5
DC2769A-A-KIT
Rev. 0
OPERATION
The DC2769A-A-KIT demonstrates operation of a mag-
netically coupled resonant Wireless Power Transfer (WPT)
system. The LTC6990 based transmitter provides efficient
wireless power for the LTC4124 receiver to charge the
Li-Ion battery.
DC2771A-A – Wireless Power Transmitter Board
Featuring the LTC6990
The DC2771A-A is a wireless power transmitter board
using LTC6990 oscillator. The NMOS, M1(see DC2771A-A
schematic on Page 7), is driven by a 50% duty cycle square
wave generated by the oscillator. During the first half of
the cycle, M1 is switched on and the current through
the LC resonant tank rises linearly. During the second
half of the cycle, M1 is switched off and the current cir-
culates through the LC resonant tank. If the transmit LC
tank frequency is set to 1.29 times the driving frequency,
which is 1MHz in this application, switching losses in
M1 are significantly reduced due to the zero-voltage
switching (ZVS). Figure 10 illustrates the ZVS operation
in DC2771A-A. The peak voltage of the transmit resonant
tank that appears at the drain of M1 is:
V
IN
=
1
.
038
•π•
V
IN
This equation is derived by performing voltage second
balance equation on the resonant tank inductor.
V
GS_M1
2V/DIV
V
DS_M1
5V/DIV
DC2769AA F10
Figure 10. ZVS Operation on M1 when f
TX_TANK
= 1.29
•
f
DRIVE
DC2775A-A – Wireless Power Receiver Board
Featuring the LTC4124
The DC2775A-A demo board implements a parallel res-
onant LC circuit that connects the LTC4124 between ACIN
and GND pins. The AC waveform on the resonant circuit
is rectified by the internal wireless power managing cir-
cuitry to DC voltage on V
CC
pin. This DC source is then
fed into the internal linear battery charger to charge a
Li-Ion battery.
As shown in Figure 11, when the LTC4124 receives more
energy than it needs to charge the battery, the wireless
power manager in the IC keeps the input voltage to the
IC, V
CC
, low by shunting the receiver resonant tank to
ground. In this way, the linear charger is highly efficient
as its input is always kept just above the battery voltage,
V
BAT
. The resonant tank also receives less power when
the shunting circuit is engaged, as the resonant frequency
is detuned from the transmitter frequency.
The LTC4124 includes a full featured CC/CV (Constant
Current/Constant Voltage) linear battery charger with
trickle current pre-charge, safety timer termination, bad
battery detection, temperature qualified safe charging and
automatic recharge. The maximum charge current sup-
ported by DC2775A-A is 10mA and the charge voltage is
programmable by VSEL
X
jumpers.
V
CC
2V/DIV
V
ACIN
2V/DIV
V
BAT
2V/DIV
DC2769AA F11
Figure 11. Rectification of AC Input and Regulation of V
CC
