OPAMP basics - How do I know much much current the power +/- need to support? - Page 1
Forum section: electronics / beginners

author: bitman
author_rank: Unknown
timestamp: October 22, 2021, 11:06:18 pm
content: For a simple OPAMP where I provide a + and - power input, how can I tell how much current these lines will draw? Particular, I was trying to generate a negative voltage for the OPAMP and I have no clue how much current draw this line will have, particular because the Charge Pumps I have found are very limited current vise (10-20 uA). And I'm coming up short trying to understand how to calculate how the output load is distributed between the two power inputs (not the inverting and non-inverting inputs).Ie inhttps://www.onsemi.com/pdf/datasheet/lm324-d.pdfit seems 60uA is the max output current - how does that get translated to Vcc and Vee? Do I need to ensure that a charge-pump can supply 60uA on the negative? Not sure how they are related.

author: Benta
author_rank: Super Contributor
timestamp: October 22, 2021, 11:13:22 pm
content: First, it's not uA (microamperes) but mA (milliamperes).For the current consumption, you need to consider two elements:1: how much current does the opamp itself consume? (datasheet, "Power Supply Current": 3.0 mA max.).2: how much current does it need to supply to the load on the output?Add those two together, and you have the ballpark figure. Add 20% for margin, and you'll be OK.

author: bitman
author_rank: Unknown
timestamp: October 22, 2021, 11:41:46 pm
content: Quote from: Benta on October 22, 2021, 11:13:22 pmSo is that the same current for Vss and Vee (in this case)?

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 22, 2021, 11:49:48 pm
content: Quote from: bitman on October 22, 2021, 11:41:46 pmIt depends. It can be the same and can be not the same. We may guess, or you may post an exact circuit so it will be easier to answer.The other way is to take into consideration max possible opamp output current. It is usually stated in the datasheet.

author: Benta
author_rank: Super Contributor
timestamp: October 22, 2021, 11:50:53 pm
content: Quote from: bitman on October 22, 2021, 11:41:46 pmNo idea. What's your load and how is it connected? My crystal ball is in the repair shop.

author: David Hess
author_rank: Super Contributor
timestamp: October 23, 2021, 11:31:32 pm
content: The operational amplifier has a roughly constant current load between the supply terminals, and the output current adds to one or the other supply terminal depending on if the output is sourcing or sinking current.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 24, 2021, 05:20:06 pm
content: Its the sum of the power supply or quiescent curent, the current in the fedback loop (the current in Rf) and the current drawn by the load, supply rails and Isc permitting.Opamps can eat more current at higher speeds but this not usually a big issue.

author: Bassman59
author_rank: Super Contributor
timestamp: October 25, 2021, 04:47:25 pm
content: Quote from: bitman on October 22, 2021, 11:06:18 pmThe current drawn from the supplies is the sum of the device's quiescent (not doing anything) draw, which is specified in the data sheet, and the current drawn by the load.Load current is simply the maximum amplitude of the output across the load. It's probably easiest to assume that, for power draw reasons, your output will always swing to the op-amp's maximum output, even if it rarely does. Or you could pick your circuit's maximum output voltage and use that.Remember the load includes both the thing you're driving with the amplifier and the op-amp's feedback network. (In an inverting amplifier, the feedback resistor connects to virtual ground, so the current in that resistor is easy to calculate.) That is, the load impedance and the feedback resistance are in parallel. The load resistance may well dominate so you might be able to ignore the feedback resistance: a 1k load impedance swamps a 100k feedback.If you assume AC signals that swing equally about 0 V, the draw from the VCC and VEE rails will be basically equal.