# TSM101A potentially has two competing feedback loops = instability?

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#### treez

Hello,
The TSM101A offers voltage and current regulation for an SMPS…

TSM101 datasheet
http://www.farnell.com/datasheets/1700438.pdf

However, does the current regulator opamp always take priority over the voltage regulator opamp.?

If it does not, and there is a ‘free-for-all’, then the control loops of the current and voltage loops may “fight” with each other, resulting in instability.

If there is no priority given by the chip to either its current or voltage regulation opamps, then it would only be possible to use the TSM101 as a…

1…Current regulator with overvoltage limitation
2….Voltage regulator with overcurrent limitation.

Neither of these are what we want…..we have a situation where we could have the output voltage being regulated to 24V and the output current being regulated to 5 amps simultaneously…..this could only work if one of the feedback loops (current or voltage) had a feedback loop bandwidth that was at least 10 times the other one…..and this is not viable for us.

So does the TSM101 give priority to either its voltage or current feedback opamp?

#### Audioguru

The circuit is normally a voltage regulator. If the current becomes higher than the current setting then the output voltage is reduced until the current does not exceed the current setting.
Then the circuit is a voltage regulator OR a current regulator since Ohm's Law says you cannot have both operating at the same time.

You cannot have 24V AND 5A at the same time unless the load is exactly 24V/5A= 4.80 ohms and its resistance never changes, but then you do not need a current regulator. If the resistance of the load increases to 10 ohms then either the voltage stays regulated at 24V but the current drops to 24V/10 ohms= 2.4A OR the current remains at 5A AND the voltage increases to 5A x 10 ohms= 50V.

You can calculate the output current or voltage if the load resistance is reduced.

treez

### treez

points: 2

#### Anna Conda

##### Banned
Lowest always wins is the way these are designed, so either current or voltage control, yes at the crossover point there can be unstable operation, this is how experienced design engineers earn their keep by creating circuits that perform this function nicely without fighting, oscillation or other funny behaviour...

points: 2