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LM51521-Q1: Vout reverse to Vin

Jeff YANG
Mastermind 7195 points
Part Number: LM51521-Q1

Hi,

We found a issue seems there have reverse current from Vout reverse to Vin. plesae see below.

After start up ~30ms, there have spike on Vin and you can see the output current is negative.

Is any possible reason to cause this issue, it casued Vin 5V power domain OVP.

Below is schematic. please help review anything os wrong. thanks!

  • 0
    TI__Mastermind 19485 points

    Hello Jeff,

    Thanks for reaching out to us via e2e.

    In your screenshot, I assume that yellow is the switch node voltage and blue is the inductor current.

    When the MODE pin is connected to VCC, the device will operate in FPWM mode.
    In this mode it is allowed that the indoctor current will become negative.

    So, yes, this reverse current is allowed in FPWM mode.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Mastermind 7195 points

    Hi Harry,

    The CH2 blue line is input currnet not inductor current.

    So we can change mode pin to GND set to skip mode device should not allow negative current, right?

    Why FPWM mode allow the negative current?

    Thanks!

  • 0 in reply to Jeff YANG
    TI__Mastermind 19485 points

    Hello Jeff,

    So, you are measuring the current at L44. Then the shape of that signal makes more sense to me.

    The LM51251 offers three Light Load Switching Modes.
    When you set mode pin to GND you select DE mode, when you leave it floating, you will select Skip mode.
    See description of the MODE pin is the datasheet:
    Device switching mode (FPWM, diode emulation, or skip) selection pin.
    The device is configured to skip mode if the pin is open or if a resistor that is greater than 500 kΩ is connected from the pin to AGND during initial power-on.
    The device is configured to FPWM mode by connecting the pin to VCC or if the pin voltage is greater than 2.0 V during power-on.
    The device is configured to diode emulation mode by connecting the pin to ground or the pin voltage is less than 0.4 V during initial power-on.

    > Why FPWM mode allow the negative current?
    Simply because this mode is defined like that.
    See section 9.4.2 Light Load Switching Mode, pages 28 to 31 in the datasheet.

    All information in this correspondence and in any related correspondence is provided “AS IS” and “with all faults” and is subject to TI’s Important Notice (www.ti.com/.../important-notice.shtml).

    Best regards
    Harry