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Biasing DC inputs to chips

Discussion in 'General Electronics Discussion' started by WPB, Dec 26, 2014.

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  1. WPB

    WPB

    2
    0
    Dec 26, 2014
    Hello there,

    I would like to use the MAX1811 for charging a Li-ion battery. The chip has some inputs for controlling its behaviour, for example selecting either a 100mA or 500mA charging current (called the "SELI" pin). The datasheet says:

    Drive SELI low to set the charge current to the 100mA
    mode. Use a 10kΩ pulldown resistor to ground on SELI,
    if necessary, to ensure that the MAX1811 defaults to
    the 100mA mode in the event that no logic signal is
    present. Drive SELI high to increase the charge current
    to the 500mA mode only if the polled USB port can provide
    the required current.

    My question is, if I always want one mode or the other, can I just tie the SELI input either directly to ground or directly to the positive supply rail, depending on which mode I want? Or is a pulldown/pullup resistor a requirement?

    Thanks in advance!
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    Yes, you can tie the SELI pin low or high. The pulldown resistor is mentioned in the data sheet because in some designs, the controlling device (the thing that interrogates the USB Master to determine its current rating, monitors the cell, and controls the SELI pin) may not always be operating, or may not be able to interrogate the USB Master. In that case, it's safest to select the low-current charging option. When the whole system is up and running, the USB port's current capability is known, and the cell's state is being monitored, the controlling device can drive SELI high, but otherwise, SELI should default to low.

    SELI is specified for an absolute maximum voltage of 7.0V (exposure to absolute maximums should be avoided for best reliability; I wouldn't use more than 5.0V) and can be powered even if the MAX1811 is not powered. As long as you don't exceed that voltage, the MAX1811 will not be damaged.
     
  3. WPB

    WPB

    2
    0
    Dec 26, 2014
    Thank you very much for your reply. It's funny, as I was writing the question, the penny dropped for me, and I suspected I knew the answer already, but I wanted to get someone else's more experienced input. Much appreciated!
     
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