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LM51521-Q1: Diode emulation

Part Number: LM51521-Q1


I'm using the LM51521 in a DC UPS boosting up the input coming from a AC/DC front end or a back-up battery.

Nominal conditions are

- Boost converter input voltage range: 2.9 - 5.8V

- Boost converter output voltage: 12.5V

- Boost converter output current: 2A

- Switching frequency: 100KHz

When input voltage goes below 4.5V, gate drive signal of upper mosfet stops be for sync rectification and appears to be the one used for diode emulation (see attached picture right and left from central).

MODE pin is connected to Vcc to force PWM mode and disable diode emulation.

Is there any particular setting to be applied here to clear this behavior (having permanent sync rectification enabled).

Picture taken at Vin = 4V; Iout = 1.5A; 



  • Hi Flavio,

    I guess SENSE-SW voltage triggered maximum reverse current limit. Refer to datasheet Forced PWM (FPWM) Mode.

    Please check:

    1. Layout: Connect VOUT, HO, and SW. These traces must be routed in parallel using a short, low inductance path. VOUT must be directly connected the drain connection of QH. SW must be directly connected to the source connection of QH.
    2. Use MOSFET with low parasitic inductance and low reverse recovery.
    3. Slow down the switching speed may help.

    Best Regards,

    Feng Ji

  • Hi Feng,

    Thanks for your feedback.

    Checking advises provided.

    I'll be back to you with results.