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参数资料
| 型号： | LTC3872ETS8#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | 1 MM HEIGHT, LEAD FREE, PLASTIC, MO-193, TSOT-23, 8 PIN |
| 文件页数： | 20/22页 |
| 文件大小： | 245K |
| 代理商： | LTC3872ETS8#PBF |
LTC3872
7
3872fb
Main Control Loop
The LTC3872 is a No RSENSE constant frequency, current
mode controller for DC/DC boost, SEPIC and flyback
converterapplications.TheLTC3872isdistinguishedfrom
conventionalcurrentmodecontrollersbecausethecurrent
control loop can be closed by sensing the voltage drop
across the power MOSFET switch or across a discrete
senseresistor,asshowninFigures1and2.ThisNoRSENSE
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
outputvoltageiscomparedtoaninternal1.2Vreferenceby
the error amplifier, which outputs an error signal at the ITH
pin. The voltage on the ITH 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
ITH 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 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
MOSFETmaximizesconverterefficiencyandminimizesthe
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 typi-
cally only 10A. With an external capacitor connected to
the RUN/SS pin an optional external soft-start is enabled.
A 0.7A trickle current will charge the capacitor, pulling
the RUN/SS pin above shutdown threshold and slowly
rampingRUN/SStolimittheVITHduringstart-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 ITH range.
Light Load Operation
Under very light load current conditions, the ITH 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
comparatorremainstripped(evenatzeroloadcurrent)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.
Figure 1. SW Pin (Internal Sense Pin)
Connection for Maximum Efficiency
COUT
VSW
VOUT
VIN
GND
L
D
+
NGATE
GND
VIN
SW
3872 F01
LTC3872
COUT
VSW
RSENSE
VOUT
VIN
GND
L
D
+
NGATE
GND
VIN
SW
3872 F02
LTC3872
Figure 2. SW Pin (Internal Sense Pin)
Connection for Sensing Resistor
operaTion
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