Turning off a LED driver (linear regulator based via opamp) - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: May 16, 2018, 07:57:50 pm
content: Hello,We have a LED driver as in the attached schematic. We sometimes wish to shut it off by turning on the PNP and putting the opamp’s rail voltage on to its inverting input. We will later turn it back on again by turning off the PNP.The opamp used is the MIC6211. The positive rail is obviously outside of its input common mode range. Some of the old opamps would malfunction if such a high voltage was placed on to one of their input pins. Do you think the MIC6211 will be OK with this?MIC6211 datasheet:http://ww1.microchip.com/downloads/en/DeviceDoc/mic6211.pdf

author: RobK_NL
author_rank: Frequent Contributor
timestamp: May 17, 2018, 07:22:41 am
content: What a bizarrely convoluted way of doing things!Can't you just turn off the reference voltage?

author: capt bullshot
author_rank: Super Contributor
timestamp: May 17, 2018, 07:33:24 am
content: Thou shall not exceed the datasheet given input range if you want to operate the OpAmp as desired. There are OpAmps that just invert their behaviour and fully turn on M1 if you pull one of its inputs outside the range. It's not a serious and reliable design to hope your chosen Amp doesn't. One can buy Amps that are specified for that or just make up your circuit correctly: A rather simple solution would be to place a resistor in series with Q1's collector to form a divider with R5, to keep the voltage at U1 - within the specified range.

author: slugrustle
author_rank: Frequent Contributor
timestamp: May 18, 2018, 02:01:47 am
content: You really only need to force the inverting input to a higher voltage than the non-inverting input. Your LEDs are only rated for 1A (consistent with R1 = 1ohm, V4 = 1V), and it looks like your MCU is running at 5V (from V3 = 5V). Why not pull the non-inverting input high with an MCU pin through a low-leakage diode like an LL4148?

author: ocset
author_rank: Super Contributor
timestamp: May 19, 2018, 09:15:22 am
content: QuoteThanks yes thats what we''ll do , as we need the shutdown signal to be off the same power rail as that that supplies the opamp.The 5V rail might be up when the opamp rail is down in our system, and then we'd have the added unpleasantness of current flow through the opamps esd diode. -This is something which no datasheet ever really elucidates on. No datasheet ever really says its ok to do this, even with sub 1mA current going through the esd diode.

author: capt bullshot
author_rank: Super Contributor
timestamp: May 19, 2018, 10:23:20 am
content: Quote from: treez on May 19, 2018, 09:15:22 amIt's not written in the common datasheet, that's right. Some microcontrollers are specified for a maximum allowable current through these diodes, usually it is about 5mA. There are two major effects: Latching up the IC (that requires typically some 10mA) or leaking into other pins (happens at lower currents also, that is e.g. the max. current some uCs specify - max. diode current based one A/D pin leaking into another). According to common knowledge (read some analog guru books), 1 ... 5mA shouldn't harm your OpAmp. That's why one puts a series resistor with these inputs.