Datasheet

LTC3873
7
3873fa
OPERATION
Main Control Loop
The LTC3873 is a general purpose N-channel switching
DC/DC converter for boost, fl yback and SEPIC applications.
Its No R
SENSE
sensing technique improves effi ciency,
increases power density and reduces the cost of the
overall solution.
For circuit operation, please refer to the Functional Diagram
of the IC and the Typical Application on the front page.
During normal operation, the power MOSFET is turned
on when the oscillator sets the PWM latch and is turned
off when the current comparator resets the latch. The
divided-down output voltage is compared to an internal
1.2V reference by the error amplifi er, which outputs an
error signal at the I
TH
pin. The voltage on the I
TH
pin
sets the current comparator input threshold. When the
load current increases, a fall in the V
FB
voltage relative to
the reference voltage causes the I
TH
pin to rise, causing
the current comparator to trip at a higher peak inductor
current value. The average inductor current will therefore
rise until it equals the load current, thereby maintaining
output regulation.
The LTC3873 can be used either by sensing the voltage
drop across the power MOSFET or by connecting the SW
pin to a conventional sensing resistor in the source of the
power MOSFET. Sensing the voltage across the power
MOSFET maximizes converter effi ciency and minimizes the
component count; the maximum rating for this pin, 60V,
allows MOSFET sensing in a wide output voltage range.
Shunt Regulator
A built-in shunt regulator from the V
CC
pin to GND limits
the voltage on the V
CC
pin to approximately 9.3V as long as
the shunt regulator is not forced to sink more than 25mA.
The shunt regulator permits the use of a wide variety of
powering schemes that exceed the LTC3873’s absolute
maximum ratings. Further details on powering schemes
are described in the Application Information section.
Start-Up/Shutdown
The LTC3873 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the
V
CC
supply pin voltage and a threshold RUN/SS pin. The
LTC3873 transitions into and out of shutdown according
to the state diagram shown in Figure 3.
The undervoltage lockout (UVLO) mechanism prevents the
LTC3873 from trying to drive a MOSFET with insuffi cient
voltage. The voltage at the V
CC
pin must exceed V
TURNON
Figure 1. SW Pin (Internal Sense Pin)
Connection for Maximum Effi ciency
Figure 2. SW Pin (Internal Sense Pin)
Connection for Sensing Resistor
Figure 3. Start-Up/Shutdown State Diagram
C
OUT
V
SW
V
OUT
V
IN
GND
L
D
+
NGATE
GND
V
CC
SW
3873 F01
LTC3873
C
OUT
V
SW
R
SENSE
V
OUT
V
IN
GND
L
D
+
NGATE
GND
V
CC
SW
3873 F02
LTC3873
LTC3873
SHUT DOWN
V
RUN/SS
< V
SHDN
(NOMINALLY 0.8V)
3873 F03
V
IN
< V
TURNOFF
(NOMINALLY 4V)
V
RUN/SS
> V
SHDN
AND V
IN
> V
TURNON
(NOMINALLY 8.4V)
LTC3873
ENABLED