LTC1871 Linear Technology, LTC1871 Datasheet - Page 8

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

OPERATIO
Main Control Loop
The LTC1871 is a constant frequency, current mode
controller for DC/DC boost, SEPIC and flyback converter
applications. The LTC1871 is distinguished from conven-
tional current mode controllers because the current con-
trol loop can be closed by sensing the voltage drop across
the power MOSFET switch instead of across a discrete
sense resistor, as shown in Figure 2. This sensing tech-
nique improves efficiency, increases power density, and
reduces the cost of the overall solution.
LTC1871
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 com-
pared to an internal 1.230V reference by the error amplifier
EA, which outputs an error signal at the I
on the I
threshold. When the load current increases, a fall in the FB
voltage relative to the reference voltage causes the I
8
Figure 2. Using the SENSE Pin On the LTC1871
TH
2b. SENSE Pin Connection for Precise
Control of Peak Current or for V
pin sets the current comparator C1 input
2a. SENSE Pin Connection for
Maximum Efficiency (V
GND
GND
V
V
IN
IN
U
GND
GND
V
V
SENSE
SENSE
IN
IN
GATE
GATE
L
L
V
V
R
SW
SW
SW
S
D
D
+
+
< 36V)
SW
TH
V
C
V
C
1871 F02
pin. The voltage
OUT
OUT
OUT
OUT
> 36V
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.
The nominal operating frequency of the LTC1871 is pro-
grammed 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 refer-
ence and comparator C2 allow the user to program the
supply voltage at which the IC turns on and off (compara-
tor 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 10 A.
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 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
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.