LTC1871EMS-7 Datasheet, PDF (8/36 Pages) Linear Technology – Wide Input Range, No RSENSE Current Mode Boost,

LTC1871
BLOCK DIAGRAM
SLOPE
COMPENSATION
FREQ
4
V-TO-I
OSC
0.6V
MODE/SYNC
I_OSC
5
50k
85mV
+ – ^OV
1.230V
+
FB
3
1.230V
I_TH
2
INTV_CC
8
– ^EA
g_m
+
5.2V
LDO
BURST
+ ^COMPARATOR
0.30V
–
1.230V
V-TO-I
SLOPE
– ^UV
+
2.00V
TO
START-UP
BIAS
CONTROL
V_IN
BIAS AND
START-UP
CONTROL
RUN
1
C2
1.248V
PWM LATCH
S
Q
R
LOGIC
CURRENT
COMPARATOR
C1
V_IN
9
INTV_CC
GATE
7
GND
SENSE
10
I_LOOP
1.230V
V_REF
R_LOOP
GND
6
1871 BD
OPERATION
Main Control Loop
The LTC1871 is a constant frequency, current mode con-
troller for DC/DC boost, SEPIC and flyback converter ap-
plications. The LTC1871 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 instead of across a discrete sense resistor,
as shown in Figure 2. This 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 Figure 1. In normal operation, the power MOSFET
8
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.
1871fe