LT8705
APPLICATIONS INFORMATION
sensing is not in use. Similarly, the CSPOUT and CSNOUT
CurrentLimiting:AsshowninFigure10,IMON_INvoltages
pins should be tied to V
and IMON_OUT should be
exceeding1.208V(typical)causetheV voltagetoreduce,
OUT
C
grounded when not in use.
thuslimitingtheinductorandinputcurrents.R
be selected for a desired input current limit using:
can
IMON_IN
Theremainingdiscussionreferstotheinputcurrentmoni-
tor circuit. All discussion and equations are applicable to
the output current monitor circuit, substituting pin and
device names as appropriate.
1.208V
RIMON_IN
=
Ω
A
V
I
RSENSE(LIMIT) •1m •R
SENSE1
Current Monitoring: For input current monitoring, cur-
rent flowing through R
develops a voltage across
SENSE1
For example, if R
is chosen to be 12.5mΩ and the
SENSE1
CSPIN and CSNIN which is multiplied by 1mA/V (typical),
converting it to a current that is forced out of the IMON_IN
desired input current limit is 4A then:
1.208V
pin and into resistor R
(Note: Negative CSPIN to
IMON_IN
RIMON_IN
=
= 24.2kΩ
A
CSNIN voltages are not multiplied and no current flows
out of IMON_IN in that case). The resulting IMON_IN volt-
age is then proportional to the input current according to:
4A •1m •12.5mΩ
V
ReviewtheElectricalCharacteristicsandtheIMONOutput
Currents graph in the Typical Performance Characteris-
tics section to understand the operational limits of the
IMON_OUT and IMON_IN currents.
A
V
V
= IRSENSE1 • RSENSE1 •1m •R
IMON_IN
IMON_IN
For accurate current monitoring, the CSPIN and CSNIN
voltagesshouldbekeptabove1.5V(CSPOUTandCSNOUT
pins should be kept above 0V). Also, the differential volt-
Overcurrent Fault: If IMON_IN exceeds 1.61V (typical), a
fault will occur and switching activity will stop (see Fault
Conditions earlier in the data sheet). The fault current is
determined by:
age V
should be kept below 100mV due to
CSPIN-CSNIN
the limited amount of current that can be driven out of
IMON_IN. Finally, the IMON_IN voltage must be filtered
1.61V
1.208V
IRSENSE1(FAULT)
=
•I
RSENSE1(LIMIT)
A
with capacitor C
because the input current often
IMON_IN
has ripple and discontinuities depending on the LT8705’s
For example, an input current limit set to 4A would have
a fault current limit of 5.3A.
region of operation. C
equation:
should be chosen by the
IMON_IN
100
Output Overvoltage
CIMON_IN
>
F
f•R
IMON_IN
If the output voltage is higher than the value set by the
FBOUTresistordivider,theLT8705willrespondaccording
to the mode and region of operation. In forced continu-
ous mode, the LT8705 will sink current into the input (see
the Reverse Current Limit discussion in the Applications
Information section for more information). In discontinu-
ous mode and Burst Mode operation, switching will stop
and the output will be allowed to remain high.
where f is the switching frequency, to achieve adequate
filtering. Additional capacitance, bringing the C
total to 0.1μF to 1μF, may be necessary to maintain loop
stability if the IMON_IN pin is used in a constant-current
regulation loop.
IMON_IN
8705p
31
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