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LM5088-Q1: LM5088-Q1

Franc Povse
Prodigy 10 points
Part Number: LM5088-Q1
Other Parts Discussed in Thread: LM5088, LM5148

Tool/software:

Subject: LM5088-1 DC/DC Converter Design – Request for Support

I’ve developed a DC/DC step-down converter design using the LM5088-1 for 48V to 24V at 5.5A. The calculations were performed using TI’s Webench Power Designer, and I also verified component values with the LM5088 Quick Start Calculator.

The results from both tools are comparable.

However, I’m currently facing three issues:

  • Significant ringing at the switching node
  • Jitter on the falling edge
  • Ripple on the output when the converter is unloaded

The low-frequency ripple is present up to a load of approximately 200mA. At higher currents, up to 5.5A, the output is stable.

Attached are scope images showing:

  • 1.Ringing at the switching node (measured with a Tektronix bayonet probe)
  • 2. Jitter at the switching node
  • 3.Output ripple voltage at a 20mA load

The PCB layout is similar to the evaluation board for the LM5088.

Could you please advise on possible solutions or improvements?

 

Fig. 1

Fig. 2

Fig. 3

The light blue trace represents the ripple on the 24V output voltage, while the purple trace shows the voltage at the switching node.

  • 0
    TI__Guru 58661 points

    Hello Franc 

    1. Please use a short ground spring and measure the voltage between the cathode of the power diode and the ground connection of the sense resistor. 

    2. Looks like you are seeing the Spread Spectrum function of the device, which modulate switching frequency. Please refer 7.3.7 Frequency Dithering section in the datasheet . 

    3. Your waveforms are not synchronized well each other. Would you please capture Inductor current , V(COMP) and output voltage together ? Please show me both zoom-in and zoom-out waveforms. 

    -EL 

  • 0 in reply to Franc Povse
    TI__Guru 58661 points

    Hello Franc 

    1. There are some ideas which help lowering the minimum load requirement which is 100mA in your case, but there is no perfect solution unless you use Synchronous controllers like as LM5148. Please try these ideas. 

    • Connect C18 to the ground connection of R6 and reduce the value (Try ~220pF)
    • Change D1. Please use a Schottky diode with smaller junction capacitance 
    • Add a dummy resistor (~ 10kOhm) from SW to ground 
    • Increase C11. Please try 2.2uF at C8, and 1uF at C11

    2. about 100-150ns duty cycle jitter is not bad. Please try these ideas if you''d like to reduce further 

    • Use smaller value inductor at L2
    • Connect the CSG pin directly to the ground connection of R6 without any other connections 
    • Place R9 close to the IC and connect the ground connection of R9 to the GND of the IC. 

    -EL