LTC4125
13
4125f
Input current limit is detected via the IMON pin. If the
voltage on the IMON pin exceeds 1.20V (V
ILIM
, typ) after
a valid exit condition is found, transmit power is reduced
to zero until the next search interval. Input current limit
is 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
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 1.20V (V
ILIM
, typ), the input current limit
is reached. Notice that for the same values of R
IN
, R
IMON
and R
IS
, this input current limit is 150% (typ) of the input
current threshold—one of the programmable valid exit
conditions.
The final fault condition used in the algorithm is the die
temperature of the LTC4125. If the internal die temperature
of the LTC4125 ever exceeds 150°C (typ), then transmit
power is immediately reduced to zero until the next search
interval. Unlike other fault conditions, the die temperature
fault is not limited to the duration of the Optimum Power
Search period.
I
TH
vs I
LIM
As noted in the previous two sections, there are two input
current parameters whose values are determined by R
IN
,
R
IMON
and R
IS
: I
TH
(input current threshold) and I
LIM
(input
current limit). When the input current exceeds I
TH
during
the optimum power search, the search will stop and the
LTC4125 maintains operation at or slightly above this
input current level. However, if the input current exceeds
I
LIM
at any point during operation, power transmission will
cease immediately until the next search interval. The input
current limit is 150% (typ) of the input current threshold.
V
PTH1
/V
PTH2
and Pulse Width
The pulse width of each half of the full bridge driver
can be monitored using the PTH1 and PTH2 pins. When
AutoResonant drive is enabled, the pulse width is:
PW
SWx
(s)
=
0.24
f
n
•
V
PTHx
⎛
⎝
⎜
⎞
⎠
⎟+
150ns
where f
n
is the full bridge resonant frequency, and 0.24 is
the typical normalized PTH voltage to Pulse Width Gain.
During the Optimum Power Search period, as the pulse
width increases, the voltage on the PTH pins increases as
well. When V
PTH1
or V
PTH2
exceeds 2.4V, the maximum
pulse width is guaranteed to have been reached, and the
end of search ramp fault condition stops power delivery
until the next search interval. Again, this typically indicates
that no receiver is present or that a conductive foreign
object is present.
PTHM
The pulse width of the first step in the Optimum Power
Search can be programmed using the PTHM pin. This fea-
ture helps the Optimum Power Search find the appropriate
pulse width when the minimum transmit power levels of
the full bridge are known. This requires characterization
of the application to know that the optimum operating
point is always above a certain pulse width for all condi-
tions. When PTHM is connected to ground, the first step
defaults to 150ns.
operaTion
