• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 62 subscribers
  • Views 281 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ2954 Input Voltage Range

Rob Knighton
Prodigy 20 points
Other Parts Discussed in Thread: BQ2954

I am trying to determine whether a commercially designed battery charging product based on the BQ2954 is capable of operation using an input voltage other than that provided by the power supply provided.

The product currently uses a mains power supply at 12VDC +/- 0.5V and is designed to charge battery packs at 8.3VDC.at max 1A.

The design is based closely on the circuit given in the datasheet and uses scaling resistors RB1=310K / RB2=100K

I have an application where I want to power the charger from a vehicle supply using a 24V-13.6V Voltage Converter which has a worst case output range of 13.6V +15/-20%  (10.88-15.64V).

I have carried out some basic tests substituting a bench power supply and varying the input voltage and found that although the circuit ceases to function below 11.2V (which I am not too worried about), the battery voltage was maintained within 0.1V for an increase in voltage to 16V.

I would be grateful for any advice on the factors that influence the line regulation that one might expect to achieve using a circuit such as the datasheet example.

In that example the input is marked as 8-24V +/- 10% - And the regulated battery voltage is stated at 4.2V - So one might assume that anywhere within this range is acceptable - So, in my application, would I be correct to believe that I can accommodate 12V-24? -  But what is the significance of the +/- 10%? - Does this mean it could be 12V - 10% to 24V +10%?

  • 0
    Prodigy 20 points

    On reflection, possibly I didn't phrase this question quite as well as I might have done:

    I think maybe what I should have asked was:

    What is the min/max headroom that can be accommodated by the circuit shown as application example in the datasheet?

    I assume this is going to depend largely on the range of the pw modulator circuit and the spec of the FET in the main current path.

    I made some measurements on my (8.4V battery) circuit looking at pin 14 'MOD' and found that at 14V input, the mark/space ratio of this output was approx 50/50.

    I increased my input to 16V which is the maximum I could foresee being applied, and the m/s ratio was still changing nicely to this point and beyond.

    On the lower input voltage side, again the m/s ratio was changing nicely down to 11.2V when the circuit ceased to regulate (shut down/ MOD inhibited) - So I assume a minimum headroom of ~4V (which ties in with the min 8V stated on the datasheet application example.

    Also, my apologies: I mis-quoted the battery pack I am looking to charge as 8.3V - Should have been 8.4V.

    Anyway, from what I have seen, I am inclined to believe that my charger will run quite happily (and safely) on the DC supply available in the vehicle at worst case upper voltage limit of the converter.

    -And,  we may expect the charger to fail in the unlikely event that the input should fall to <11.2V - But it will do so in a safe and predictable manner, and the condition will be indicated by fault led on the unit.  

    I would however, be grateful for any advice.

    Best Regards,

    Rob