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# BQ51003: How to tune resonant capcitors value.

Part Number: BQ51003

Hello.

I found a equation on bq510003 datasheet on page 27.  if use fs=10KHz, fd=1MHz, and Ls value is fixed. therefore C1, C2 will be worked out.

But I use EVM parameter Ls=10uH to work out C1,  get C1= 3900000F. it doesn't match EVM C1=68nF + 68nF +47nF = 183nF.

Could you please check it and give me a explaination. thank you!

Any side, I didn't find the equation on TX chip datasheet (bq50002A). how to work out TX resonant capcitors?

• Hello
On data sheet page 27 equation 6 shows fs = 100kHz and fd = 1MHz. I do not see fs=10kHz.

For C1 calculation the Ls' value should be used. This is measured with the coil mounted on a TX coil simulator , Fig 34.
Ls' will be higher than free space inductance Ls used for calculation for C2 (Cd).

For BQ51003 the Ls' value measured 13.5uH, this will calculate a value for C1 187nF, in the ckt C1/C2/C3 is 68nF/68nF/47nF = 183nF.

The TX side is set by WPC standard for A11 TX. The L value is set at 6.3uH and resonant caps are set at 400nF.

Best Regards,

Bill Johns,  Applications Engineer

Texas Instruments Inc

Dallas, Texas

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