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LM74502-Q1: Vout goes too low when Vin around 4.8V

Egemen Aksoy
Intellectual 305 points
Part Number: LM74502-Q1
Other Parts Discussed in Thread: LM74502

Hi,

In my design, I am using LM74502 for load dump protection. My circuit is attached

When the Vin is around 4.8V the output voltage that supplies the SMPS goes too low. The scopes are attached.

1)

1) Blue: Vin          Yellow: Vout

2)

2) Blue: Pin 5 (Vs)      Yellow: Vin

Is this normal or what is wrong with this design?

Also the overvoltage clamping scopes are attached.

Vin=37.6, Vout=36.5

Vin=38.5, Vout=34.7

Vin=45, Vout=34.7

Is there any problem?

Best regards.

  • +1
    TI__Guru 57737 points

    Hi Egemen,

    At input voltages less than 5V, the current drawn into the VS pin (quiscent current)  of the IC  is high. This current multiplied by the VS pin series 10k resistance causes a drop in voltage that is seen at the VS pin. In the waveform shared you can see that the VS pin voltage is dropped to below VS POR threshold where the IC will not operate as expected. 

    To resolve this issue, customer needs to add a diode between the output/ IC SRC pin as shown below. This diode will provide a low impedance path to the VS pin when HGATE is high. 

    The OV Clamp behavior is as expected. 

  • 0 in reply to Praveen GD
    Intellectual 305 points

    Hi, thanks for the reply,

    There will be a 200V load dump, so what should be the voltage and current rating of the selected diode?

    Should the selected diode be a Schottky diode?

    Best Regards.

  • +1 in reply to Egemen Aksoy
    TI__Guru 57737 points

    Hi Egemen,

    The Diode is placed between VS pin and SRC. The VS pin is clamped to  around 60V by the zener. So, the max voltage across the diode would be 60V -65V. So, using a 75V rated should be fine. The current flowing through the diode will be very less in the order of 1mA. So, you can choose a really low current rated schottky diode.