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BQ24703: Some Questions in BQ24703 Design

Qiang Huang
Prodigy 55 points
Part Number: BQ24703
Other Parts Discussed in Thread: TPS54622

Hello, TI experts,

I plan to use BQ24703 as the AC adaptor and battery charger controller in my new system design.  I would appreciate it very much if you could help me to understand more detailed requirement in design and application.  Please see my question as follows;

(1) If my Li-Ion battery pack is 14.8V, is there any limit in selection of the minimum output voltage of the AC adaptor when using with BQ24703?

Is that OK if I use the AC adaptor with the same 14.8VDC-output, or better to use an AC adaptor with 1-2V higher than the voltage of the battery pack in this application?

(2) In BQ24703 datasheet/Page-10/Application Diagram;

    - Any special requirement for D4?  If the battery is equipped with OVP/OCP, do I still need to use U4 here?

    - How to select Zener diodes D5&D6, if they are used with smaller R19&R21 resistors?  Should I select 30V Zener diodes for protection of the SRP&SRN input range (-0.3 to 30V)?

    - How to select the Zener diodes (no reference numbers here) connected to U4/SI4435DY?

       Are these Zener diodes needed only if VS is grounded/disabled?

       If used, should I select 15V Zener diodes for clamping the voltage over +/-15V if a 14.8V battery is used here?

    - Is it better in design, if the same current flow from the Source to Drain of U4/SI4435DY is kept as that of U2&U3/SI4435DY?  Or there is no big difference to have the S-to-D or D-to-S current flow of U4 here. 

(3) In BQ24703 datasheet/Page-21/Selecting Output&Input Capacitors:

Are there the formulas available for directly calculating the input/output/filtering capacitance (C12,C11,C8), like those formulas given in other TI power ICs application notes?

When using Formulas (8)&(9) given for calculation of the in/output ripple currents, how to select the D value (duty cycle) here?

Thanks a lot for your time and help!

QH

  • 0
    TI__Genius 11850 points

    Hi QH,

    1) Since the BQ24703 is a buck charger, the adapter voltage must be higher than the maximum battery charge voltage.

    Can you provide some more information about your battery pack? I'm assuming that it's a 4s pack with a nominal voltage of 3.7 V/cell, which gives the 14.8 V that you mentioned. However, Li-ion battery packs are typically charged to 4.2 V/cell, so the charge voltage would be 16.8 V. The adapter should be at least 1-2 V above this in order to provide enough margin even after the voltage drops across D1 and R14.

    2) Before we get too deep into component selection, may I ask why you chose the BQ24703 for your application? This is a legacy device that is 17 years old.

    Since the BQ24703 is an asynchronous buck charger, the low side switch is a diode, which can hurt the efficiency. I strongly recommend one of our newer chargers instead, which can provide higher efficiency, a simpler BOM without so many external diodes, and more features. If you let me know your requirements, then I can help suggest another charger which may be a better fit for your application.

    3) You may refer to formulas 8-11 for guidance, but we don't have formulas to directly calculate a specific capacitor value. In general, I recommend sticking to the component values used on the EVM. These values were selected to provide stable operation across a wide range of operating conditions. These components are listed in the Bill of Materials on Section 5.1 of the EVM User's Guide here:

    http://www.ti.com/lit/ug/sluu160a/sluu160a.pdf

     For a buck converter, the relationship between the input voltage and the output voltage is defined by Vout = D*Vin. Based on your adapter voltage and your battery voltage, you can calculate the duty cycle.

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 55 points

    Hello, Angelo,

    Thanks a lot for your time and kind help for my BQ24703 questions. Also thanks for the attached EVM info.

    This is a new design with the power consumption (est.100W-120Wmax) and consideration of using both of an AC-adaptor and a Li-Ion battery pack (with 2X4cells-in-series, mfr. battery voltage range:12V-14.8V) and the DCDCs (TPS54622/65265/54821), if suitable.

    But we still can consider of using other battery packs and appreciate very much for your advice on the new TI Power/PM ICs with higher efficiency and better performance for our application. Can I email you the detailed requirements of our design for your references in the new Power/PM IC selection for our application? 

    Thanks!

    QH

  • 0 in reply to Qiang Huang
    TI__Genius 11850 points

    Hi QH,

    Sure, I have accepted your friend request and PMed you my email address. We can discuss your design in more detail privately.

    Best regards,

    Angelo