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LM5088: Design inquiry using LM5088-1 device

Part Number: LM5088

I currently design the LM5088QMH-1 device for power design.
VIN = 25V to 70V
Vout = 24V
Iout = max. 10A
Fsw = 580 kHz

As designing according to the design example shown in the datasheet,
It is difficult to derive the value of RAMP Resistor and leave a question.

On page 22, I would like to design as Rramp = (Vvcc - Vramp) / (Ios - 25uA).
Ios = Vout x 5uA/V = ​​24V x 5uA/V = ​​120uA/V
I can get it.

The Vvcc value, as shown on page 6,
Vvcc(reg) = typ. 7.8V at Vvcc = open
Can I apply a value?

In addition, the Vramp value, as seen on page 7,
Cycle-by-cycle current Limit Delay = typ. 280ns ​​at Vramp=2.5V
If so, is Vramp = 2.5V?

I would appreciate your feedback on how to obtain the Rramp value.
Thank you.

  • Hello,

    The VCC value will be typical 7.8V when left open (no external connection). In order to power with an external supply, use at least 8.2V to power the VCC. This will disable the internal regulator and improve device efficiency.
    switch.

    Second for the Vramp value, the Vramp value is 2.5V during this test for cycle by cycle current limit delay (280ns typ).

    Please let me know if anything remains unclear or if you have any additional questions.

    Best Regards,
    Katelyn Wiggenhorn
  •  


    Q1)

    For the design example in the LM5088 datasheet,
    Vcs(th)=typ. 120mV at 25 deg.C
    Rs (Current Sense Resistor)=typ. 9.85mohm

    In page 22, "The current limit value(ILIM) is set by the current sense resistor(Rs)"
    So, I solved the Current limit value as following
    ILIM = typ. 12.18A

    And then, Power of Rs = ILIM x Vcs(th) = 1.46W

    However, at the EVB schematics of LM5088,
    Resistor rating of Rs is selected as "RL3720WT-R010-F"
    Its product has a 1Watt of maximum Power rating.

    I think that its resistor rating should be a minimum 2Watt product.
    Am i wrong ? 


    Q2)

    On page 30 of the datasheet,

    At the Figure 27. LM5088-1 Application Schematic,
    There is a diode D2 (NU)
    First, I want to know the purpose of this diode.
    Also, if it is under my design conditions, whether or not this diode must be used and if so
    I want to know which device to use.


    plz reply 
    Thank you

  • Hello,

    Q1) The current will not directly flow through the ILIM pin. The diode will prevent the high currents to be seen by the RS resistor. Consequently, you do not need to rate this resistor for maximum current.

    Q2) This diode is for tying the output to VCC. This diode prevents the interference of current or voltage from VCC to Vout. If you are tying VCC to Vout, you should use this diode. If not, you do not need to use this diode.

    Best Regards,
    Katelyn
  • Dear Katelyn Wiggenhorn

    I'm planning to design power board using a LM5088 IC

    If you wouldn't have any concern, I'd like to crosscheck my schematics about that

    Please let me know your email

    And then, I will send mine.

    Thank you

  • Hello,

    You can share your schematic through e-mail. My e-mail is k-wiggenhorn@ti.com .

    Best Regards,
    Katelyn

  • Hello,

    Due to inactivity, I am going to close this thread. Please feel free to e-mail me your schematic if you have any questions or open a new E2E thread.

    Best Regards,
    Katelyn