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Original question:

LP8555: What is maxium delta voltage between each string in one port and LED qunatity that can support

LP8555: Different Number of LED's on Channel A and B with the same current/brightness on each channel

Eric201902
Prodigy 120 points
Part Number: LP8555
Other Parts Discussed in Thread: , LM3697EVM

Hi,

We are evaluating the feasibility of using LP8555 to drive two LED bars on Channel A and Channel B, each with different number of LED's.

For our design, we need to keep the brightness of the LED's on both channels on the same level.

 

We have tested the design on LP8555EVM connected to the LED's on  LM3697EVM(Also from TI),  where SML312WBCW1 LED's with forward voltage of 3V are been used.  We connected BOOSTA, BOOSTB, and GND

from the LP8555EVM to the LM3697EVM LED channels as shown below:

Where the number of LED's drive by each Boost channel is adjusted through the configuration of HL1A,  HL1B, etc star jumpers.

We used a 5V DC voltage from external power supply for the BATT voltage:

From the test, we have measured a great variance between the current of the two channels and instability in the current.

Please give advice on how to keep the brightness of the LED's on both channels on the same level with stability.

  

The Pictures below shows our test setup and measurement:

Below is our setting for the LP8555 Eval Software:

Where the circuit is configured to operate without 12C control:

  • 0
    TI__Mastermind 22745 points
    Hi Eric,
    here are some suggestion for you,
    1. you can try to use the same LED counts for Back A and B to see whether LP8555 can output same current;
    2. if you can observe same current form the first test above, then this means the set up is ok;
    3. If you can't get the same current from the same LED counts, you need to check your connection and configuration, there must be something wrong;
    4. What's the LED couns for Bank A and Bank B?
    5.can you test the each bank seperately to see whether there is any mistake?
    Regards
    Sean
  • 0 in reply to Sean Zhou
    Prodigy 120 points

    1. I tried to use the same counts, the current is the same your A and B.

    2. That means the set up is OK.

    3. N/A

    4. Last time I tested 4+9, today I test 4/6/9 on each side.

    5. I tried to drive only one channel while leave other empty, it seems like it cause a current overflow, which has destroyed the LED.

    From the result, I think it means we cannot drive different channel of LED's under current configuration.

  • 0 in reply to Eric201902
    TI__Mastermind 22745 points
    Hi Eric,
    Actually, it can support different LED couns with Bank A and Bank B with the same current. I think your connection may be wrong, you need to connect "HVLED2"point to LEDA1, and "HVLED2" to LEDB2, not GND. Let's try again.
    Regards
    Sean
  • 0 in reply to Sean Zhou
    Prodigy 120 points

    Yes, I have fixed my connection, and I am able to drive different number of LED's on each Bank.  Thank You for our advice.