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bq51050b Recommended Voltages

Other Parts Discussed in Thread: BQ51050B, BQ51013B, TIDA-00318, BQ25100, BQ51003, BQ500212A, TIDA-00762

What are the recommended voltages for Vrect using the default Schematic in bq51050b datasheet? 

  • Evagoras,

    The RECT pin is used to add capacitance to the bq51050B.  It is not a power pin.  The voltage on RECT is created from the AC to DC conversion of the AC1/AC2 signal.  Voltages on RECT depend on the operating conditions and will vary from slightly over the BAT voltage to over 8V.  I recommend capacitors with 16V or higher ratings.



  • With the schematic provided in the datasheet i have very low voltage on the RECT pin, when i replace the C2 capacitor with approximately 150nF (the default is 1,8-2,2 nF) , the voltage at the RECT pin increases and the LED D5 on the BQ500212AEVM turns on but randomly.
  • Evagoras,

    RECT is a direct reflection of the coupling between the TX and RX.  The startup of power transfer requires VRECT to increase above the nominal voltage required on OUT.  A good example of this is in the bq51013B datasheet in figure 17.  The pulses seen on the RECT pin show the initial communication from the RX to the TX.  When VRECT begins to increase, that is an indication that the TX has received the proper identification from the RX and is beginning to change its level of power transfer.

    Take a look at VRECT on power up (when the RX is placed on the TX).  

    Based on your post, I expect you are using the schematic in figure 32 of the bq51050B datasheet.  Have you made any modifications to the schematic?  What about the coil?



  • Yes I use the schematic in figure 32 of the bq51050b datasheet. I use the coil WE 760308101208 13μΗ.
    The modifications are:
    C1=162nF , C2=1,9nF , RFOD=200Ω , R1=205Ω , RTERM=2,4kΩ , 42,4kΩ , R4=100Ω, C3=20,1μF , C4=1,1μF.
    With these modifications the Vrect is too low so that the power transfer begin.
    When I replace the C2 capacitor to approximately 120-150nF , the LED D5 on BQ500212AEVM turns on but the Vrect is not regulated at all.

  • Evagoras,

    The WE 760308101208 coil is only 10mm in diameter.  In most cases, that is too small to work with a standard Qi transmitter (at typically 43mm TX coil). At 13uH, it is also difficult to set up with a smaller coil as seen in the TIDA-00415 transmitter reference design (non-Qi due to the coil size).

    A larger RX coil would give better results.



  • So the recommended coil is the Vishay IWAS-4832FF-50 ?
    What can I use for small applications like wearables ?
    This is the Waveform of Vrect.

  • Evagoras,

    There are many standard 5W coils we have evaluated on our evaluation modules.  Many of those are listed in the bq51013B datasheet (Table 7), including the Vishay coil.

    The WPC specifications were set up to have very rigid controls on the TX coils, but not on the RX.  This allows for great flexibility.  As a result, we cannot test every RX coil.  We have done testing with many coils, but do not have a recommended list.  

    As the RX coils get smaller, it make is very difficult to design a wearable product that can be Qi-certified due to the RX:TX coil size ratio.  That is part of why we have the reference designs with smaller TX coils (Like the TIDA-00415 mentioned in the previous post) and the TIDA-00334.

    Examples of an RX designed for a wearable solutions are the TIDA-00318 and the TIDA-00329.  Both use a 22mm RX coil.  Both have been tested on a standard TI TX EVM and on the TIDA TX boards.

    One point on the TIDA-00318 is that is uses the bq51003 for the RX and adds the bq25100 for the charger.  The bq51050B is optimized for 5W and is not as accurate with the charge current and termination currents for some of the low capacity solutions.  

    As for the plot showing the RECT voltage.  What is the setup for this test (coils used)?  What is the battery voltage during this testing?



  • For this test I used Wurth 760308101208, C1=150nF , C2=100nF because using the values from the equations the connection didn't establish.
  • I suggest you get your system working with a full size coil before trying the 10mm coil. The 10mm coil will not work with a standard Qi transmitter due to lack of coupling.


  • I try a custom coil with 13mm diameter and C1=1,8nF C2=100nF . And I have a full rectified voltage at RECT at 5V.
  • Evagoras,

    Great news!  Does the full system now operate with the new coil?

    There are a few other tools that may be helpful here.

    This an an article on wearable coils:

    We recently updated which you may find interesting.

    Finally, there is an FAQ section in the E2E that can be helpful.



  • Dick,
    Thank you for the useful information.
    There are Transmitter EVMs for low power ? Or I have to modify my existing BBQ500212AEVM ?
  • Evagoras,

    The bq500212AEVM will work for the lower power levels you are dealing with.  However, as noted earlier, the small coil size may be an issue.  See the TIDA-00334 or TIDA000415 which will offer ideas on smaller TX coils that can be used.  The schematics will show any differences between the coil sizes and you would be able to modify the bq500212AEVM accordingly.



  • Nice, but what I have to change from BQ500212AEVM if I want to change the TX coil?
    I have to modify all the system or I have to change some capacitors only?
  • Hello Evagoras

    When the TX coil is changed on the bq500212A solution the inductance value should stay at 6.3uH +/- 10%.  The tuning capacitors should stay at 400nF.

    Coil resistance needs to be low and litz wire should be used, See TIDA-00762.

  • So there is nothing component to be changed, the only thing that I have to do if I want to use a smaller in size Coil is to use the same inductance (6.3uF +-10%) but with lower DCR and with litz wire ?
  • Smaller TX coils can be used without additional changes to the circuit, see TIDA-00762 and TIDA-00334.  The smaller coils are recommended for lower power applications 2.5W or less.  The smaller coils do not meet Qi specifications and are not recommended for 5W.  The design should be proprietary, TX and RX are design to work with each other and not interoperable. 

    FOD performance will be degraded due to coil change.  But with lower power and proprietary design this can be managed and provide good performance.

    The best reason to use small TX coil is to improve performance with small RX coil. Coupling factor between coils is improved with similar diameter, 20mm RX will work better with a 20mm TX vs a standard Qi TX coil of 50mm.

    Coil we have tested and would recommend are:

    1.) 30mm round Wurth  P/N 760308101103

    2.) 30mm Round TDK P/N WT303012-12F2-ID

    3.) 20mm round Wurth P/N 760308101104

    4.) 20mm Round TDK P/N WT202012-15F2-ID

    Note---TIDA-00415 is a half bridge design and the coil inductance is lower, should only be used with this design.

  • Can I use 20nF instead of 22nF for COMM capacitors ?
  • Evagoras,

    Yes, you can use 20nF capacitors instead of 20nF.  The COMM capacitors set the depth of the communication pulses.  Lower capacitor values will mean lower depth.  Verify your communication robustness with the new capacitors.