LTC3873 Datasheet, PDF (7/16 Pages) Linear Technology – No RSENSETM Constant Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller

LTC3873
OPERATION
Main Control Loop
The LTC3873 is a general purpose N-channel switching
DC/DC converter for boost, flyback and SEPIC applications.
Its No R_SENSE sensing technique improves efficiency,
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 amplifier, 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.
L
V_IN
V_CC
SW
LTC3873
NGATE
GND
GND
D
+
V_SW
V_OUT
C_OUT
3873 F01
Figure 1. SW Pin (Internal Sense Pin)
Connection for Maximum Efficiency
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 efficiency 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 insufficient
voltage. The voltage at the V_CC pin must exceed V_TURNON
LTC3873
SHUT DOWN
L
V_IN
V_CC
NGATE
LTC3873
SW
GND
GND
D
V_{SW +}
V_OUT
C_OUT
R_SENSE
3873 F02
Figure 2. SW Pin (Internal Sense Pin)
Connection for Sensing Resistor
V_IN < V_TURNOFF
(NOMINALLY 4V)
V_RUN/SS < V_SHDN
(NOMINALLY 0.8V)
V_RUN/SS > V_SHDN
AND V_IN > V_TURNON
(NOMINALLY 8.4V)
LTC3873
ENABLED
3873 F03
Figure 3. Start-Up/Shutdown State Diagram
3873fa
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