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LTC3704 数据表(PDF) 9 Page - Linear Technology

部件名 LTC3704
功能描述  Wide Input Range, No RSENSE??Current Mode Boost, Flyback and SEPIC Controller
Download  32 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC3704 数据表(HTML) 9 Page - Linear Technology

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LTC1871X
9
1871xf
For more information www.linear.com/LTC1871X
OPERATION
The nominal operating frequency of the LTC1871X is pro-
grammed using a resistor from the FREQ pin to ground
and can be controlled over a 65kHz to 900kHz range. In
addition, the internal oscillator can be synchronized to
an external clock applied to the MODE/SYNC pin and can
be locked to a frequency between 100% and 130% of its
nominal value. When the MODE/SYNC pin is left open, it
is pulled low by an internal 50k resistor and Burst Mode
operation is enabled. If this pin is taken above 2V or an
external clock is applied, Burst Mode operation is disabled
and the IC operates in continuous mode. With no load (or
an extremely light load), the controller will skip pulses
in order to maintain regulation and prevent excessive
output ripple.
TheRUNpincontrolswhethertheICisenabledorisinalow
current shutdown state. A micropower 1.248V reference
and comparator C2 allow the user to program the supply
voltage at which the IC turns on and off (comparator C2
has 100mV of hysteresis for noise immunity). With the
RUN pin below 1.248V, the chip is off and the input supply
current is typically only 10µA.
An overvoltage comparator OV senses when the FB pin
exceeds the reference voltage by 6.5% and provides a
reset pulse to the main RS latch. Because this RS latch is
reset-dominant, the power MOSFET is actively held off for
the duration of an output overvoltage condition.
The LTC1871X can be used either by sensing the voltage
drop across the power MOSFET or by connecting the
SENSE pin to a conventional shunt resistor in the source
of the power MOSFET, as shown in Figure 2. Sensing the
voltage across the power MOSFET maximizes converter
efficiency and minimizes the component count, but limits
theoutputvoltagetothemaximumratingforthispin(36V).
By connecting the SENSE pin to a resistor in the source
of the power MOSFET, the user is able to program output
voltages significantly greater than 36V.
Programming the Operating Mode
For applications where maximizing the efficiency at very
light loads (e.g., <100µA) is a high priority, the current
in the output divider could be decreased to a few micro-
amps and Burst Mode operation should be applied (i.e.,
the MODE/SYNC pin should be connected to ground).
In applications where fixed frequency operation is more
critical than low current efficiency, or where the lowest
outputrippleisdesired,pulse-skipmodeoperationshould
be used and the MODE/SYNC pin should be connected
to the INTVCC pin. This allows discontinuous conduction
mode (DCM) operation down to near the limit defined
by the chip’s minimum on-time (about 175ns). Below
this output current level, the converter will begin to skip
cycles in order to maintain output regulation. Figures 3
and 4 show the light load switching waveforms for Burst
Mode and pulse-skip mode operation for the converter
in Figure 1.
Burst Mode Operation
Burst Mode operation is selected by leaving the MODE/
SYNC pin unconnected or by connecting it to ground. In
normaloperation,therangeontheITHpincorrespondingto
no load to full load is 0.30V to 1.2V. In Burst Mode opera-
tion, if the error amplifier EA drives the ITH voltage below
0.525V, the buffered ITH input to the current comparator
C1 will be clamped at 0.525V (which corresponds to 25%
of maximum load current). The inductor current peak is
then held at approximately 30mV divided by the power
Figure 2. Using the SENSE Pin On the LTC1871
COUT
VSW
VSW
2a. SENSE Pin Connection for
Maximum Efficiency (VSW < 36V)
VOUT
VIN
GND
L
D
+
COUT
RS
1871x F02
2b. SENSE Pin Connection for Precise
Control of Peak Current or for VSW > 36V
VOUT
VIN
GND
L
D
+
GATE
GND
VIN
SENSE
GATE
GND
VIN
SENSE


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