LTC1700_ 查看數據表（PDF） - Linear Technology
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LTC1700_ Datasheet PDF : 32 Pages
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LTC1871-7
U
OPERATIO
resistors R1 and R2 are typically chosen so that the error
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 LTC1871-7 contains an independent, micropower
voltage 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:
VIN(OFF)
=
1.248V
•
1+
R2
R1
VIN(ON)
=
1.348V
• 1+
R2
R1
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” operation.
+
R2
INPUT
SUPPLY OPTIONAL
FILTER
R1
CAPACITOR
–
VIN
RUN
+
6V
–
RUN
COMPARATOR
BIAS AND
START-UP
CONTROL
1.248V
µPOWER
REFERENCE
GND
18717 F8a
Figure 8a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin
RUN
EXTERNAL
LOGIC CONTROL
6V
1.248V
RUN
COMPARATOR
+
–
18717 F08b
Figure 8b. On/Off Control Using External Logic
12
+
R2
1M
INPUT
SUPPLY
VIN
RUN
+
6V
–
RUN
COMPARATOR
–
GND 1.248V
18717 F08c
Figure 8c. External Pull-Up Resistor On
RUN Pin for “Always On” Operation
18717f
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