Datasheet

LTC1871-1
12
18711fb
APPLICATIONS INFORMATION
error caused by the current fl owing into the FB pin dur-
ing 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 LTC1871-1 contains an independent, micropower volt-
age reference and comparator detection circuit that remains
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.
The turn-on and turn-off input voltage thresholds are
programmed using a resistor divider according to the
following formulas:
V
IN(OFF)
= 1.248V • 1+
R2
R
1






V
IN(ON)
= 1.348V • 1+
R2
R
1






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 con-
nected to the input voltage through an external 1M resistor,
as shown in Figure 8c, for “always on” operation.
Figure 8a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin
Figure 8c. External Pull-Up Resistor On
RUN Pin for “Always On” Operation
Figure 8b. On/Off Control Using External Logic
–
+
RUN
COMPARATOR
V
IN
RUN
R2
R1
INPUT
SUPPLY
OPTIONAL
FILTER
CAPACITOR
+
–
GND
18711 F8a
BIAS AND
START-UP
CONTROL
1.248V
µPOWER
REFERENCE
6V
–
+
RUN
COMPARATOR
1.248V
18711 F08b
RUN
6V
EXTERNAL
LOGIC CONTROL
–
+
RUN
COMPARATOR
V
IN
RUN
R2
1M
INPUT
SUPPLY
+
–
GND
1.248V
18711 F08c
6V