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LTC4125EUFD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28
LTC4125
19
4125f
For more information www.linear.com/LTC4125
Figure12 shows the difference in LTC4125 behavior when
a conductive foreign object is placed on the transmit coil,
with or without a frequency limit programmed at the FTH
pin. Again, the same circuit in Figure7 is used.
Note that without the FTH pin programmed (tied to V
IN
),
the LTC4125 does not detect a valid receiver circuit, and
therefore limits the power delivered to a foreign object to
only pulses of power that are generated during a search
interval. Without a valid receiver, the search fails to find
a valid exit condition until it reaches the end of the power
search ramp fault condition, which causes the transmitter
to stop delivering power before the next search interval.
TIME (s)
0.00
V
PTH
(V)
2.5
1.5
0.5
2.0
1.0
0.0
V
FB
(V)
1.0
0.6
0.2
0.8
0.4
0.0
0.800.40
4125 F12
1.000.600.20
V
FB
WITHOUT FTH
V
FB
WITH
FTH
V
PTH
WITHOUT
FTH
V
PTH
WITH FTH
Figure12. Comparison of the PTH and FB Pins Waveforms
with and without the FTH Pin Programmed to Detect the
Presence of a Conductive Foreign Object
Therefore, without using FTH, these pulses of power will
continue to deliver a limited amount of power to the foreign
object. To eliminate even this small amount of transmitted
power, the FTH pin can be programmed to about 10% to
15% higher than the expected resonant frequency (as
determined by the tank inductance and capacitance). If
this frequency limit is exceeded at any point during the
search interval (typically at the first step), the LTC4125 will
cease to deliver any power to the object and the STAT pin
will be set to high impedance to indicate that the transmit
coil is not delivering any power.
In the example shown in Figure7, the tank frequency is
103kHz, and the frequency threshold is set to be 119kHz,
with R
FTH2
=59kΩ and R
FTH1
=100kΩ.
An internal frequency to voltage converter creates a volt-
age representation of this AutoResonant Drive frequency
(
Block Diagram
). When a foreign conductive object is
brought close to the transmit coil, the apparent inductance
of the transmit coil is dramatically reduced and the driving
frequency of the LTC4125 adjusts to a higher frequency.
Figure11 shows the contrast between the tank voltage
frequency with and without the presence of a small con
-
ductive foreign object. The circuit in Figure7 is
used to
generate this figure with the two PTH pins shorted together
and driven at 0.5V, and a 15mm × 15mm copper square
plate placed directly on top of the coil as a conductive
foreign object.
TIME (µs)
0
VOLTAGE (V)
50
–20
30
10
0
–10
40
20
–50
–30
–40
35 4015 20
4125 F11
5025 3010 455
f = 101kHz
f = 301kHz
V
PTH1
= V
PTH2
= 0.5V
Figure11. Comparison of the LC Tank Voltage Frequency without
and with the Presence of a Conductive Foreign Object
The frequency limit is programmed via the FTH pin with
the following formula:
f
LIM
=
V
FTH
V
IN
• 320kHz =
R
FTH2
R
FTH1
+ R
FTH2
• 320kHz
Note that the internal frequency to voltage converter is
discretized to 7 bits with a full input range between 0kHz
and 320kHz. Therefore, the accuracy of the frequency
threshold input is limited to ±2.5kHz. The total resistance
of R
FTH1
plus R
FTH2
is recommended to be in the order
of 100kΩ.
applicaTions inForMaTion
LTC4125
20
4125f
For more information www.linear.com/LTC4125
Referring to Figure6 and Figure13, the two timing intervals
that use CTS frequency are T1—the wait time after the
initial reset at the beginning of the search, and T2—the
settling time after each pulse width step. The timing interval
that uses CTD frequency is T3—the delay time from the
end of one search to the beginning of the next search. The
three values are related to the timer frequencies as follows:
T1=
256
f
CTS
T2 =
32
f
CTS
=
T1
8
T3 =
65 • 10
3
f
CTD
For the recommended C
TS
=4.7nF and C
TD
=470pF, these
timing intervals are T1=144ms, T2= 18ms, and T3=3.7s.
The values of T1 and T2 need to be large enough such that
the system has time to settle back to its zero value after
reset (T1), and to settle to its new value after each step
(T2). For the recommended resonant frequency range of
50kHz to 250kHz, a starting value for the recommended
C
TS
capacitor value is 4.7nF.
TIMER CAPACITORS—C
TS
AND C
TD
The capacitor connected to the CTS pin (C
TS
) sets the CTS
frequency (f
CTS
) which determines the step settling time
in the Optimum Power Search. This CTS frequency can
be programmed as follows:
f
CTS
=
10µA
C
TS
• 1.2V
where 10µA is the typical I
CTS,PU
and I
CTS,PD
.
Similarly the capacitor connected to the CTD pin (C
TD
) sets
the CTD frequency that can be programmed as follows:
f
CTD
=
10µA
C
TD
• 1.2V
where 10µA is the typical I
CTS,PU
and I
CTD,PD
.
applicaTions inForMaTion
4125 F13
T1 T 2 T2 T2 T2 T 2 T2 T2 T2 T2 T2T3
OPTIMUM SEARCH DURATION OPTIMUM SEARCH DURATION
T1 T3
V
PTH
1/V
PTH2
V
FB
Figure13. Timing Diagram of Typical Search Cycles
LTC4125
21
4125f
For more information www.linear.com/LTC4125
In some applications users may find that across all operat-
ing conditions, the pulse width never falls below a particular
value at the end of a sear
ch cycle. This indicates that the
lowest transmit power levels of the full bridge are not
required. If this is the case, the PTHM pin can be used to
program the size of the first step of the pulse width sweep
in the Optimum Power Search to reduce the search time.
This minimum pulse width value can be set according to
the following formula:
MINPW =
0.576
f
n
•
V
PTHM
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
+150ns
where 0.576 is the product of 0.24V
–1
(the typical normal-
ized PTH voltage to pulse width gain) and 2.4V (the typical
maximum output voltage at the P
TH pin).
Using a resistor divider between V
IN
and GND to set the
voltage at the PTHM pin, the formula is simplified as follows:
MINPW =
0.576
f
n
•
R
PTHM2
R
PTHM1
+R
PTHM2
⎛
⎝
⎜
⎞
⎠
⎟
+150ns
where f
n
is the resonant frequency of the LC tank.
Figure15 contrasts the Optimum Power Search behavior
when using PTHM versus when PTHM is grounded. The
circuit in Figure7 is used to generate Figure15, with PTHM
set to 1.6V in one case and grounded in the other. Again,
remember that V
PTHx
corresponds to the full bridge pulse
width while V
FB
corresponds to the transmit tank voltage.
TIME (s)
0.0
V
PTH
(V)
V
FB
(V)
1.0
0.6
0.2
0.8
0.4
0.0
2.5
1.5
0.5
2.0
1.0
0.0
0.40.2
4125 F15
0.60.30.1 0.5
V
PTH
SEARCH
TIME
WITHOUT
PTHM SET
SEARCH
TIME WITH
PTHM SET
PTHM
LEVEL
V
FB
V
FB
V
FB
V
PTH
Figure15. Comparison of the PTH Pins Voltage Steps During a
Sweep with PTHM at GND and Programmed at a Particular Value
The value of T3 determines the delay interval time between
each search. A starting value of 470pF for the C
TD
capacitor
sets this delay time between each search to 3.7s.
Figure 14 shows the voltage stepping at FB, PTH1 and
PTH2 for the circuit in Figure7 with C
TS
=4.7nF, showing
a successful sweep in finding an optimum power point.
Note that V
PTHx
corresponds to the full bridge pulse
width while V
FB
corresponds to the transmit tank voltage.
MINIMUM PULSE WIDTH (PTHM PIN)
In a typical search as shown in Figure14, the first pulse
width step is about 150ns. This corresponds to the mini
-
mum voltage on the PTHx pins (see the earlier V
PTH1
/V
PTH2
and Pulse Width section for more information).
TIME (s)
0.00
VOLTAGE (V)
2.5
1.5
0.5
2.0
1.0
0.0
0.400.20
4125 F14a
0.600.300.10 0.50
V
PTH1
= V
PTH2
V
FB
TIME (s)
0.40
VOLTAGE (V)
2.5
1.7
0.9
2.1
1.3
0.5
0.480.44
4125 F14b
0.500.460.42
V
PTH1
= V
PTH2
V
FB
Figure14. FB, PTH1 and PTH2 Pins Voltage Stepping
During a Sweep with C
TS
= 4.7nF
applicaTions inForMaTion
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28

LTC4125EUFD#PBF

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Buy LTC4125EUFD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Wireless Charging ICs 5W AutoResonant Wireless Pwr Transmitter
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