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LM25145: About product concept

Tsukui Yusuke
Guru 21930 points
Part Number: LM25145

Hi,

Why did TI choose voltage mode for this product?
Nowadays, products using current mode are mainstream.
I think the voltage mode is inferior in load response characteristics and phase compensation parts.
Could you tell me the benefits and reasons for selecting the voltage mode.

Best Regards,
Yusuke



  • 0
    TI__Guru 74012 points

    Hi Yusuke-san,

    Voltage-mode control provides very low-noise PWM operation and a shunt resistor for current sensing is not required. Line transient response is very good with the line feedforward feature where the ramp amplitude is proportional to Vin. Also, load transient response can be quite fast as it has second-order power stage response (whereas current-mode control has single-order response and may have slower settling time).

    Overall, voltage-mode control provides high noise immunity, low cost and small solution size.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Guru 21930 points

    Tim-san,

    Thank you for your kind support.
    Let me discuss your comments.

    ”Line transient response is very good with the line feedforward feature where the ramp amplitude is proportional to Vin. Also, load transient response can be quite fast as it has second-order power stage response (whereas current-mode control has single-order response and may have slower settling time).”

    1.About " line feedforward feature"
    　I think voltage mode is inferior to current mode in load response performance.
    　Could you tell me about the features added to the normal voltage mode?

    2.About "second-order power stage response"
    　I couldn't really understand that.
    　Could you be more specific? 

    3.Voltage mode phase compensation circuit is complex.
    　Please advise on the advantages over the disadvantages.

    　

    Best Regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Guru 74012 points

    Hi Yusuke-san,

    The operation, features and advantages of voltage-mode control are discussed extensively in the datasheet. in general, voltage-mode control provides as good if not better transient response than current-mode control.

    Here are some brief response to your questions:

    1. Line feedforward makes the ramp amplitude proportional to Vin. Thus, the loop gain is constant as Vin varies. The advantage is that the DC level of the COMP voltage does not need to change when a line transient occurs -- the duty cycle automatically changes as the ramp slope adjusts as required.

    2. The power stage response is second order based on the LC output filter. In contrast, the power stage response of a current-mode controlled system is based on single-pole dynamics -- the current-mode system effectively converts the output to a current source, albeit with non-ideal behavior requiring slope comp, etc.

    3. Please use the quickstart calculator file to assist with compensation component selection - no manual calculations required. This file is available by download from the product folder under "Design and Development".

    Regards,

    Tim