LTC4124 datasheet(17/24 Pages) AD

17 / 24 page

LTC4126-ADJ

Rev. 0

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APPLICATIONS INFORMATION

resonant capacitance and RL-AC is the equivalent AC load

resistance.

One simplification is as follows:

RL–AC ≈

RL–DC

which assumes that the drop across the Schottky diode is

muchsmallerthantheamplitude|VRX|.Additionally,RL-DC

can be approximated as the ratio of the output voltage

(VOUT) to the output current (IOUT):

RL–DC =

VOUT

IOUT

The amplitude of the current in the transmit coil |ITX| can

be either measured directly or its initial (no receiver) value

can be calculated based on the transmitter circuit. This

initial value is a conservative estimate since the amplitude

of the transmitter coil current will drop as soon as the

receiver, with a load, is coupled to it.

The coupling factor (k) between the two coils could be

obtained by running a finite element simulation inputting

the coil dimensions and physical configurations. An easier

methodtoobtainthiscouplingnumber,istousetheseries-

aidingandseries-cancellingmeasurementmethodfortwo

loosely coupled coils as shown in Figure 13.

And:

LAIDING = LAB

LCANCELLING = LCD

LAIDING –LCANCELLING

4 LTXLRX

Figure 11. Typical Acceptable Voltage Waveform

on the ACIN Pin with VCC – VACIN < 16V.

Figure 12. Modeling Parallel Resonant Configuration

and Half Wave Rectifier on the Receiver

Figure 13. Series-Aiding and Series-Cancelling Method

Configurations Used for Measuring the Coupling Factor k

Asanalternativetousingtheempiricalmethodtodetermine

themaximumnegativevoltageontheACINpin,thefollowing

formula can be used in conjunction with Figure 12, which

shows a parallel resonant configuration on the receiver:

VRX =

ωk LTXLRX

1–

ω2LRXCRX

(

)2+ ω LRX

RL–AC

⎛

⎝⎜

⎞

⎠⎟

ITX

VCC

ACIN

–10

–6

–2

4126-ADJ F11

TIME (100µs/DIV)

4126-ADJ F12

LRX

CRX

RL–AC

LTX

ITX

VRX

LRX

CRX

CRECT

RL–DC

LTX

ITX

IOUT

VOUT

•

4126-ADJ F13

LTX

LRX

|VRX| is the amplitude of the voltage on the receiver coil,

|ITX| is the amplitude of the current in the transmit coil,

k is the coupling factor between the transmit and receive

coils,  is the operating frequency in radians per second,

LTX is the self-inductance of the transmit coil, LRX is the

self-inductance of the receive coil, CRX is the receiver

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