Hi Team,
My customer would like to know footroom of LMV844 under very light load condition?
Do you have any date of that?
They expects to have 10mV or more footroom in almost no load condition
Also, In recent devices, this value has become very small, but what is the principle behind this?
Regards,
Takashi Onawa
Onawa-san,
The LMV844 datasheet only provides output swing to the supply rails under specific 2 k and 10 k load conditions. We certainly expect with an even lighter load that the output will swing somewhat closer, but the datasheet doesn't make any assurances just how close that will be. The Typical Characteristics, Output Swing Voltage vs Supply Voltage curves shown below provide information on how close the output will typically swing to rail. It clearly shows that the swing is a function of supply voltage and its swings closer at lower supply voltages.
The output swing to the rail gets closer as the RL increased from 2 k to 10 k. Closer swing to the rail should be expected as RL is increased to 100 k, and higher, but there will be some limit to just how close it gets. It may come close to 10 mV from the rail, but no assurances are made in the datasheet that it will.
How close the output comes to the rail is certainly dictated by the op amp output stage design and the on resistance (Ron) of the output MOSFETS. If the application is doesn't use a supply over +5.5 V, the OPA4340 has the closest output swing to the rail of our PRAMPS op amps.
Regards, Thomas
Precision Amplifiers Applications Engineering
Onawa-san,
Adding to Thomas' explanation, one may use sinking or sourcing graph shown in the datasheet to calculate LMV844 Ron resistance for a given supply voltage and junction temperature – I have done this for sinking current for 5V (blue) and 10V (red) supply voltage at 25C - see calculation below.
You may see that the Ron resistance actually decreases as one would expect with an increase of the supply voltage from 30ohm (5V) to 22ohms (10V). Since in case of light load (or no load) the total current in the output lower N-channel transistor is dominated by the quiescent current of LMV844, you may estimate the output swing to rail assuming that about half of LMV884 quiescent current of ~1mA flows through the output transistor. Therefore, the estimated output voltage swing to negative rail for light (or no load) condition would be:
Vswing (for Vs=5V ) = IQ/2*Ron = 0.5mA*30ohm = 15mV
Vswing (for Vs=10V ) = IQ/2*Ron = 0.5mA*22ohm = 11mV
All of the above calculations have been done for 25C junction temperature, which is a prudent assumption under light load conditions. Just keep in mind that these numbers are just typical and may vary up to +/-50% due to process variations.
