LTC4124 datasheet(24/30 Pages) AD

24 / 30 page

LTC4124

Rev. A

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APPLICATIONS INFORMATION

7. Determine the value of CTS for step duration during

the LTC4125 load detection.

Set the step duration to 50ms, approximately about

5 times the observed VIMON settling time.

CTS = 5 •

10µA • TS

32 • 1.2V

= 1.3 × 10

–6 • T

S s

( )

≈ 15nF (standard capacitor value)

8. Determine the value of RFB1 and RFB2 for no receiver

detection.

With the minimum pulse width as determined in

Step 3 and no receiver, measure IIN and VLC-TX ampli-

tude. In this particular example, with no RX present, at

VPTH = 0.43V: IIN = 580mA and |VLC-TX| = 40V.

The same measurement is done with a receiver at

the worst coupling condition (largest gap between

the TX and RX coils) with the battery pin set at 4.3V

(zero charge current → lowest output power). In this

particular example, using setup 4 at VPTH = 0.43V,

IIN = 450mA and |VLC-TX| = 25V .

At the VPTH determined in step 3, the voltage at the

transmitter resonant tank with no receiver is 40V

(VLC-TX-MAX), and with a receiver at minimum load, it

is 25V (VLC-TX-MIN). Using the formula provided in the

LTC4125 data sheet, RFB1 and RFB2 can be calculated

as follows:

VLC−TX−MIN – VD

VIN

– 1<

RFB1

RFB2

VLC−TX−MAX – VD

VIN

– 1t

25V – 1V

– 1<

RFB1

RFB2

40V – 1V

– 1

3.8 <

RFB1

RFB2

< 6.8

Therefore, set the ratio to be 6. Since the recom-

mended total RFB1 + RFB2 resistance is about 100kΩ,

set RFB1 = 100kΩ and RFB2 = 16.5kΩ.

9. Determine the value of RFTH1 and RFTH2 for foreign

object detection.

As outlined in steps 1 and 2, the resonant frequency

selected is 240kHz. However, the operating frequency

of the two coupled resonant LC at the TX and RX under

heavy load at the best coupling condition is as high

as 300kHz. Therefore, set the foreign object detection

frequency threshold at 310kHz, about 3% above the

highest operating frequency. Set RFTH2 = 100kΩ and

RFTH1 according to the following formula:

RFTH1 =

320kHz

fLIM

– 1

⎛

⎝⎜

⎞

⎠⎟

•RFTH2

320kHz

310kHz

– 1

⎛

⎝⎜

⎞

⎠⎟

• 100k

Ω = 3.23kΩ

≈ 3.24kΩ (standard 1% resistor value)

10. Determine the value of CTD for the delay between each

load detection attempt.

Set the wait time to 5 seconds. Then, the CTD value is

calculated using the following formula:

CTD =

10µA • TD

65 × 103 • 1.2V

= 1.3 × 10

–10 • 5 ≈ 680pF

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