the top MOSFET off for
about 1/10 of the clock period every tenth cycle to allow
C
B
to recharge. However, it is recommended that there is
always a load present during the drop-out transition to
ensure C
B
is recharged.
Shutdown and Start-Up (RUN and TK/SS)
The LTC3851A-1 can be shut down using the RUN pin.
Pulling this pin below 1.1V disables the controller and
most of the internal circuitry, including the INTV
CC
regula-
tor. Releasing the RUN pin allows an internal 2µA 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 6V on this pin.
The start-up of the controller’s output voltage, V
OUT
, is
controlled by the voltage on the TK/SS pin. When the
voltage on the TK/SS pin is less than the 0.8V internal
reference, the LTC3851A-1 regulates the V
FB
voltage to
the TK/SS pin voltage instead of the 0.8V reference. This
allows the TK/SS pin to be used to program a soft-start
by connecting an external capacitor from the TK/SS pin to
GND. An internal 1µA pull-up current charges this capacitor
creating a voltage ramp on the TK/SS pin. As the TK/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 TK/SS pin can be used to cause
the start-up of V
OUT
to track another supply. Typically,
this requires connecting to the TK/SS pin an external
resistor divider from the other supply to ground (see the
Applica tions Information section). When the RUN pin
is pulled low to disable the controller, or when INTV
CC
drops below its undervoltage lockout threshold of 3.2V,
the TK/SS pin is pulled low by an internal MOSFET. When
in undervoltage lockout, the controller is disabled and the
external MOSFETs are held off.
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Continuous Conduction)
The LTC3851A-1 can be enabled to enter high efficiency
Burst Mode operation, constant frequency pulse-skipping
mode or forced continuous conduction mode. To select
forced continuous operation, tie the MODE/PLLIN pin to
INTV
CC
.
To select pulse-skipping mode of operation, float
the MODE/PLLIN pin or tie it to GND. To select Burst Mode
operation, tie MODE/PLLIN to INTV
CC
through a resistor
no less than 50k, but no greater than 250k.
When the controller is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-forth of the maximum sense voltage even though
the voltage on the I
TH
pin indicates a lower value. If the
average inductor current is higher than the load current,
the error amplifier, EA, will decrease the voltage on the I
TH
pin. When the I
TH
voltage drops below 0.4V, the internal
sleep signal goes high (enabling “sleep” mode) and both
external MOSFETs are turned off.
