LTC3851 LINER [Linear Technology], LTC3851 Datasheet - Page 10

• Manufacturer Part Number: LTC3851
• Description: Synchronous Step-Down Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC3851
Main Control Loop
The LTC3851 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
RS latch. The peak inductor current at which I
the RS latch is controlled by the voltage on the I
which is the output of the error ampli fi er EA. The V
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
relative to the 0.8V reference, which in turn causes the
I
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
next cycle.
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
internal 5V low dropout linear regulator supplies INTV
power from V
The top MOSFET driver is biased from the fl oating boot-
strap capacitor, C
off 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
detec tor detects this and forces the top MOSFET off for
about 1/10 of the clock period every tenth cycle to allow
C
always a load present during the drop-out transition to
ensure C
Shutdown and Start-Up (RUN and TK/SS)
The LTC3851 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
the RUN pin allows an internal 2μA current to pull up the
10
TH
B
to recharge. However, it is recommended that there is
voltage to increase until the average inductor current
CC
Power
B
OUT
is recharged.
, the loop may enter dropout and attempt
IN
.
B
, which normally recharges during each
REV
IN
, or the beginning of the
, decreases to a voltage
CC
regulator. Releasing
CMP
, resets the
CMP
CC
pin. An
TH
resets
FB
pin,
pin
CC
FB
pin and enable that control ler. 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
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 LTC3851 regulates the V
TK/SS pin voltage instead of the 0.8V reference. This al-
lows 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
value. Alternatively, the TK/SS pin can be used to cause
the start-up of V
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
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 LTC3851 can be enabled to enter high effi ciency 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
To select pulse-skipping mode of operation, fl oat the
MODE/PLLIN pin or tie it to GND. To select Burst Mode
operation, tie MODE/PLLIN to INTV
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
average inductor current is higher than the load current,
OUT
OUT
TH
rises smoothly from zero to its fi nal
to “track” another supply. Typically,
pin indicates a lower value. If the
CC
through a resistor
FB
voltage to the
OUT
3851f
, is
CC
CC
.