LTC3872-1_15 Datasheet, PDF (7/20 Pages) Linear Technology – No RSENSE Current Mode Boost DC/DC Controller

LTC3872-1
Operation
Main Control Loop
The LTC3872-1 is a No R_SENSE constant frequency, cur-
rent mode controller for DC/DC boost, SEPIC and flyback
converter applications. The LTC3872-1 is distinguished
from conventional current mode controllers because the
current control loop can be closed by sensing the voltage
drop across the power MOSFET switch or across a discrete
sense resistor, as shown in Figures 1 and 2. This 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 Block Diagram
of the IC and the Typical Application on the front page. In
normal operation, the power MOSFET is turned on when
the oscillator sets the RS 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 FB voltage relative to the reference voltage causes the
I_TH pin to rise, which causes 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 LTC3872-1 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
L
V_IN
V_IN
SW
LTC3872-1
NGATE
GND
GND
D
+
V_SW
V_OUT
C_OUT
38721 F01
Figure 1. SW Pin (Internal Sense Pin)
Connection for Maximum Efficiency
component count; the maximum rating for this pin, 60V,
allows MOSFET sensing in a wide output voltage range.
The RUN/SS pin controls whether the IC is enabled or is
in a low current shutdown state. With the RUN/SS pin
below 0.85V, the chip is off and the input supply current is
typically only 8µA. With an external capacitor connected to
the RUN/SS pin an optional external soft-start is enabled.
A 0.7µA trickle current will charge the capacitor, pulling
the RUN/SS pin above shutdown threshold and slowly
ramping RUN/SS to limit the V_ITH during start-up. Because
the noise on the SW pin could couple into the RUN/SS
pin, disrupting the trickle charge current that charges the
RUN/SS pin, a 1M resistor is recommended to pull-up
the RUN/SS pin when external soft-start is used. When
RUN/SS is driven by an external logic, a minimum of 2.75V
logic is recommended to allow the maximum I_TH range.
Light Load Operation
Under very light load current conditions, the I_TH pin volt-
age will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will assure that the current
comparator remains tripped (even at zero load current) and
the regulator will start to skip cycles, as it must, in order
to maintain regulation. This behavior allows the regulator
to maintain constant frequency down to very light loads,
resulting in low output ripple as well as low audible noise
and reduced RF interference, while providing high light
load efficiency.
L
V_IN
V_IN
NGATE
LTC3872-1
SW
GND
GND
D
V_{SW +}
V_OUT
C_OUT
R_SENSE
38721 F02
Figure 2. SW Pin (Internal Sense Pin)
Connection for Sensing Resistor
For more information www.linear.com/LTC3872-1
38721f
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