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TPS22975: Load switch SOA

Anatoly Melichov
Prodigy 30 points
Part Number: TPS22975

Hello,

Durring the design of a power seuence, I use the TPS22975 load switch.

The ramp time controll used to limit the inrush current followed by a capacitive load.

I miss some information about the SOA (Safe operating area) of the internal MOSFET that necessary for  “safe design” (Glitch – free turn on of a switch) and the  amount of dissipated energy during  a “soft start” feature of the load switch.

Absence of this information  limits the valuable Load capacitance at the load switch output and makes some designing use-cases totally unclear.

Can you please help me with that issue?

Thank you,

best regards,

  • 0
    TI__Genius 10855 points

    Hi Anatoly, 

    Thanks for reaching out on E2E!

    We don't provide SOA information for our load switches, due to the controlled slew rate that is achievable using our soft-start functionality. The biggest concern that can arise with capacitive loads is inrush current. The equation below can be used to calculate the amount of inrush current:

    I = C * dV/dT

    Where:

    I = inrush current

    C = load capacitance

    dV = output voltage

    dT = rise time, calculated from CT capacitor

    Using this equation, we can calculate the amount of inrush current through the device. As long as this doesn't exceed the abs max rating of the device, it should survive the inrush current event.

    Thanks,

    Arthur Huang

    To find the latest information on Power Switches, visit www.ti.com/powerswitch

  • 0 in reply to Alpha H
    Prodigy 30 points

    Hi Arthur!
    Thank you for the fast response,
    My question is about the energy dissipated on the device durring the turn on.

    I'll give an example according my design:

    vin = vbias = 3.3v (vin comes from a powerfull Power supply with large output capacitance).
    Cload = 200uF. // load capacitance
    CT = 1nF. // rise time is approximately 1.5 ms

    questions:

    1. Is this design is glich-free (no thermal shutdown will occur durring start up time) and how can I estimate a " problematic load capacitor value under the same conditions ?
    2. How can I estimate that the design is not critical in terms of part's reliability (working on high heat but no thermal shutdown durring start up) ?

    Thank you,

    Best regards,

  • 0 in reply to Anatoly Melichov
    TI__Genius 10855 points

    Hi Anatoly, 

    Using these values, we can approximate the inrush current over the 1.5ms:

    Iinrush = Cload * dV/dT

    = 200uF * 3.3V / 1.5mS

    = 0.44A

    With 0.44A of inrush current, I don't see any issues with this setup. Calculating the power dissipation, the junction temperature is still within operating conditions during the inrush event. 

    However, there are two system level considerations that could affect your solution. With your output capacitance at 200uF, I recommend adding as much input capacitance or more, if possible. This is for two reasons:

    1. Normally, the CIN capacitor is used to help the power supply handle large amounts of inrush current. During startup, if CIN isn’t large enough, the sudden change in current can cause the input voltage to sag. Is your input supply strong enough?
    2. If VIN suddenly drops while the device is enabled, there could be a brief moment where VOUT > VIN. If this happens, this could potentially bias the body diode inside the device and cause reverse current to flow upstream. Will there be a condition where VIN could suddenly be removed while the device remains enabled?

    Thanks,

    Arthur Huang

  • 0 in reply to Alpha H
    Prodigy 30 points
    Hi Arthur,
    can you please provide an information and the means you used to perform :

    "Calculating the power dissipation, the junction temperature is still within operating conditions during the inrush event".

    Thank you,
    best regards,
  • 0 in reply to Anatoly Melichov
    TI__Genius 10855 points

    Hi Anatoly, 

    We don't provide SOA curves because the current carrying capability of our load switches are ultimately dependent on the junction temperature and self-heating of the device. Taking a look at the TPS22975, we can calculate the self-heating and thermal junction (Tj) temperature using the following equations. I'm assuming your load current is 5A.

        Iout= 5A
        Ron=~16mohm, Figure 9
        Tj max= 125C
        θj-a=74.8 C/W 

        delta Tj = Iout^2 * Ron * θj-a
                      = 5^2 * .016 * 74.8
                      = 29.92

    Afterwards:

        Tjmax = delta Tj + Ta max
           125C = 29.92 + Ta max
            = 95C

    Therefore, even if your load current is 5A, you could still run the TPS22975 at roughly 95C ambient. In summary, I don't see 0.44A of inrush current affecting the device, unless you're operating out of the recommended temperature range.

    Thanks,

    Arthur Huang

    To find the latest information on Power Switches, visit www.ti.com/powerswitch