LTC1871 Linear Technology, LTC1871 Datasheet - Page 12

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

APPLICATIO S I FOR ATIO
caused by the current flowing into the FB pin during
normal operation is less than 1% (this translates to a
maximum value of R1 of about 250k).
Programming Turn-On and Turn-Off Thresholds
with the RUN Pin
The LTC1871 contains an independent, micropower volt-
age reference and comparator detection circuit that re-
mains active even when the device is shut down, as shown
in Figure 8. This allows users to accurately program an
input voltage at which the converter will turn on and off.
The falling threshold voltage on the RUN pin is equal to the
internal reference voltage of 1.248V. The comparator has
100mV of hysteresis to increase noise immunity.
LTC1871
12
LOGIC CONTROL
Figure 8b. On/Off Control Using External Logic
EXTERNAL
U
RUN
SUPPLY
Figure 8a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin
INPUT
U
6V
1.248V
+
–
CAPACITOR
OPTIONAL
FILTER
W
+
–
1871 F08b
COMPARATOR
RUN
R2
R1
U
V
RUN
GND
IN
REFERENCE
6V
1.248V
POWER
The turn-on and turn-off input voltage thresholds are
programmed using a resistor divider according to the
following formulas:
The resistor R1 is typically chosen to be less than 1M.
For applications where the RUN pin is only to be used as
a logic input, the user should be aware of the 7V
Absolute Maximum Rating for this pin! The RUN pin can
be connected to the input voltage through an external 1M
resistor, as shown in Figure 8c, for “always on” operaton.
V
V
–
+
IN OFF
IN ON
SUPPLY
(
(
INPUT
)
)
COMPARATOR
Figure 8c. External Pull-Up Resistor On
RUN Pin for “Always On” Operation
+
–
1 348
1 248
RUN
.
.
1M
R2
V
V
•
•
START-UP
BIAS AND
CONTROL
1
V
RUN
GND
1
IN
1871 F8a
R
R
R
R
2
1
2
1
1.248V
6V
+
–
COMPARATOR
RUN
1871 F08c