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Current source feedback capacitor
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Jay_Diddy_B:
Hi OM2220 and the group,

I have been thinking about the differences between the idealized model and the full model. I have arrived at the following conclusions:

Ideal Model






The loop gain has a large variation depending on the current range. This is not desirable.

Darlington Transistor Gain

The current gain of the Darlington Transistor falls with lower currents:



In this applications, this reduction in gain at low currents helps.

If I reintroduce the Darlington transistor and measure the loop response I get:





You can see how the gain is compressed at low current by the Darlington transistor.


To be continued ….

Regards,

Jay_Diddy_B



Jay_Diddy_B:
Opamp

If I reintroduce the opamp. I am using the LTC2050 that is included with the LTspice library:





The LTC2050 is sufficient close to an ideal opamp in this application that it does not have any impact in the loop gain.

To be continued …

Regards,
Jay_Diddy_B
Jay_Diddy_B:
MOSFET Switches

Only the MOSFET switches are left.

When the MOSFET switches are off, the parasitic component is dominated by Coss. Coss is the output capacitance:



If I build a simple AC model to explore Coss I get:






This is a single pole at 1kHz.

C= 1/(2 x pi x F x R)

C= 1.6nF

Coss = 1.6nF

This matches the Coss from the Si4412DY datasheet:



To be continued …

Regards,

Jay_Diddy_B

Jay_Diddy_B:
Coss

If I replace the ideal switches with a short or capacitor on the 1A range I get this:





This is a major deviation from the ideal result. The Coss has a large impact on the loop gain.
I have only added the Coss for the low current switch. If I add all the capacitors I get:






There is very little difference.

Observation

The Coss of the MOSFETs on the low current range is the most important.

To be continued ...

Regards,
Jay_Diddy_B
Jay_Diddy_B:
Smaller MOSFET

If I select small MOSFETs, higher RDSon, but smaller Coss on the lower current ranges I can get better performance from the circuit.





The Coss of the 2N7002 MOSFET is 100pF

If I use these MOSFETs for the lower current ranges, I get:






This is a good result. The circuit is very well behaved on all the current ranges.


I have attached the LTspice model.

Regards,

Jay_Diddy_B

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