LTC4120 datasheet(13/32 Pages) LINER

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LTC4120/LTC4120-4.2

4120ff

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Figure 5. Resonant Receiver Tank

C2S

1:n

C2P

DHC

LTC4120

CIN

4120 F05

OPERATION

of a basic power transmitter that may be used with the

LTC4120.Thistransmittertypicallyoperatesatafrequency

of approximately 130kHz; though the operating frequency

varies depending on the load at the receiver and the cou-

pling to the receiver coil. For LX = 5µH, and CX = 300nF,

the transmitter frequency is:

fO ≈

2 • π • LX •CX

= 130kHz

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,configuredinaresonant

circuitfollowedbyarectifierandtheLTC4120.Thereceiver

coilpresentsaloadreflectedbacktothetransmitterthrough

the mutual inductance between LR and LX. The reflected

impedance of the receiver may influence 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 automatically 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 coupling,

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,anddisconnectwhentheDHCpinishighimpedance.

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

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