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LM3549: LM3549 LED current setting

Shoji Nakayama
Prodigy 90 points
Part Number: LM3549

On the X-axis in Figure 7, the current setting looks like 9 bits up to 1023, while the LED current setting registers from 01h to 12h have a 10-bit (8-bit and 2-bit) allocation.
Does the D1 bit of the even address (02h, etc.), which is the MSB, affect the LED current?
Also, I cannot understand the relationship between the Y-axis output current in Fig. 7 (maximum 720mA) and the Y-axis in Fig. 13 and Fig. 14 (red is maximum 600mA, green is maximum 500mA, blue is maximum 400mA).

On the LM3549 evaluation board, when the registers are set as shown below, the LED DC current of each R/G/B channel seems to be 400mA.

Addr  Value
00h   00h         using Bank0
01h   FFh         IR0 LSM
02h   01h or 11h  IR0 MSB
03h   FFh         IG0 LSM
04h   01h or 11h  IG0 MSB
05h   FFh         IB0 LSM
06h   01h or 11h  IB0 MSB
14h   00h         No brightness control

I don't know how to set the DC current of the R_OUT pin to 600mA.

  • 0
    TI__Genius 9255 points

    Hi Shoji,

    FOR Q1:

    please refer to the following pic. it shows that current setting is up to 10 bits. D1 of 02H is the MSB of the current setting.

    FOR Q2:

    The maximum output current is 705(690-720)mA.

    the current range show in the Y-axis in Fig. 13 and Fig. 14 (red is maximum 600mA, green is maximum 500mA, blue is maximum 400mA)  is setting by current setting register(which can range from 107-705). The final output current = current register * pwm control duty.

    FOR Q3:

    00h   00h         using Bank0
    01h   FFh         IR0 LSM
    02h   01h or 11h  IR0 MSB

    currenting setting = 01 1111 1111

    target current in mA = (Iset - 710) * 0.479 + 550mA = (511-710)*0.479+550=454.679mA

    if you want to set the DC current of the R_OUT pin to 600mA, set current register (Iset = (600mA - 100mA)/(650mA/1024)=788 ) to 11 0001 0100. write  0x14 to address 01h and write 0x03 to address 02h.

    Best Regard

    Monet Xu

  • 0 in reply to Monet Xu
    Prodigy 90 points

    Hi Monet-san,

    Thank you for your reply.
    The current setting registers can be set up to 1023, but do you mean that the input values should be up to 814 for red, 630 for green, and 473 for blue?

    Target    Formula            Dec    Bin        MSB(Bin)LSB(Bin)    MSB(Hex)LSB(Hex)
    100mA    (100-100)/(650/1024)    0    0000000000    00    00000000    00    00
    150mA    (150-100)/(650/1024)    79    0001001111    00    01001111    00    4F
    200mA    (200-100)/(650/1024)    158    0010011110    00    10011110    00    9E
    250mA    (250-100)/(650/1024)    236    0011101100    00    11101100    00    EC
    300mA    (300-100)/(650/1024)    315    0100111011    01    00111011    01    3B
    350mA    (350-100)/(650/1024)    394    0110001010    01    10001010    01    8A
    400mA    (400-100)/(650/1024)    473    0111011001    01    11011001    01    D9
    450mA    (450-100)/(650/1024)    551    1000100111    10    00100111    02    27
    500mA    (500-100)/(650/1024)    630    1001110110    10    01110110    02    76
    550mA    (550-100)/(650/1024)    709    1011000101    10    11000101    02    C5
    600mA    (600-550)/0.479+710    814    1100101110    11    00101110    03    2E
    650mA    (650-550)/0.479+710    919    1110010111    11    10010111    03    97
    700mA    (700-550)/0.479+710    1023    1111111111    11    11111111    03    FF

    And when driving with the maximum current of each color, is it OK to set the current limit of the buck-boost converter to 500mA on the positive side and 550mA on the negative side?

    Best regards,
    Nakayama

  • 0 in reply to Shoji Nakayama
    TI__Genius 9255 points

    Hi Nakayama,

    FOR Q1:

    Yes, you are right~

    FOR Q2:

    if you setting maximum current of R/G/B to 600/500/400mA,  the current limit of the buck-boost converter should at least set to 1100mA/1000mA.

    Best Regard

    Monet Xu

  • 0 in reply to Monet Xu
    Prodigy 90 points

    Hi Monet-san,

    I got a good understanding of the current setting.

    Thank you and est regards,
    Nakayama

  • 0 in reply to Shoji Nakayama
    TI__Genius 9255 points

    Hi Nakayama,

    If you have any question, please contact me~

    Best Regard 

    Monet Xu