LTC4125
12
4125f
For more information www.linear.com/LTC4125
operaTion
Exit Conditions
The Optimum Power Search employs many exit conditions
to ensure that the optimum transmit power is found during
a search across many different operating situations. The
primary exit conditions are not user programmable. Under
most operating conditions, these primary exit conditions
will produce the optimum transmit power.
However, two user programmable exit conditions are
provided to enable additional functionality and improved
performance in some scenarios: input current threshold
and differential tank voltage threshold. Input current
threshold is programmable using R
IN
, R
IMON
and R
IS
:
I
TH
=
IN
R
IMON
•
ITH
R
IS
=
IN
R
IMON
•
R
IS
Referring to the Block Diagram, V
IMON
is a gained up ver-
sion of the differential voltage across R
IS
. When V
IMON
is
greater than 0.80V (V
ITH
, typ), the input current threshold
is reached. When this occurs during an Optimum Power
Search interval, the search stops and the pulse width is
held until the next search interval.
The second user programmable exit condition sets a dif
-
ferential FB pin voltage threshold using the DTH pin.
D
uring
the Optimum Power Search, this threshold is compared
to the FB pin voltage increase resulting from one pulse
width step to the next. If the threshold is exceeded, the
exit condition is met. As described previously, when an
exit condition is met, the pulse width (i.e. transmit power
level) is held until the next search interval.
The DTH threshold is a useful exit condition when coupling
between the transmit and receive coils is poor. Shorting
the DTH pin to the IN pin will ensure that this exit condi
-
tion is ignored. This default setting is sufficient in most
applications.
Please refer to the Applications Information
section for details on how to program this pin.
Fault Conditions
A fault condition will cause the Optimum Power Search
to stop transmitting power immediately by keeping the
pulse width at zero until the next search interval. There
are six fault conditions: frequency (foreign object), NTC
(external temperature), over voltage, end of search ramp,
input current limit and internal (die) over temperature.
The frequency threshold is programmed by the FTH pin. If
the AutoResonant Drive frequency exceeds the frequency
threshold during the power search, then the search stops
and the pulse width is reduced to zero. This condition may
indicate the presence of a conductive foreign object. No
power is delivered until the next search interval.
An external over temperature condition is detected via
the NTC pin. If V
NTC
falls below the NTC Hot Threshold
(typically 35% of V
IN
) during the power search, then
the search stops and the pulse width is reduced to zero.
No power is delivered until the next search interval. The
NTC thermistor can be used to monitor the temperature
of the transmit coil to ensure safe operation of the coil.
Furthermore, the presence of a conductive foreign object
that generates heat when placed in the magnetic field of
the coil can also be sensed with this technique.
Excessive tank voltage is detected via the FB pin voltage. If
V
FB
exceeds V
IN
during the power search, then the search
stops and the pulse width is reduced to zero. No power is
delivered until the next search interval.
Another fault condition exists when the power search
ramp has reached its maximum pulse width (50% duty
cycle) and no optimum transmit power has been found.
This typically indicates that no receiver is present or that a
conductive foreign object is present between the transmit
and receive coils preventing any significant power from
being delivered to the receiver. Transmit power is reduced
to zero until the next search interval.
