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BQ24650: Regarding application report SLVA437A Revised OCT 2011.

Dave Whitaker
Prodigy 190 points
Part Number: BQ24650

Hello,

I have been looking through the Application Note # SLVA437A   - Revised Oct 2011. 

Please clarify the following ...

1)   On Page  5,  Step 5    it states that a resistor R23 needs to be added between  the junction  D5 and R21 and Gnd to protect the FET Q3 from over voltage if Vin exceeds 20V.  However the schematic on page 2      shows this resistor as being  R24 - NOT R23.  Is this a typographical error

2) There are 2 gnds shown in the schematic on Page 3.   The earth Grd ( three horizontal  slashes) appear  to be for the purpose of noise immunity but are the system load and the battery sharing the same gnd ? 

3) It's my intent that this circuit be design around a 24 volt lead acid battery.   Apart from the difference in my Ah specs for the battery, the higher  float and bulk charge voltages it would appear that I could  use the same resistance parameters  provided I use the same  Vref   and the 40mV differential  constants.  Would you agree ?

Regards

Dave

  • 0
    TI__Mastermind 24075 points
    Hi Dave,
    Yes that is a typo, R24 of figure and R23 of calculation refer to the bottom resistor of the voltage divider, so max Vgs of fet is not reached. Make sure you look at max charge voltage of lead acid battery and not nominal voltage, as BQ24650 supports up till 26V battery regulation voltage. VREF is internal 3.3V, and 40mV differential is also internal and based on full scale differential voltage between SRP and SRN (refer 8.3.3 of d.s)
  • 0 in reply to Kedar Manishankar
    Prodigy 190 points

    Hi Kedar,

    Thanks for the reply - however I'm still confused.  The equation in step 2 on page 5 has  Vref as being 2.1 volts, but the data sheet has the number 2.1 V,  but that is referred to as the Vreg.  The 3.3 volts that you are  referring to is listed as being Vref_reg.   Can you please clarify the data sheet parameters against the equations/s  on page 5 ?  

    Regards

    Dave

  • 0 in reply to Dave Whitaker
    TI__Mastermind 24075 points
    Hi Dave,
    I can see your confusion. We have a pin VREF on the IC which is a 3.3V voltage reference output. The vref in equation in step 2 on page 5 (SLVA437A) is referring to the internal 2.1V feedback reference voltage, used to monitor battery voltage. This 2.1V is the feedback regulation voltage and is referred to as VREG in the E.C table of the d.s. The datasheet and the app note are not uniform and have a little ambiguity. Was I able to clear up the differences?
  • 0 in reply to Kedar Manishankar
    Prodigy 190 points

    Hi Kedar,

    Yes,   that clear's up my confusion but I do have another question regarding the current capability and settings.

    We were discussing at work  the possibly of a design ( with reference to the same application note)  such that the user would be able to select one of several bulk current settings by the selection of different  resistance values used ( switched in or out)  in the SRP/SRN  circuit.  For example,  one customer may have a larger Ah battery rating than other customer.   Based on the fact that table 1 in the data sheet has different  inductor and capacitance  values for a range of currents  it would seem to me that this would be either be impractical or would simply  just not work.  Is this possible with one chosen inductor and cap value ?

    Please give me your comments.

    Regards
    Dave

  • 0 in reply to Dave Whitaker
    TI__Mastermind 24075 points
    Hi Dave,
    With BQ24650 the charge current is set just by the sense resistor, so that will physically have to be replaced for the different charge current settings you wish to achieve. The table is just recommended values based off desired inductor current ripple (Inductor saturation current must be higher than the charging current (ICHG) plus half the ripple current (IRIPPLE)), desired output voltage ripple. DCR and ESR are also tradeoffs with inductor and capacitor values, and have an effect on efficiency (more effect at higher charge currents). Essentially choosing the inductor and capacitor values come with tradeoffs (another one being size of component for board space) and the table simplifies choice for recommended component values at different charge currents.
    For BQ24650 as long as the tradeoffs are considered for your customer's desired applications, and the the resonant frequency of the
    output inductor and output capacitor is designed between 12 kHz and 17 kHz, this should be fine. (Frequency range is specified for good loop stability based off of BQ24650 internal loop compensation).
    For more information on inductor and capacitor selection you can refer to section 9.2.1 Design Requirements of BQ24650 datasheet.