Adjustable current source (milliAmps) - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: April 14, 2020, 12:20:45 am
content: Hi,I need an adjustable type of current source that can generate current pulses but I have very little idea of what is normally used. I can control the device with an FPGA and/or a DAC (for example if it is a voltage controlled current or something like a "frequency-to-current converter"). I read about transconductance amplifiers but they seem a little power hungry.I need to generate current pulses at 20kHz, and adjustable current levels in the range 1mA-5mA. The supply is 3.3V and supply current as little as possible (microamps if even possible).I don't know the specifics of the load yet, but it is a passive load in the range 1k-5k.Thank you

author: PTR_1275
author_rank: Frequent Contributor
timestamp: April 14, 2020, 01:04:10 am
content: Analog devices made some modules that did all sorts of conversions. I’ve used some of them before and they worked well for the task.One of them is a 5B390-5v in and either 0-20mA or 4-20mA outputs but they only look like they’re good to 400Hz.While it might not suit what you need, the datasheet might give some clues as to how to do it or other ways it can be done.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 14, 2020, 02:16:06 am
content: 5k load at 5mA needs at least 25 V power supply voltage (higher a bit of cause). So you need some step-up dc-dc too.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 14, 2020, 02:19:05 am
content: Current source design depends on accuracy, speed, load type etc.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2020, 03:25:42 pm
content: Quote from: raff5184 on April 14, 2020, 12:20:45 amWhy does it need micropower operation when the minimum output current is so much larger?QuoteWhat kind of transition time is needed? 20 kHz implies about 2 microseconds.The standard operational amplifier and transistor current source or sink will work great with an ADC but micropower operation will limit speed. Something like an LT1077 would be about as slow as feasible and require 60 microamps of supply current.Quote5 milliamps into 1k is 25 volts so a 3.3 volt supply is not adequate.Is the load grounded or floating or connected to the positive supply?

author: raff5184
author_rank: Regular Contributor
timestamp: April 14, 2020, 04:28:17 pm
content: Quote from: David Hess on April 14, 2020, 03:25:42 pmI am not sure about transition time. Is there a way to calculate it? I know that I want to generate a PWM at 20kHz (60us of pulse duration).The reason for micropower is that my device has a very limited amount of energy (small rechargeable battery or supercapacitor).The load is floating. How would floating or grounded load change the design?However it is not connected to positive supply

author: raff5184
author_rank: Regular Contributor
timestamp: April 14, 2020, 04:31:30 pm
content: Quote from: David Hess on April 14, 2020, 03:25:42 pmWould an OpAmp + MOSFET work?A BJT can sink currents in the order of hundreds of mA, right? i don't need that much

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2020, 06:02:08 pm
content: Quote from: raff5184 on April 14, 2020, 04:28:17 pmRoughly, rise or fall time is 0.35/bandwidth. This applies for a single pole frequency roll-off but most stable systems are pretty close to this. I just took 20 kHz, multiplied by 10 for 200 kHz to figure the needed bandwidth, and then got the transition time from that.QuoteSo I guess you are only pulsing the load for short durations but need an accurate and stable output current.QuoteA floating load allows more flexibility in how the current source is designed. For instance if the load is floating or connected to the positive supply, then the current source can be referenced to the negative supply (current sink) which is convenient.A floating load could also be attached between the operational amplifier's output and inverting input with the non-inverting input grounded or attached to some reference voltage. Then any current applied to the inverting input, which is at a fixed voltage of the same level as the non-inverting input, is mirrored through the load. So now not even an output transistor is not required. This configuration is sometimes used to use a zener diode as a voltage reference.Quote from: raff5184 on April 14, 2020, 04:31:30 pmA small MOSFET will work fine and is probably the best choice. If a bipolar transistor was used, then it would need to be driven by a JFET or MOSFET to remove base current errors. Or a JFET could be used.

author: BB3553AM
author_rank: Contributor
timestamp: April 14, 2020, 06:11:45 pm
content: Hello you might want to use the LT3092 :https://www.analog.com/media/en/technical-documentation/data-sheets/3092fc.pdfThan you could just put a mosfet in series with Rset that you would switch on or off. The active current mirror might act as buffer to prevent kickback (if you want have done at the output).

author: raff5184
author_rank: Regular Contributor
timestamp: August 26, 2020, 05:07:45 pm
content: Quote from: BB3553AM on April 14, 2020, 06:11:45 pmI finally went back to this project. I tried the LT3092. On a small load, of few ohms, I can source 5mA, but for a 1k-2k ohm load I need to boost the source voltage to about 8V. If I add a 300 ohm resistor between IN supply pin and OUT pin I can go up to 2 mADo you think it makes sense? Also, the mosfet that you suggested was only to turn ON/OFF the source, but not to increase the current, right?EDIT: I tried to use two LT3092's in parallel. I was expecting to double the current, but the second one only adds 200 uA