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dc2181afb
DEMO MANUAL
DC2181A-A/B
theory of operation
Figure 7. DC1968A Basic Transmitter
The DC2181A demo board demonstrates operation of a
double tuned magnetically coupled resonant power transfer
circuit. The DC2181A demo Board must be used in con-
junction with either the DC1968A wireless power basic
transmitter or the PowerByProxi ProxiPoint transmitter.
For theory of operation of the PowerByProxi ProxiPoint
transmitter, please refer to the ProxiPoint documentation.
DC1968A – Basic Transmitter
The DC1968A basic transmitter is used to transmit wire-
less power and is used in conjunction with the DC2181A
wireless power receiver board featuring the LTC4120.
The DC1968A is configured as a current fed astable multi-
vibrator, with oscillation frequency set by a resonant tank.
The DC1968A basic transmitter is set to 130kHz operation
and the DC1967A LTC4120 demonstration board resonant
frequency is 127kHz with DHC enabled and 140kHz with
DHC disabled. For the DC1968A basic transmitter the
resonant components are the 2X 0.15µF PPE film capaci-
tors (Cx1 and Cx2) and the 5.0µH (Lx) transmit coil (see
Schematic: Basic Inductive Transmitter with Pre-
Regulator). This gives a resonant frequency of 129.95kHz.
The tolerance on the transmit coil and resonant capacitors
is ±2%, or 2.6kHz. Inductors L1 and L2 are used to make
the resonant structure current fed.
The current fed topology makes the peak-to-peak voltage
on the resonant tank equal to 2πV
CC
. V
CC
is 5V, so the
peak-to-peak tank voltage is 31.5V, see Figure 7.
2µs/DIV
V
Cx-Cy
20V/DIV
V
Cx
10V/DIV
V
Cy
10V/DIV
DC2181A F07
The blue and green traces are the drains of the transmit-
ter MOSFETs M1 and M2 (see Schematic: Basic Inductive
Transmitter with Pre-Regulator), respectively. The red
trace is the difference (V
CX
– V
CY
) of those two nodes,
and shows that the resonant tank is producing a sine
wave. The peak-to-peak voltage of 2πV
CC
= 31.5V, results
from the current fed topology. This in turn determines
the breakdown of the MOSFETs and diodes D2 and D3.
To increase transmit power by raising V
CC
, you must also
change M1, M2, D2 and D3, to reflect the higher voltages
on the C
X
and C
Y
nodes.
The magnitude of the magnetic field is directly proportional
to the current in the transmit coil. For a resonant system
this current is Q times the input current. So the higher the
Q the larger the magnetic field. Therefore the transmit coil
is constructed with Litz wire, and the resonant capacitors
are very low dissipation PPS film capacitors. This leads
to a Q of approximately 10 at 130kHz, and a circulating
current of approximately 6A
P-P
, at full load.
DC2181 – Wireless Power Receiver Board Featuring
the LTC4120
The DC2181 LTC4120 wireless power receiver IC imple-
ments dynamic harmonization control (DHC), which tunes
or detunes the receive circuit to receive more or less
power as needed. The primary receive tank is composed
of AE1, and C2S, although it must be noted that C2S is AC
grounded through C5, the LTC4120 decoupling capacitor,
to be in parallel with AE1. C2S also serves to tap power
off the resonant circuit and send it to the LTC4120, (see
Schematic: 400mA Wireless Synchronous Buck Battery
Charger).
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