ltc3851-1 Linear Technology Corporation, ltc3851-1 Datasheet - Page 10

• Manufacturer Part Number: ltc3851-1
• Description: Synchronous Step-down Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

OPERATION
LTC3851-1
Main Control Loop
The LTC3851-1 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
resets the RS latch. The peak inductor current at which
I
the I
The V
compared to the internal reference voltage by the EA. When
the load current increases, it causes a slight decrease in
V
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 LTC3851-1 can be shut down using the RUN pin. Pull-
ing this pin below 1.1V disables the controller and most
of the internal circuitry, including the INTV
Releasing the RUN pin allows an internal 2μA current to
pull up the pin and enable that control ler. Alternatively,
the RUN pin may be externally pulled up or driven directly
10
CMP
TH
FB
B
to recharge. However, it is recommended that there is
voltage to increase until the average inductor current
relative to the 0.8V reference, which in turn causes the
TH
CC
resets the RS latch is controlled by the voltage on
FB
pin, which is the output of the error ampli fi er EA.
Power
pin receives the voltage feedback signal, which is
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
CC
regulator.
pin. An
CMP
CC
,
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 LTC3851-1 regulates the V
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
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 LTC3851-1 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
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,
the error amplifi er, EA, will decrease the voltage on the I
pin. When the I
sleep signal goes high (enabling “sleep” mode) and both
external MOSFETs are turned off.
CC
. To select pulse skipping mode of operation, fl oat
TH
OUT
OUT
voltage drops below 0.4V, the internal
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
OUT
38511f
, is
CC
TH