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LP55231 total current

Mark Graham
Prodigy 235 points
Other Parts Discussed in Thread: LP55231

Hello

I would like to drive 9 LEDs with the LP55231. I have 5V input, so do not need the charge pump which has a maximum output of 100mA.

Can I drive all of the LEDs at 25mA? There is no calculation of heat dissipation for the chip in this scenario.

Thanks, Mark

  • 0
    TI__Intellectual 1985 points

    Hi Mark,

    Yes, you can drive all 9 channels at 25mA with your 5V input and set the charge pump mode to 1X. However, depending on the forward voltage of the led, 5V supply might have too much headroom.

    Regards
    H L

    ,

  • 0 in reply to Hak-Leong Ng
    Prodigy 235 points

    Thanks for the reply.

    My LEDs are 2V for Red and 3.3V for Blue and Green. I am trying to work out if it is better to supply the driver with 3.3V (with Charge pump enabled) or 5V (which would be easier). Assuming I use 5V, will this be a problem to the red LEDs? I don't understand why the 3V of headroom for the red LED is a problem if the current is controlled to, for example, 20mA.

    I would also like to know how to calculate the power dissipated by the LP55231. For a red channel, for example, if the device is supplying 2V at 20mA from a 5V supply, will it dissipate 60mW (like a linear regulator), or very much less due to the nature of the PWM (based on the duty cycle and Rdson of the output drive fet)?

    Thanks gain

    Mark

  • 0 in reply to Mark Graham
    TI__Intellectual 1985 points

    Hi Mark,

    In my opinion, it will be a better option to supply 3.3V and use the charge pump 1.5X mode (4.5V) for blue and green.

    Assuming you have a 5V supply, red LED vf is 2V and the current is set to 20mA. If the PWM is 100%, a continuous 20mA will flow from the supply through  the LP55231 and the red LED. So 20mA x 2V is consumed by the LED and 20mA  x 3V is consumed and dissipate 60mW by LP55231. This is for the case of 100% PWM, if the PWM is only 50%, then LP55231 only needs to dissipate 30mW.

    Regards

    H L