• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 559 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LP8863-Q1: LP8863-Q1 SEPIC Mode

taichi
Expert 1395 points
Part Number: LP8863-Q1

Hello,

We have a question from customer about LP8863 -Q1 .

The following description is on page 73 of the datasheet.

========================

9.2.3 SEPIC Mode Application
When LED string voltage can be above and below the input voltage level, use the SEPIC configuration. In SEPIC
mode, the SW pin detects a maximum voltage equal to the sum of the input and output voltages, a consideration
when selecting components. This fact limits the maximum output voltage to 24 V. External frequency can be
used to synchronize boost or SEPIC switching frequency through the BST_SYNC pin.

 ======================

Q1）

Does the SW pin here refer to LED 0 to LED 5 pins?

Q2）

Although it states that the maximum output voltage is limited to 24 V, can you use up to 36 V when the input voltage is 12 V?

Q3)

For details, is it appropriate for use with LP8663-Q1?

Vin : 4.5V ~ 37V (Included load dump & cranking) / 12V operating voltage

Vout : 27V ~ 36V(9 Strings) × 6P 

Iout : 120mA

Regards

Taichi Nakamura

  • 0
    TI__Expert 4565 points

    Hi Nakamura-san,

    Please see my answers below for your questions.

    Q1）

    Does the SW pin here refer to LED 0 to LED 5 pins?

    - No, it's switch node of SEPIC mode. Voltage of this SW node can be Vin + Vout, so max voltage rating of SEPIC components should consider this

    Q2）

    Although it states that the maximum output voltage is limited to 24 V, can you use up to 36 V when the input voltage is 12 V?

    - Yes, if Vin level is higher, voltage at SW node will be higher too at given max Vout

    Q3)

    For details, is it appropriate for use with LP8663-Q1?

    Vin : 4.5V ~ 37V (Included load dump & cranking) / 12V operating voltage

    Vout : 27V ~ 36V(9 Strings) × 6P 

    Iout : 120mA

    - Yes, LP8863 itself won't see this high voltage(max Vin + max Vout), so it should be fine, but external components such as SW FET, coupling cap, diode, etc will need to be this high voltage rating.

    And typical conversion ratio at high load current is ~5, so normal Vin condition(CR = 36/12 = 3) will be OK, but min Vin condition(CR = 36/4.5 = 8)  may cause OCP condition temporarily during cranking period.

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Expert 1395 points

    Hi, Sung-san

    Thank you for your quick reply.

    I understood , so I will propose the SEPIC circuit while paying attention to Current Limit.

    As an additional question, Vout voltage with SEPIC circuit is limited while 6 ~ 24V in datasheet P8.

    What is the reason for this?

    Is it due to restrictions on peripheral parts?　ex. RFBTOP = 560 kΩ, RFBBOT = 170 kΩ ...

    or

    Does it affect the protection function?

    Regards,

    Taichi Nakamura

  • 0 in reply to taichi
    TI__Expert 4565 points

    Hi Nakamura-san,

    The efficiency of SEPIC mode is lower than boost only mode due to high conversion ratio at SW node, so switch duty cycle will be more limited at same output voltage condition than boost. So, we set FB divider resistors to limit max Vout lower than boost mode to keep boost SW from too high conversion ratio. Theoretically, you can increase or decrease Vout level further by adjusting FB resistor values, but we don't recommend that kind of use outside of spec range. 

    Protection can be achieved by choosing SEPIC(or boost) components with high enough voltage rating. LP8863 itself can support high voltage up to 48V.

    Hope this helps,