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PCA9555: PCA9555 design questions

Janet Lin
Intellectual 2590 points

Part Number: PCA9555
Other Parts Discussed in Thread: PCA9535, TCA9535

Hi E2E,

 

Here are some questions for designing PCA9555.

  1. I would like to ask whether the PCA9555 can actually use the circuit shown in Figure 9.3 to control the LED.

    Assuming that the VCC of the PCA9555 is 3.3 V, the voltage on the Pn pin should theoretically not exceed 3.3 V. In this case, there would be a 1.7 V difference between the 5 V supply and the Pn pin. The yellow-highlighted section indicates that the LED will only turn off when the voltage across it is less than 1.2 V. Therefore, based on my understanding, the LED in Figure 9.3 would still remain ON.

  2. Also, I noticed that Figure 9.2 has a current-limiting resistor above the LED, while Figure 9.3 does not. Since the voltage across the LED in Figure 9.3 is higher, the LED current would generally be higher as well. Would a current-limiting resistor also be required in Figure 9.3?

 

BR,

Janet

 

  • 0
    TI__Guru 61061 points

    Hi Janet,

    We are still looking into this and will follow up shortly.

    Regards,

    Jack

  • 0
    Guru 326790 points

    Section 9.1.1.2.1 was copied from section 9.2.1.1 of the PCA9535 datasheet, but mistakes were made.

    Figure 9-2 shows a parallel resistor, but the PCA9555 already has such a resistor internally.

    Figure 9-3 was copied wrongly; the PCA9535 datasheet indeed has a current-limiting resistor in series.

    And what this section assumes (but not actually says) is that the LED is disabled by configuring the port pin as an input. In figure 9-2, a better way to disable the LED is to configure the port pin as an output driving high, then no parallel resistor is needed. (In figure 9-3, configuring the port pin as an input is the only way to make this work, but the PCA9555 still has the internal pull-up to 3.3 V, so a very small current will still flow.)

  • 0 in reply to Clemens Ladisch
    TI__Prodigy 470 points

    Hi Clemens,
    Could you clarify what's the correct circuit looks like?

    Customer still need a series current-limiting resistor with LED.

    And can remove the parallel resistor and set the port pin as output driving high.

    If the voltage is 5V, Vcc is 3.3V, the LED still flow small current but can not cause LED light on.

    Am I correct?

  • 0 in reply to Stanley Wang
    TI__Guru 64738 points

    Hi Stanley,

    Current limiting series resistor is still needed for the LED. 

    If you set output port driving HIGH when VCC = 3.3V, the max voltage that can appear on the output is 3.3V.

    A 5V on the LED with 3.3V reference may see the LED power on slightly depending on its forward voltage and the size of the series current limiting resistor. 

    Can you use the TCA9535 w/o internal pull-up resistors? Then the figure would align? 

    Regards,

    Tyler