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AFE4490 connection for LED in common anode mode

Royan Ong
Prodigy 20 points
Other Parts Discussed in Thread: AFE4490, AFE4403, AFE4490SPO2EVM

Hi there,

I am developing an AFE4490 circuit with the red and IR LED (Marubeni SMT735-890) in a common anode mode. The AFE4490 is mainly powered from 3.0V (regulated), while the TX section is directly from a LiPo battery (3.7V, unregulated).

There are a few ambiguities that I need clarified, which has yet been answered in post http://e2e.ti.com/support/applications/high_reliability/f/30/t/383235. The nearest answer was in http://e2e.ti.com/support/applications/high_reliability/f/30/p/393157/1388344 , but it was for the AFE4403.

Clause 1) Figure 67 of the AFE4490 datasheet shows the anode of the LED connected to "External Supply" and LED_DRV_SUP left unconnected.

Clause 2) Figure 132 shows the anode of the LED connected to LED_DRV_SUP. Is this LED_DRV_SUP the same as that in Clause 2?

Clause 3) The table in Section 7.3 shows that LED_DRV_SUP and TX_CTRL_SUP must be within +-0.3V of each other. Must they be connected?

Does it mean that:

* Clause 1 is incorrect (LED_DRV_SUP must be connected)?

* LED_DRV_SUP has to be connected to "External Supply" (Clause 2)?

* TX_CTRL_SUP has to be connected to LED_DRV_SUP (Clause 3)?

 

Hope to hear from you soon.

 

Best regards,

 

Royan

  • 0
    TI__Mastermind 49566 points

    Hello Royan,

    LED_DRV_SUP cannot be left floating in common anode configuration.
    You could use a separate supply to connect to the anode of the LEDs in common anode configuration.
    Figure 132 shows that you could also use the LED_DRV_SUP to connect to the "External supply".

    LED_DRV_SUP and TX_CTRL_SUP must be within +-0.3V of each other. What this condition means that if you plan to use separate supplies for LED_DRV_SUP and TX_CTRL_SUP, then they need to be +/- 0.3V of each other. You can also connect LED_DRV_SUP and TX_CTRL_SUP together.
    Refer to the AFE4490SPO2EVM schematics (section 12.2 in the link below) if you would like to use it as a reference design.
    http://www.ti.com/lit/ug/slau480c/slau480c.pdf

  • 0 in reply to Praveen Aroul
    Prodigy 20 points
    Dear Praveen,

    Thank you very much for your speedy reply. Glad that it cleared up this ambiguity. Could I suggest that Figure 67 be made less ambiguous especially by not leaving LED_DRV_SUP unconnected.

    Thanks again.

    Best regards,

    Royan
  • 0 in reply to Royan Ong
    TI__Mastermind 49566 points
    Hello Royan,

    As I mentioned in my other post (related to AFE4403) Figure 67 is an illustration to show the common anode LED configuration and does not portray the internal architecture of the transmit block.
    Thanks for pointing it out anyways. We will be resolving this in the next datasheet revision.