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LM2623: Vin:8V, Vout:12V/Iout:2A

R.Fukunaga
Expert 2485 points
Part Number: LM2623
Other Parts Discussed in Thread: TPS61178

Tool/software:

Hi all,

I have few questions about configuration of LM2623.

My customer designed the circuit with the following configuration:

Vin:8V

Vout:12V

Iout:2A

1) Is the above I/O configuration possible with this device?

If an efficiency is 80% based on the input/output relationship, the input current can be calculated to be approximately 4A, but is it possible?

2) If the Vout is set to 12V, 12V will be applied to the VBOOT pin, which is outside the range of the absolute maximum ratings.

Is it not possible for this device to output 12V?

3) My customer is designing RF1: 120kΩ, RF2: 13.5kΩ.

This is outside the recommended range of 50k to 100kΩ, but is this OK?

Best Regards,

Ryusuke

  • 0
    TI__Prodigy 70 points

    Hi Ryusuke,

    1. I think the customer's IO configuration is impossible with this device. When Vin=8V, Vout=12V, L=4.7uH,f=2MHz,the maximum output current of LM2623 is about 0.6A. And the input current is about 1.2A.
    2. If the Vout set to 12V, VBOOT pin should disconnect with Vout.
    Considering the absolute maximum ratings of BOOT pin, customer should connect BOOT pin to a voltage that is less than 10V.
    3. If RF2 is lower than the recommended voltage, it can increase the robustness against noise injection, but the quiescent current will increase and the efficiency at light load will reduce.

    Best regards,

    Miya

  • 0 in reply to Miya Zhang
    Expert 2485 points

    Hi Miya,

    Thank you for your reply.

    I made a mistake with the device. It is LM2623A, not LM2623.Below is the circuit. Please let me know if there is anything wrong.

    1. I think the customer's IO configuration is impossible with this device. When Vin=8V, Vout=12V, L=4.7uH,f=2MHz,the maximum output current of LM2623 is about 0.6A. And the input current is about 1.2A.

    　→Can I assume that even the LM2623A is impossible? Iin≒4A is at the current limit and I think I cannot output 2A.
    2. If the Vout set to 12V, VBOOT pin should disconnect with Vout.
    Considering the absolute maximum ratings of BOOT pin, customer should connect BOOT pin to a voltage that is less than 10V.

    　→When 12V is applied to the BOOT pin, is there any problem with short circuit BOOT and (Vdd: 3.3V)?
    3. If RF2 is lower than the recommended voltage, it can increase the robustness against noise injection, but the quiescent current will increase and the efficiency at light load will reduce.

    　→Okay, thank you.

    Best Regards,

    Ryusuke

  • +1 in reply to R.Fukunaga
    TI__Prodigy 70 points

    Hi Ryusuke，

    1. Even the LM2623A is not available in this work condition. The input current limit is 2.2A. When Vin=8V, Vout=12V, L=4.7uH,f=2MHz,the maximum output current of LM2623A is about 1A.

    For the customer's application conditions, the TPS61178 is recommended. When Vin=8V, Vout=12V, the maximum output current is about 3.6A, which can meet customer's need.


    2. When 12V is applied to the BOOT pin, the device could be damaged. The BOOT pin can be connected to VDD.
    3. The absolute maximum ratings of EN pin is 10V, so when EN is applied to 12V, the device could be damaged.

    Best regards,

    Miya

  • 0 in reply to Miya Zhang
    Expert 2485 points

    Hi Miya,

    2. When 12V is applied to the BOOT pin, the device could be damaged. The BOOT pin can be connected to VDD.

    →Can the MOSFET gate of the internal circuit be driven even when BOOT=3.3V?

    Can the MOSFET gate be driven even when BOOT pin is connected to 3.3V?

    Best Regards,

    Ryusuke

  • +1 in reply to R.Fukunaga
    TI__Prodigy 70 points

    Hi Ryusuke,

    LM2623 is a very old device, we don't have a database to verify if MOSFET gate can be driven even when BOOT pin is connected to 3.3V.

    But from our experience, it should be possible to  drive MOSFET gate when BOOT=3.3V.

    Best regards,

    Miya Zhang

  • 0 in reply to Miya Zhang
    Expert 2485 points

    Hi Miya,

    Thank you for your advice.

    It was really helpful.

    Best Regards,

    Ryusuke