| Electronics > Projects, Designs, and Technical Stuff |
| Current source feedback capacitor |
| << < (8/9) > >> |
| 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 |
| Navigation |
| Message Index |
| Next page |
| Previous page |