5.1 Volt Output instead of 5 Volt [TPS54331 DC-DC]

[zuirgham]

Refer to the schematic.

In the previous version of the board, I'm getting 5.068 volts as output voltage which is also designed using TPS54331. The newer version of the board also has the same components, but I'm getting 5.111 volts as output voltage.

VDM=Vin=8.084V(Older Version)

VDM=Vin=8.606V(Newer Version).

I removed L4 so that the circuit is isolated from the rest of the board, still, I'm getting 5.111 V as output. Also I Tried the same thing with the older version board, i'm getting 5V with no issues. Could it be something to do with the design of P.C.B? I consulted the P.C.B designer, he said both versions have a similar layout. No changes. Can anyone suggest what might be happening?

 

[Easy peasy]

if the resistors are only 1% and the ref in the chip is 2% then this explains the differences ...

 

[KlausST]

Hi,

Did you read the datasheet thoroughly ... and calculated the expectable output voltage range?

You should be able to do the math on your own:
With your feedback resistors of 1k91 and 10k2...

Nominal output voltage is 0.8V × (1+ 10k2 / 1.91) = 5.072V. (This is very close to what you see.)

Calculate the theoretical minimum output voltage with:
* resistor 1k91 to be at it's maximum tolerated value
* resistor 10k2 to be at it's minimum tolerated value
* feedback voltage at it's minimum tolerated value
* minimum value of load regulation chart (Fig. 13, your load current range)
* minimum value of line regulation (Fig. 14, your line voltage conditions)

Calculate the theoretical maximum output voltage with:
* resistor 1k91 to be at it's minimum tolerated value
* resistor 10k2 to be at it's maximum tolerated value
* feedback voltage at it's maximum tolerated value
* maximum value of load regulation chart (Fig. 13, your load current range)
* maximum value of line regulation (Fig. 14, your line voltage conditions)

The statistical expectable output voltage range is not that bad, because it is not expectable that all values are at their worst case at the same time.
We can't do (and don't like to) the math for you, because we don't know your exact part tolerances.

But you will see the output voltage is well within the specified range.
--> Nothing to complain about.

A supply voltage regulator is no voltage reference. It is made to supply the IC's with power. It is not meant as ADC reference.


Klaus

 

[FvM]

TPS54331 reference tolerance is actually 3.5%. Means the observed voltage deviation is much smaller than worst case expectable.