LT1970A for a simple, linear benchtop power supply - Page 1
Forum section: electronics / projects

author: Kalvin
author_rank: Super Contributor
timestamp: January 23, 2015, 07:57:30 am
content: Hi,I was looking for a power op amp with adjustable current limit, and bumped into this LT1970A device. It seems to be quite ideal device for simple, linear benchtop power supply:- Max. 36V input voltage- Max. +/- 500mA source/sink current without external power transistors.- Fail-safe current limit, thermal protected.- Preregulator friendly design for improved efficiency and less heat.- Adjustable output voltage.- Adjustable source current.- Adjustable sink current.- Can be used as a current source and current sink.- Status outputs for current limit indicators.- Output enable.- Microcontroller friendly: Parameters set by external voltages using voltage output DAC or PWM DAC.- Can be used as power op amp with 3.6MHz GBW, 1.6V/us slew-rate.- Current limiting bandwidth 2MHz.- TSSOP 20, Price $7.- Datasheethttp://www.linear.com/product/LT1970A- Application and design notes can be found in the LT1970 page:http://www.linear.com/product/LT1970I am not sure whether this qualifies as ghetto-style benchtop power supply, but it sure looks versatile, nice linear power supply circuit with lots of built-in features suitable for hobby and semi-professional lab power supply design: Using external power transistors the output current can be increased from 500mA to 5A. Using a tracking preregulator, the heat dissipation can be reduced considerably. The device can be controlled using potentiometers or it can be driven by a microcontroller with a simple PWM DACs. Using two of these chips, a dual-output tracking regulator is possible.This is completely new chip for me, so I cannot say how well this behaves and how easy it will be used in practice. Also, I am not sure how well this behaves driving large capacitive or inductive loads, or what kind of compensation network it might need to keep it stable, and how it behaves when powering up or powering down.Any comments or ideas?Br,KalvinEdit: Changed device type to LT1970A which is improved version of LT1970.

author: Paul Price
author_rank: Super Contributor
timestamp: January 23, 2015, 09:18:03 am
content: Setting current limit with an external pass transistor to get higher current output will be problematical since the op-amp requires a large value of resistance to set current limiting. The current setting resistor will get hot, need to adjustable, big so difficult to adjust.

author: prasimix
author_rank: Unknown
timestamp: January 23, 2015, 10:42:26 am
content: A little bit exotic for my taste. As you already pointed out for usable/attractive solution it needs some real "dressing up" (pre-regulator, boosting output stage for higher current) that dissolve its initial attractiveness.

author: Kalvin
author_rank: Super Contributor
timestamp: January 23, 2015, 12:16:47 pm
content: Quote from: prasimix on January 23, 2015, 10:42:26 amI do agree. However, the 500mA may be suitable and sufficient for the low power op amp circuits requiring low noise and adjustable benchtop power supply.

author: dannyf
author_rank: Super Contributor
timestamp: January 23, 2015, 12:40:31 pm
content: You may want to look into some audio power amps with programmable current limit, as a potential source for this exercise. OPA54x comes to my mind.

author: Kalvin
author_rank: Super Contributor
timestamp: January 23, 2015, 01:36:52 pm
content: Quote from: dannyf on January 23, 2015, 12:40:31 pmThanks, dannyf. Those OPA54x's look good too. At least they will be able to provide quite a few Amps of current without any external transistors, and their maximum operating voltage spans well beyond the LT1970A.

author: dannyf
author_rank: Super Contributor
timestamp: January 23, 2015, 02:06:40 pm
content: Power amps in general make good regulators for a power supply: they are designed to dissipate lots of heat; they offer gains so the output can be controlled from a low-voltage source (mcu for example); they usually have some protection built in; they offer a much larger voltage range; they are bi-directional (sink and source current); and they are inexpensive;...

author: BravoV
author_rank: Super Contributor
timestamp: January 23, 2015, 02:47:58 pm
content: Why reinvent the wheel, just copy this proven design from Linear Technology, check this thread ->Linear Technology DC2132A CV&CC Adj. Bench Power Supply Board

author: dannyf
author_rank: Super Contributor
timestamp: January 23, 2015, 03:06:50 pm
content: What about a smps straddling a ccs?The smps (LM2596 for example) would sample the output from a LM317-based CCS. Would be interesting to see how that works out.

author: Kalvin
author_rank: Super Contributor
timestamp: January 24, 2015, 01:57:13 pm
content: Quote from: BravoV on January 23, 2015, 02:47:58 pmThis LT1970A got my attention as it looks to be a $7 single chip solution for a versatile, linear benchtop power supply with adjustable current limit. The chip seems to be intended to be used in the automated test equipment (see:http://www.ni.com/white-paper/4297/en/). It can be controlled with either potentiometers or interfaced easily to a microcontroller. It has few outputs to signal overload situation etc. which can drive LEDs and/or wired to microcontroller inputs. Of course, you can use LM317, handful of discrete components and one or two cheap quad op amps to do something similar.