LTC4125
18
4125f
For more information
is reduced to zero until the next search interval. Input cur-
rent limit is also programmable using R
IN
, R
IMON
and R
IS
:
I
LIM
=
R
IN
R
IMON
•
V
ILIM
R
IS
=
R
IN
R
IMON
•
1.20V
R
IS
where 1.20V is the typical V
ILIM
.
As mentioned in the Operation section, for the same values
of R
IN
, R
IMON
and R
IS
, this input current limit is 150% of
the input current threshold.
Notice that the user has the ability to set the input current
threshold and limit by choosing values for three different
components. For most applications, the voltage drop across
R
IS
at the current limit threshold is recommended to be
less than 50mV, and the ratio of R
IMON
to R
IN
to be in the
range of 10-40, with R
IN
in the order of 10kΩ.
In the Figure 7 example, the desired current threshold and
limit are 1.3A and 1.95A respectively. The R
IS
is set to
be 20mΩ to limit the drop across it to 40mV at the input
current limit. With R
IN
set to 11.3kΩ, the R
IMON
value is
348kΩ, yielding the final current threshold and limit of
1.3A and 1.95A respectively.
If the input current is time varying or noisy, as would be
expected of a sinusoidal load of an LC tank, add filtering
capacitors C
IF
and C
IMON
to obtain a time average voltage
at the IMON pin that corresponds to the time average value
of the current through the input current sense resistor.
The value of C
IF
and C
IMON
should be selected such that
the time constants R
IN
C
IF
and R
IMON
C
IMON
are less than
T2—the settling time interval between each step in the
Optimum Power Search algorithm (Figure 6). This is to
ensure that a current threshold exit condition can be de-
tected within a single step in the search. In the example
IF
and C
IMON
are set to 10nF.
FREQUENCY THRESHOLD (FTH PIN)
As discussed in the Operation section, the AutoResonant
Drive used in the LTC4125 drives the external LC tank at
its resonant frequency. The frequency threshold input
(FTH) serves as the primary protection feature against
inadvertently transmitting power into a foreign object.
INPUT CURRENT LIMIT SETTING AND MONITORING
IS+
IS–
R
IS
3V TO 5.5V
C
IF
C
IMON
R
IMON
R
IN
LTC4125
4125 F10
IMON
1V
A1
A2
Figure 10. Input Current Limit and Monitoring
Figure 10 shows the architecture employed by the LTC4125
for the input current monitoring. The input current thresh-
old, used as one of the exit conditions in the proprietary
Optimum Power Search algorithm, is set using a combi-
nation of R
IS
, R
IN
and R
IMON
resistors according to the
following formula:
I
TH
A
( )
=
R
IN
R
IMON
•
V
ITH
R
IS
=
R
IN
R
IMON
•
0.80V
R
IS
where 0.80V is the typical V
ITH
.
The input current through the sense resistor R
IS
is avail-
able for monitoring through the IMON pin. The voltage at
the IMON pin varies with the current through the sense
resistor (R
IS
) as follows:
V
IMON
=
R
IMON
•
R
IS
R
IN
•
I
RIS
One of the fault conditions, the input current limit, is also
detected via the IMON pin. If the input current limit is
reached after a valid exit condition is found, transmit power
applicaTions inForMaTion
