XTR111: Current limiting PNP transistor selection

Hi Tomasz,

Tomasz Jastrzebski said:

Hello, I am designing a DAC-controlled 150 µA - 25 mA current source, compliance 10V.

This is 150 µA - 25 mA current loop transmitter for XTR111.

XTR111_0-10V_to_0.15-25mA 06042024.TSC

Tomasz Jastrzebski said:

I am still uncertain how to select PNP transistor required to limit the current.

The selection of PNP in Figure 37a has the current limit at approx. 37mA. 15Ω*37mA = 0.56V will forward biasing the emitter and base and turned on Q1 and pull the Q1's gate high to protect Q1 NMOS. You should select PNP that is capable to dissipate the power when Q1 is turned on. Or use 2N2907 BJT. 

Tomasz Jastrzebski said:

Are there any at least semi-official guidelines?

Please see the paragraph above, and the guidelines how to pick the external mosfet.

If you have other questions, please let me know. 

Best,

Raymond

Hello Raymond,
Thank you for your response and TINA file. I studied the docs, I understand how this current limiter works - it is actually very simple.
The reason why I still asked this question was because I came across posts describing current leakage problem with this config.
I try to better understand why 2N2907 transistor is recommended.
What particular properties/characteristics make it well suitable? - besides max values ofc.
Can you say that I_lim value set by R6 should be (i.e.) 10% higher than I_set to avoid leakage caused by non-perfect pnp transistor V_EB/I_B diode-like characteristics?
Do I correctly guess that transistor with the most sharply curved V_EB/I_B characteristics which does not change much with the temperature should be used?
These are kind of guidelines I am looking for.

Hi Raymond, thank you, exactly, it seems to me that collector cut-off current (I_CEX) @ V_BE=0.5V and its thermal stability is the parameter I need to pay particular attention to. Most transistor specs do not provide this value but at least DigiKey allow search by collector cut-off (Mouser does not). This thread mentions MMBT2907A as a good alternative with I_CB0=10nA which allows to hope for a low I_CEX - it is 50nA in case of Diodes Inc product, they specify it while NXP does not.
The problem with 2N2907 transistor is that it is large and expensive, sometimes very expensive.
Figure 37b and two pnp transistor limiter is much different story - a second choice since my preferences are simplicity and small PCB area.

I hope this brief summary helps anyone who comes across this thread.