LTC3335 datasheet(13/32 Pages) AD

13 / 32 page

LTC4120/LTC4120-4.2

Rev. G

For more information www.analog.com

Figure 5. Resonant Receiver Tank

C2S

1:n

C2P

DHC

LTC4120

CIN

4120 F05

OPERATION

frequency varies depending on the load at the receiver

and the coupling to the receiver coil. For LX = 5µH, and

CX = 300nF, the transmitter frequency is:

fO ≈

2 • π • LX • CX

= 130 kHz

This transmitter typically generates an AC coil current of

about 2.5ARMS. For more information on this transmitter,

refer to AN138: Wireless Power Users Guide.

The receiver consists of a coil, LR, configured in a reso-

nant circuit followed by a rectifier and the LTC4120. The

receiver coil presents a load reflected back to the trans-

mitter through the mutual inductance between LR and

LX. The reflected impedance of the receiver may influ-

ence the operating frequency of the transmitter. Likewise,

the power output by the transmitter depends on the load

at the receiver. The resonant coupled charging system,

consisting of both the transmitter and LTC4120 charger,

provides an efficient method for wireless battery charging

as the power output by the transmitter varies automati-

cally based on the power used to charge a battery.

Dynamic Harmonization Control

Dynamic harmonization control (DHC) is a technique for

regulating the received input power in a wireless power

transfer system. DHC modulates the impedance of the

resonant receiver to regulate the voltage at the input to

the LTC4120. When the input voltage to the LTC4120 is

below the VIN(DHC) set point, the LTC4120 allows more

power to appear at the receiver by tuning the receiver

resonance closer to the transmitter resonance. If the input

voltage exceeds VIN(DHC), the LTC4120 tunes the receiver

resonance away from the transmitter, which reduces the

power available at the receiver. The amount that the input

power increases or decreases is a function of the cou-

pling, the tuning capacitor, C2P, the receiver coil, LR, and

the operating frequency.

Figure 5 illustrates the components that implement the

DHC function to automatically tune the resonance of the

receiver. Capacitor C2S and inductor LR serve as a series

resonator. Capacitor C2P and the DHC pin of the LTC4120

form a parallel resonance when the DHC pin is low imped-

ance, and disconnect when the DHC pin is high imped-

ance. C2P adjusts the receiver resonance to control the

amount of power available at the input of the LTC4120.

C2P also affects power dissipation in the LTC4120 due

to the AC current being shunted by the DHC pin. For this

reason it is not recommended to apply total capacitance

in excess of 30nF at this pin.

Using DHC, the LTC4120 automatically adjusts the power

received depending on load requirements; typically the

load is battery charge current. This technique results in

significant power savings, as the power required by the

transmitter automatically adjusts to the requirements at

the receiver. Furthermore, DHC reduces the rectified volt-

age seen at the input of the LTC4120 under light load

conditions when the battery is fully charged.

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