2
\$\begingroup\$

I am trying to understand how to use this configuration of the LTC2992 power measure IC and the only part I am confused with so far is the two transistors which I have circled.

enter image description here

What I know so far about this part is the ORGND will change between being rather ground or a maximum of -95V. The GPIO pins are set to input they are both open drain. I don't understand the point of these transistors even after going through the datasheet unless I have missed something. I only theory is that they are acting as diodes so far. Heres a link to the datasheet

https://www.analog.com/media/en/technical-documentation/data-sheets/ltc2992.pdf

The IC itself is meant to be able to sense the power across the sense resistor and then send this data to a master.

\$\endgroup\$
  • 1
    \$\begingroup\$ my best guess: they wanted a pair of matched diodes. \$\endgroup\$ – Marcus Müller Aug 12 at 9:13
  • \$\begingroup\$ As the notes state "MAX EMITTER-BASE BREAKDOWN VOLTAGE OF Q4, Q5 SHOULD BE LESS THAN 7V" they may be protection (sacrificial) devices. \$\endgroup\$ – Peter Smith Aug 12 at 9:15
  • \$\begingroup\$ @MarcusMüller Can't be matched. Those have only one device per package. The LTC4371 is for a dual negative supply (as its VSS) while the LTC2992 is a positive supply device. The first two GPIOs are for monitoring the two supplies. \$\endgroup\$ – jonk Aug 12 at 16:28
  • 1
    \$\begingroup\$ @PeterSmith I don't think they are sacrificial. They are depending on the fact that BJTs (and in particular these BJTs) will avalanche when their base-emitter junctions are sufficiently reverse-biased. They are preceded by a resistor divider that seriously divides down the voltage that is followed by a seriously sized current limit resistor, as well. So the BJT avalanche is strongly current-limited and won't damage them. \$\endgroup\$ – jonk Aug 12 at 16:32
  • \$\begingroup\$ Trak, do you understand what the LTC4371 does and what the LTC2992 is doing, as well? If not, then those BJTs will be difficult to explain. But if you do understand what those two ICs are there for, then the two BJTs aren't hard to understand. \$\endgroup\$ – jonk Aug 12 at 16:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.