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Original question:

LM51581: Sepic Controller Design for PoE-Application

LM5158: Sepic Controller Design for PoE-Application

Maik Halbe
Prodigy 150 points
Part Number: LM5158
Other Parts Discussed in Thread: LM5155

Hello,

i am using this configuration for my project and have an external mosfet between SW and the inductor Lm because of the high current:

Step 1: Design Specifications

  • Minimum Input Supply Voltage: 20,0 V
  • Typical Input Supply Voltage: 30,0 V
  • Maximum Input Supply Voltage: 60,0 V (marked red)
  • Output Target Voltage: 54,0 V
  • Maximum Output Current: 1,2 A
  • Free Running Switching Frequency: 1000 kHz
  • Free running Oscillator Set Resistor: 21,15 kΩ
  • Output Power: 64,8 W
  • SEPIC Regulator Duty Cycle Limit by LM5158 at V(supply): 90%
  • Ideal Duty Cycle at V(supply): 69,2 %

Step 2: Filter Inductor

  • Required Mode of Operation: CCM
  • Desired Maximum Inductor Current Ripple Ratio: 30%
  • Recommended Inductance: 14,88µH
  • User Selection Inductance: 15,0µH (Würth 744873150)
  • Peak Inductor current: 5,01A

Step 3: Device Selection

  • Device Recommendation: No Device
  • Device Selected: LM5158
  • Peak Current Limit Margin: -40% (marked red)
  • Slope Compensation Check: Pass

Step 4: Cac Capacitor Selection

  • Minimum Coupling capacitor value: 0,28µF
  • Selected Coupling Capacitor: 10µF
  • Minimum require C(ac) RMS current Rating: 1,8A

Step 5: Output Capacitor Selection

  • Desired load step ripple voltage: 150mV
  • Minimum output capacitance: 58,50µF
  • Capacitor RMS current: 1,8A
  • Selected output Capacitance: 68,0µF
  • Equivalent COUT ESR: 0,3mΩ

Step 6: Soft-Start Capacitor Selection

  • Suggested minimum soft-start capacitor: 30,60nF
  • Desired soft-start time at minimum input voltage: 15ms
  • Calculated soft-start capacitor: 270nF

Step 7: UVLO Resistor Divider Selection

  • Desired Voltage On: 3,8 V
  • Desired Voltage Off: 3,5 V
  • Calculated top UVLO resistor value: 34,92kΩ
  • Selected top UVLO resistor value: 34,8kΩ
  • Bottom UVLO Resistor: 22,696kΩ

Step 8: Loop Compensation

  • Voltage selected: 24.0 V
  • Select a top feedback resistor: 51.1 kΩ
  • Select a bottom resistor based on calculated value: 1.0 kΩ
  • Selected band width: 5 kHz
  • R(comp): 82 k
  • C(Comp): 2,2 nF
  • C(HF): 33 pF

I think the maximum input voltage for the device is OK (60V / abs max 63V) but what about the Peak current limit margin? Is the calculator correct for the device LM5158?

The device LM5158 has no CS-input (like the LM5155), so is this signal not needed and i connect the drain direct to ground? 

Thanks for your help.

Maik

  • 0
    TI__Genius 10385 points

    Hi Maik,

    Thank you for using E2E forums, yes the LM5158 quickstart calculator alongside the datasheet can be used to determine whether your design is correct or not.

    Regarding your other queries allow me some time I will get back to you as soon as possible.

    BR,

    Haroon

  • 0
    Prodigy 150 points

    Hi Haroon,

    any news about my project? Thanks.

    Maik

  • 0 in reply to Maik Halbe
    TI__Genius 10385 points

    Hi Maik,

    Thank you for your patience, regarding your questions yes if you have used the calculator tool available on the website then your design should be fine. You can find it here https://www.ti.com/product/LM5158#design-tools-simulation

    Regarding your "what about the Peak current limit margin?" what do you mean by that, peak current limit margin is usually set between 20 to 30%.

    Regarding the CS, this device senses switch current, which flows into the SW pin and provides a fixed internal slope compensation ramp, which helps prevent subharmonic oscillation at high duty cycle. The internal slope compensation ramp is added to the sensed switch current for the PWM operation. But, no slope compensation ramp is added to the sensed inductor current for the current limit operation to provide an accurate peak current limit over the input supply voltage. So the CS pin is not required and what do you mean by drain connected to the ground?

    BR,

    Haroon