Q: BQ2002 for higher voltage input by 369796

One of our customers needs a NiMH charger solution.  They’re using 5-series cells (about 6V output) and want to charger it from a reasonable wall-transformer, probably about 9V.  

 

We’ve shown them BQ2002, but because its ABS_MAX input voltage is 7V, that’s not going to work.  Do you have any other ideas? New devices?

 

 


A: Re: BQ2002 for higher voltage input by 1090425

All of the older bq200x parts are controllers that need a 5V bias.  The bq2002 controls an external current source--simply switching it on or off.  On the EVM, this current source is an LM317.  So, these bq200x parts will need a separate 5V supply--i.e. a resistor and zener.  The power stage can be any voltage.

A 5 series cell pack will have a maximum voltage in fast charge of about 8V.  This assumes the maximum cell voltage is 1.6V which could be the case as the cells age, etc.  So, you need to design a current source that can provide 8V at whatever fast charge current you want and not be in dropout.  Given the likely tolerance on your 9V input, this may not be realistic/feasible.  You may need to boost the input to give enough headroom to your current source.

I would recommend going to a 4 cell pack or different battery chemistry.  But if they want to use nickel, the bq2002 is your best bet.  No new nickel chargers are available.