ltc3873 Linear Technology Corporation, ltc3873 Datasheet - Page 6

• Manufacturer Part Number: ltc3873
• Description: No Rsense Constant Frequency Current Mode Boost/flyback/sepic Dc/dc Controller
• Manufacturer: Linear Technology Corporation

LTC3873
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
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
the current comparator input threshold. When the load
current increases, a fall in the V
reference voltage causes the I
current comparator to trip at a higher peak inductor cur-
rent value. The average inductor current will therefore
rise until it equals the load current, thereby maintaining
output regulation.
OPERATION
6
SENSE
Figure 1. SW Pin (Internal Sense Pin)
Connection for Maximum Effi ciency
Figure 2. SW Pin (Internal Sense Pin)
Connection for Sensing Resistor
GND
GND
V
V
IN
IN
sensing technique improves effi ciency,
TH
LTC3873
LTC3873
GND
V
GND
pin. The voltage on the I
V
NGATE
NGATE
CC
CC
SW
SW
L
L
TH
FB
V
V
R
SW
pin to rise, causing the
SW
SENSE
D
D
voltage relative to the
+
3873 F01
+
3873 F02
V
C
V
C
OUT
OUT
OUT
OUT
TH
pin sets
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
the voltage on the V
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
VCC 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
(NOMINALLY 4V)
V
IN
< V
TURNOFF
Figure 3. Start-Up/Shut Down State Diagram
CC
pin to approximately 9.3V as long as
SHUT DOWN
ENABLED
LTC3873
LTC3873
CC
V
(NOMINALLY 0.8V)
RUN/SS
pin must exceed V
3873 F03
< V
SHDN
CC
pin to GND limits
V
AND V
(NOMINALLY 8.4V)
RUN/SS
IN
> V
> V
TURNON
SHDN
TURNON
3873f