Datasheet

LT8705
31
8705fb
For more information www.linear.com/LT8705
applicaTions inForMaTion
Also, because of their use with the Boost Capacitor
Charge Control block, tie the CSPIN and CSNIN pins
to V
IN
and tie the IMON_IN pin to ground when the input
current sensing is not in use. Similarly, the CSPOUT and
CSNOUT pins should be tied to V
OUT
and IMON_OUT
should be grounded when not in use.
The remaining discussion refers to the input current moni
-
tor circuit. All discussion and equations are applicable to
the output current monitor circuit, substituting pin and
device names as appropriate.
Current Monitoring: For input current monitoring, cur
-
rent flowing through R
SENSE1
develops a voltage across
CSPIN and CSNIN which is multiplied by 1mA/V (typical),
converting it to a current that is forced out of the IMON_IN
pin and into resistor R
IMON_IN
(Note: Negative CSPIN to
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:
V
IMON_IN
=I
RSENSE1
• R
SENSE1
•1m
A
V
•R
IMON_IN




For accurate current monitoring, the CSPIN and CSNIN
voltages should be kept above 1.5V (CSPOUT and CSNOUT
pins should be kept above 0V). Also, the differential volt
-
age V
CSPIN-CSNIN
should be kept below 100mV due to
the limited amount of current that can be driven out of
IMON_IN. Finally, the IMON_IN voltage must be filtered
with capacitor C
IMON_IN
because the input current often
has ripple and discontinuities depending on the LT8705’s
region of operation. C
IMON_IN
should be chosen by the
equation:
C
IMON_IN
>
100
f•R
IMON_IN




F
where f is the switching frequency, to achieve adequate
filtering. Additional capacitance, bringing the C
IMON_IN
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.
Current Limiting: As shown in Figure 10, IMON_IN voltages
exceeding 1.208V (typical) cause the V
C
voltage to reduce,
thus limiting the inductor and input currents. R
IMON_IN
can
be selected for a desired input current limit using:
R
IMON_IN
=
1.208V
I
RSENSE(LIMIT)
•1m
A
V
• R
SENSE1




Ω




For example, if R
SENSE1
is chosen to be 12.5mΩ and the
desired input current limit is 4A then:
R
IMON_IN
=
1.208V
4A •1m
A
V
•12.5mΩ
=24.2kΩ
Review the Electrical Characteristics and the IMON Output
Currents graph in the Typical Performance Characteris-
tics section to understand the operational limits of the
I
MO
N_OUT and IMON_IN currents.
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:
I
RSENSE1(FAULT)
=
1.61V
1.208V
•I
RSENSE1(LIMIT)




A
For example, an input current limit set to 4A would have
a fault current limit of 5.3A.
Output Overvoltage
If the output voltage is higher than the value set by the
FBOUT resistor divider, the LT8705 will respond according
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
an
d the output will be allowed to remain high.