LTC4125
6
4125f
For more information
pin FuncTions
IN (Pin 1):
Input Supply Voltage: 3V to 5.5V. Supplies
power to the internal circuitry. A local 1µF bypass capacitor
to GND is recommended on this pin.
CTS (Pin 2):
Transmit Power Search Settling Time
Capacitor. Attach a capacitor from the CTS pin to GND
to program the transmit power search settling time.
Recommended settling times are typically between 1ms
and 20ms. See Applications Information for programming
instructions. While not recommended, short to IN to disable
the AutoResonant driver.
IS
–
(Pin 3):
Input Current Sense Negative Input. Connect
a current sense resistor (R
IS
) between the supply volt-
age and IS
–
using Kelvin Sense practices to monitor the
input supply current. Tie this pin to the IS
+
pin if no input
current monitoring is desired. Refer to the Applications
Information section for complete details.
IS
+
(Pin 4):
Input Current Sense Positive Input. Connect
this pin via an input current sense gain resistor (R
IN
) to the
supply voltage connected to the R
IS
sense resistor. This
pin sinks a current proportional to the voltage across the
sense resistor (R
IS
) which is used to generate the IMON
output (see Block Diagram):
I
IS
+
=
I
RIS
•
R
IS
R
IN
Tie this pin to the IS
–
pin if no input current monitoring
is desired. Refer to the Applications Information section
for complete details.
IMON (Pin 5):
Input Current Monitor. The IMON pin
sources a current that is proportional to the sense volt-
age across the sense resistor (R
IS
). With an output gain
resistor (R
IMON
), the voltage on this pin is expressed as
follows and corresponds directly to the input current (see
Block Diagram):
V
IIMON
=
I
RIS
•
R
IS
R
IN
•
R
IMON
=
R
IMON
R
IN
• Δ
V
RIS
Connect an appropriate capacitor in parallel with R
IMON
on
this pin to obtain a time-averaged voltage representation
of the input current (see Applications Information for more
details). If the voltage on this pin reaches 0.80V (V
ITH
, typ)
during a power search, an internal comparator indicates
that the input current threshold has been exceeded, and the
search is paused at this state until the next search interval.
The programmed input current threshold is determined
using the following formula:
I
TH
=
R
IN
R
IMON
•
V
ITH
R
IS
=
R
IN
R
IMON
•
0.80V
R
IS
If the voltage on the IMON pin exceeds 1.20V (V
ILIM
, typ)
at any point during the pause/delay time between the
search intervals, an internal comparator indicates that
input current limit has been exceeded, the power delivery
is immediately stopped, and a new search interval is initi-
ated. The programmed input current limit is determined
using the following formula:
I
LIM
=
R
IN
R
IMON
•
V
ILIM
R
IS
=
R
IN
R
IMON
•
1.20V
R
IS
Short this pin to GND to disable the input current monitor
feature.
NTC (Pin 6):
Thermistor Input. Connect a thermistor
from NTC to GND, and a corresponding resistor from IN
to NTC. The voltage level on this pin determines if the
thermistor temperature is within an acceptable range.
The power delivery is stopped if the thermistor indicates
a temperature that is too hot. This feature may be used
to detect the presence of a foreign metal object or other
transmission fault. Once the temperature returns to the
safe region, power delivery resumes. Refer to the Appli-
cations Information section for suggested usage. Leave
this pin open to disable the temperature qualified power
delivery feature.
DTH (Pin 7):
Delta FB Threshold Input. This pin is used
to adjust the minimum detected power step size in the
transmit power search to find the optimum transmitter
power operating point. The default setting (pin shorted to
IN) ensures proper operation in most systems. However,
in very low power or very weakly coupled systems a
smaller step size may be desired. Connect this pin to the
center tap point of a resistor divider between IN and GND.
Please refer to the Operation and Applications Information
sections for more details.
