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BQ24079 Out

MarcosCba
Prodigy 140 points
Other Parts Discussed in Thread: BQ24079, BQ24073Normal 0 21 false false false ES X-NONE X-NONE MicrosoftInternetExplorer4

 

Hello, I’m using BQ24079 operated from USB port (5v). In the datasheets states that  " OUT provides a regulated output when the input is below the OVP threshold and  above the regulation voltage (5,5V for BQ24079).  So the question is...Can I Have the regulated output if the power source for charging the battery and running the system is a USB port? I think not, so which output will I have? Should i change it for a BQ24073 which has a regulation voltage 4.4V?

 

Ps: Sorry for my bad English is not my first language.

  Thanks!

  • 0
    TI__Guru**** 150650 points

    You are correct in your thinking.  Since your 5V input is less than the regulation voltage of 5.5V, the voltage on the OUT pin will not be 5.5V, but will be less than 5V.  How much less will depend on your load.  The IC will try to regulate the OUT pin to 5.5V by turning Q1 on all the way--passing the input voltage to the OUT pin minus the drop across Q1.  This drop is specified on page 4 of the datasheet as 300 mV at 1A load current.  So, the OUT pin voltage will vary will your load (which includes the charging current) but will always be less than your 5V input.

    Whether you need an OUT voltage regulated at a specific point depends on what voltage your downstream circuitry is capable of handling and what voltage it functions best at.  Using the bq24079 with a 5V input will dissipate less power in the 24079 than the bq24073 would with the same 5V input--the 24073 would lose more power in Q1 than the 24079 since the bq24073 will drop typically 600 mV (5V-4.4V) across Q1.  Power loss in Q1 is equal to voltage across Q1 * current in Q1 (load current + battery charge current).

    Note that the decision of whether to switch to the 24073 will affect not only the output voltage but also the optional IC specific feature--either SYSOFF or TD.