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Current source feedback capacitor
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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