LTC4125
15
4125f
For more information www.linear.com/LTC4125
(I
RMS-MAX
) before thermal rise (from 25°C ambient) in the
die causes the internal thermal shutdown to stop power
delivery in the coil.
In the specific application shown in Figure7, a 24μH coil
(760308100110) from Würth is used. It has a 50mm di
-
ameter, a Q value of 140 at 100kHz as well as a saturation
current greater than
10A.
T
RANSMITTER RESONANT CAPACITOR SELECTION
The factors to consider when selecting the transmitter
capacitor are similar to the factors discussed previously
when making the inductor choice: the capacitance value,
the capacitor quality factor (Q
C
), and the voltage rating
of the capacitor. The physical dimension of the capacitor
is usually not a big factor since overall application size is
driven mainly by the size of the transmit coil.
First and foremost the parameter to consider is the ca
-
pacitance value itself.
T
he LTC4125 is designed to work
with resonant frequencies between 50kHz and 250kHz.
The AutoResonant feature of the LTC4125 ensures that
the series LC network is driven at the resonant frequency
of the LC network:
f
o
=
2π LC
Another important factor is the parasitic dissipative com-
ponent of the capacitance. As with the inductor, one way
to measure this component is by looking at the quality
factor of
the capacitor. The capacitor quality factor is
described as:
Q
C
=
ωCR
C
=
2πfCR
C
where ω is the target frequency in radians, f is the target
frequency in Hz, and R
C
is the capacitor effective series
resistance. The higher the Q, the more ideal that particular
capacitor is at that frequency.
For a given value of inductance, frequency and current
amplitude, the voltage that is developed across the inductor
and the capacitor is well defined. The capacitor voltage
Table 1. Recommended Transmit Coils
MANUFACTURER PART NUMBER
INDUCTANCE
(µH)
SIZE
(mm)
QUALITY
FACTOR AT
100kHz
Würth 760308110 24 53 x 53 140
Würth 760308100110 24
Dia. 50
140
Würth 760308100111 6.3
Dia. 50
100
Inter Technical L41200T06 5 52 x 52 80
TDK WT505090-
20K2-A10-G
24
Dia. 50
50
TDK WT505090-
10K2-A11-G
6.3
Dia. 50
100
Another important parameter to consider is the inductance
value of the coil itself. This value needs to be considered in
relation to the receive coil inductance value and the overall
wireless power system coupling between the transmit and
the receive coil. The ratio of the two inductance values
together with the coupling factor determines the voltage
and current possible on the receive coil, and therefore the
power delivery capability of the system.
The quality factor of an inductor at a particular frequency
is defined as follows:
Q
L
=
ω
R
L
=
π
R
L
where ω is the target frequency in radians, f is the target
frequency in Hz, and R
L
is the inductor effective series
resistance. The higher the Q, the more efficient that par-
ticular inductor is in carrying current at that frequency.
A
typical 24µH
transmit coil that is used to deliver power
up to 5W across a 1mm to 15mm distance has a quality
factor of ≈50 to 150 at 100kHz operating frequency.
Many commercially available transmit coils use ferrite
material to help boost the inductance value as well as
shape the magnetic field created by the transmit coil to
increase coupling and power delivery. However, ferrite
material limits the saturation current level. The satura
-
tion current level needs to be higher than the maximum
current amplitude generated in the LC resonant structure
to ensure predictable inductance values and prevent po
-
tential thermal runaways. The monolithic switches inside
the LTC4125 allow switches RMS current of up to 3.5A
applicaTions inForMaTion
