Theory Of Operation - Linear ADI Power DC3080A-KIT Demo Manual

DEMO MANUAL DC3080A-KIT

THEORY OF OPERATION

The DC3080A-KIT demonstrates the operation of
LTC4126-ADJ, wireless battery charger. This kit is com-
posed of a DC3081A transmitter board, a DC3078A
receiver board and two DC3079A 6mm application-sized
receiver boards.
DC3078A Wireless Charger Receiver Board
DC3078A features the LTC4126-ADJ, a wireless Li-Ion
charger with 1.2V step-down DC/DC converter. The charge
current in CC mode is 10mA. This charge current is pro-
grammed by the resistor on PROG pin of LTC4126-ADJ.
To efficiently receive energy from the transmitter, the
resonant frequency of this receiver board is set up to be
203kHz, matching the drive frequency of the transmitter
board. The LTC4126-ADJ rectifies the resonant AC volt-
age to generate V . When enough energy is received
CC
by the LTC4126-ADJ, V
CC
wireless power manager. As V
the resonant tank will be shunted to GND. In this way,
receiver stops receiving energy and V
until it reaches 5V. The resonant tank also receives less
power when the shunting circuit is engaged, as the reso-
nant frequency is detuned from the transmitter frequency.
V
CC
2V/DIV
V
BAT
2V/DIV
V
ACIN
2V/DIV
Figure 13. Rectification of AC Input and Regulation of DC Rail
The transmitter board also includes an external circuit
to achieve the trickle charge function. When a low volt-
age (<2.8V) battery is connected, the charge current is
reduced to 1mA until BAT pin voltage rises over 2.8V.
6
is kept between 5-5.5V by a
voltage reaches 5.5V,
CC
starts to drop
CC
Voltage
When BAT pin is lower than 2.8V before charging, the
comparator on board will inject current to the PROG pin
resistor once V is available. The effective current com-
CC
ing out from PROG pin is reduced. As PROG pin output
current is proportional to charge current, the charge cur-
rent is reduced. When the battery voltage is charged up
to above 2.8V, the comparator stops injecting current and
the charge current increases to the original programmed
level.
DC3081A Wireless Charger Transmitter Board
The DC3081A is a wireless power transmitter board,
using LTC6990 TimerBlox. The NMOS M1 is driven by
a 50% duty cycle square wave generated by the oscilla-
tor. During the first half of the cycle, M1 is switched on
and the current through the resonant tank rises linearly.
During the second half of the cycle, M1 is switched off and
the current circulates through the resonant tank. When
the transmit resonant tank frequency is set to about 1.29
times the driving frequency, M1 is turned on at zero volt-
age, and the switching loss is significantly reduced. The
peak voltage of the transmit resonant tank that appears
at the drain of M1 is :
V = 1.038 × π × V
IN 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
Figure 14. ZVS Operation on M1
(1)
Rev. 0