LTC1871-7
8
18717fd
operaTion
Main Control Loop
The LTC1871-7 is a constant frequency, current mode
controller for DC/DC boost, SEPIC and flyback converter
applications. With the LTC1871-7 the current control loop
can be closed by sensing the voltage drop either across
the power MOSFET switch or across a discrete sense
resistor, as shown in Figure 2.
The nominal operating frequency of the LTC1871-7 is
programmed using a resistor from the FREQ pin to ground
and can be controlled over a 50kHz to 1000kHz 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.
The RUN pin controls whether the IC is enabled or is in a low
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 LTC1871-7 can be used either by sensing the volt-
age 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
the output voltage to the maximum rating for this pin (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 microamps
and Burst Mode operation should be applied (i.e., the
MODE/SYNC pin should be connected to ground).
For circuit operation, please refer to the Block Diagram of
the IC and Figure 1. In normal operation, the power MOSFET
is turned on when the oscillator sets the PWM latch and
is turned off when the current comparator C1 resets the
latch. The divided-down output voltage is compared to an
internal 1.230V reference by the error amplifier EA, which
outputs an error signal at the I
TH
pin. The voltage on the
I
TH
pin sets the current comparator C1 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 C1 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.
Figure 2. Using the SENSE Pin On the LTC1871-7
C
OUT
V
SW
V
SW
2a. SENSE Pin Connection for
Maximum Efficiency (V
SW
< 36V)
V
OUT
V
IN
GND
L
D
+
C
OUT
R
S
18717 F02
2b. SENSE Pin Connection for Precise
Control of Peak Current or for V
SW
> 36V
V
OUT
V
IN
GND
L
D
+
GATE
GND
V
IN
SENSE
GATE
GND
V
IN
SENSE
