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BQ24092: Why use 1.5k on PG and CHG lines with LED?

MikeH
Guru 15580 points
Part Number: BQ24092

I am considering the BQ24092 for a new design and have run into a question on the recommended design. The typical application schematic shows a 1.5k resistor in series with an LED on both the CHG and PG lines. Why was this value chosen? with a 3.6V battery and a 1.5k resistor in series with an LED that has a 2.1V drop across it, the current through the LED will be ~1mA. This is barely enough to turn on the LED. Can someone explain this choice of resistor value?

  • 0
    TI__Guru* 75906 points
    This LED configuration (1.5k) is widely used in the bq240XX series of EVMs not only on the bq24092. We want to keep the current in optional circuits such as LEDs to a minimum. In the bq24092 configuration it will add an additional load to OUT possibly skewing test results.
    No problem in increasing the LED current with a smaller resistor.
  • 0 in reply to Bill Johns
    Guru 15580 points
    Thanks Bill. I'll have to see how much light I get with 1mA. May not be enough in direct sunlight, which is where my product will be used.

    Thanks!