LTC4124 datasheet(16/24 Pages) AD

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LTC4126-ADJ

Rev. 0

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APPLICATIONS INFORMATION

Note that since fDRIVE can be easily adjusted, it is best

practicetochoosefRX-TANKusingtheminimumcomponent

count (i.e. CRX) and then adjust fDRIVE to match.

The amount of AC current in the transmit coil can be

increased by increasing the supply voltage (VIN). Since

the amount of power transmitted is proportional to the AC

current in the transmit coil, VIN can be varied to adjust the

power delivery to the receive coil.

The overall power transfer efficiency is also dependent

on the quality factor (Q) of the components used in the

transmitter and receiver circuitry. Select components with

low resistance for transmit/receive coils and capacitors.

CHOOSING TRANSMIT POWER LEVEL

As discussed in the previous section, the supply voltage

(VIN)canbeusedtoadjustthetransmitpowerofthetrans-

mitter shown in Figure 7. Transmit power should be set as

low as possible to receive the desired output power under

worst-case coupling conditions (e.g. maximum transmit

distance with the worst-case misalignment). Although the

LTC4126-ADJ is able to shunt excess received power to

maintain the VCC voltage in the desired range, it has the

adverse effect of raising the die temperature and possibly

the battery temperature, and if the battery temperature

exceeds the Too Hot temperature threshold set by the

thermistor, the charger pauses charging the battery.

Using the rated current of the transmit inductor to set an

upper limit, transmit power should be adjusted downward

until charge current is negatively impacted under worst-

case coupling conditions. Once the transmit power level

is determined, the transmit and receive coils should be

arranged under best-case coupling conditions with a

fully-charged battery or a battery simulator to make sure

that the shunting of excess power does not raise the die

temperature too much.

Inadditiontotemperature,anotherparameterthatneedsto

be checked is the maximum negative voltage on the ACIN

pin. Following the procedure above, when evaluating the

rise in temperature of the LTC4126-ADJ under the best-

case coupling conditions, ensure that VCC – VACIN does

not exceed 16V. Figure 11 shows a typical waveform on

ACIN showing VCC – VACIN < 16V.

Figure 8. Current Through Transmit Coil

Figure 9. Voltage on the Drain and Gate of NMOS

M1 when fTX_TANK = fDRIVE

Figure 10. Voltage on the Drain and Gate of NMOS

M1 when fTX_TANK = 1.29 • fDRIVE

2µs/DIV

4126-ADJ F08

500mA/DIV

2µs/DIV

GATE

VOLTAGE

2V/DIV

4126-ADJ F09

DRAIN

VOLTAGE

5V/DIV

2µs/DIV

DRAIN

VOLTAGE

5V/DIV

GATE

VOLTAGE

2V/DIV

4126-ADJ F10

The LC tank at the receiver, LRX and CRX, is tuned to the

same frequency as the driving frequency of the transmit

LC tank:

fRX-TANK = fDRIVE

where fRX-TANK is given by,

fRX−TANK =

2 •

π LRX •CRX

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