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LP8556 Low Vin, High Vout Condition

tateo yamashiro
Guru 10890 points
Other Parts Discussed in Thread: LP8556, LP8555

Hi,

My customer has questions about LP8556.
Could you tell me about questoins?

[Question]
 My customer consider to use LP8556 under the following condition.

  Vin=3.3V
  Vout=38.5V 11S6P
  Iled=20mA/ch
  Dimming=PWM

 So my customer has questions.

 - How much is the maximum switching duty cycle?
   Can the device regulate in this condition?

 - My customer concern about thermal performance.
   Is there thermal data in close to this condition?

 - Could you write a new device option for this condition?
   My customer doesn't have I2C interface.
   But my customer need to change EEPROM setting to output 38.5V.

Best Regards,
tateo

  • 0
    TI__Expert 4565 points

    Hi,

    Max SW duty is 93.3%, but the apps condition above may hit max SW current limit.

    In EC table on datasheet, max continuous current with 3V in/30V out condition is 120mA, which is same with this apps condition for current value.

    The conversion ratio itself is OK as LP8556 supports up to 15 at 625kHz, but SW current can reach its max value.

    I've checked this condition on EVB, and Vin could go down to 3.2V until Vboost starts collapsing.

    So, it's very marginal operation.

    For other devices, I think we don't have any device can support conversion ration higher than 11 and this high power.

    Could you increase input voltage up to 4V?

  • 0 in reply to Sung Ho Yoon
    Guru 10890 points

    Thank you for your support.

    I talked with my customer. My customer also considers to use LED 6S11P.
    So I will suggest LP8555. Could you confirm about LP8555?

     - What is default EEPROM value? I think default value is "9.2.1.2 LP8555YFQR EPROM Configuration" in datasheet.
       Is it correct? *VINIT_x=23.26V and VMAX_x=28V?

     - Does the continuous load current mean each boost convertor current? *BoostA=180mA max and BoostB=180mA max?
       Can the device regulate in this condition?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Expert 4565 points
    LP8555 will support 6s11p LED config.
    And this device doesn't have EEPROM, but just EPROM and the default value of section 9.2.1.2 is correct.
    The max LED current is 50mA, so max boost load current can be 300mA as long as max SW current is not higher than limit(typ 3.1A). I believe that LP8555 can support this apps.