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TPSM365R6EVM: Efficiency difference between datasheet and measurement

Theresia Paramitha
Prodigy 20 points
Part Number: TPSM365R6EVM

Tool/software:

Hello TI Teams,

we have tried to measure the efficiency of the Evalboard using this parameters:

Vin = 48,05 V (measured)

Iin = 36 mA

Vout = 3,33 V (measured)

Iout = 333 mA (measured)

fsw = 800 kHz

From the result we only get an efficiency of about 64%. According to the datasheet it should be >70%. Can you please explain the difference? How can we get a better efficiency?

  • 0
    TI__Mastermind 31185 points

    Hi Theresia,

    One possible cause of lower efficiency is power dissipated through the internal regulator. Typically, internal regulator regulates to internal supply voltage through an internal LDO, so a higher input voltage would mean more power is dissipated through the regulator. If you use a fixed output version, the input of the LDO is instead connected through the output voltage which can supply the internal regulator with a lower voltage. 

    Also, just as a tip, if you measure light load, you will need a device that has a longer averaging time, since the device is not switching constantly. 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 20 points

    Hi Richard,

    thanks for your reply. The evalboard from TI uses this TPSM365R6RDNR in adjustable mode.  I suppose TI measure the efficiency also using this evalboard?

    The input voltage in my measurement was measured near by the TPSM365R6RDNR input pin. There was no other loss in other part. 

    With the device with longer averaging time, do you mean the DMM? 

    It's still open for me to get the same efficiency result as stated in the datasheet. Do you have any idea that could reach the same efficiency result like in the datasheet?

    Regards,

    Theresia

  • 0 in reply to Theresia Paramitha
    TI__Mastermind 31185 points

    Hi Theresia,

    Yes, we measure efficiency of the evalboard. The loss I'm referring to is internal in the device. 

    Basically, the only way I see for you to get a similar efficiency as stated in the datasheet is to swap the device for fixed output voltage. The reason for this is that you are seeing massive power dissipation from the LDO, as the loss in the LDO can be approximated by the voltage drop across the pass element (48V-3.15V) multiplied by the current (Qg*fsw). 

    For instance, let's say to turn on the high-side MOSFET, it needs a Qg of 3nC. Calculating losses with 800kHz switching frequency means you have seen about 0.1W loss while running at 3V, 0.3A, which is about 10% of your output power. 

    But you are also right, clearly, there is some modification needed to the datasheet to be clear on what device is used to take the test. 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 20 points

    Hi Richard,

    what's the reason for the difference in the LDO input voltage connection? Why can't the adjustable device has its LDO input also be connected to the output voltage? I see some other devices have a fixed lower voltage connected to the its LDO input. It would mean, the efficiency would improve right?

    Is it possible to tie the MODE/SYNC pin of the TPSM365R6V3 (fixed device) to GND (low) to get the device always in auto mode after power up? 

    Thanks,

    Theresia

  • +1 in reply to Theresia Paramitha
    TI__Mastermind 31185 points

    Hi Theresia,

    It can be a few reasons. 

    Assuming you are not using a standard 3.3V or 5V output rail, but instead wanted a 1.8V rail or 18V rail, you would see the efficiency loss at running a higher voltage, or not being able to power the LDO from the output. Only for specific output voltage rail would this feature be helpful.

    You may also consider a matter of noise. Adjustable device has external resistor dividers which has an associated noise with it, and can couple in with the LDO to affect other parts of the circuit. However fixed output voltage device already have internal resistor divider network, which has higher noise immunity. 

    You can connect MODE/SYNC pin to GND to force PFM if it is fixed frequency device. 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 20 points

    Hi Richard,

    thanks for the explanation. It helps.

    Regards,

    Theresia