J
jdhar
I have implemented a slight variant on the circuit in this datasheet,
Page 21, Figure 69:
http://www.analog.com/static/imported-files/data_sheets/AD8551_8552_8554.pdf
The problem I am having is that as I lower the voltage at the node
Rsense/R1 are connected to (lets called that Vreg), and increase the
load current past a certain point, the output voltage Vout eventually
maxes out and stays constant regardless of the load current (IL). This
is because the voltage at the source is probably low enough that the
MOSFET can't operate in saturation anymore.
To give some numbers... at about Vreg of 0.8V, I start having problems
when Vsense (drop across Rsense) is 100mV. This would put the Voltage
at Opamp Pin 2 at 0.7V, and the Vgs of the MOSFET is now very close to
the threshold since the Opamp output is near zero. Increasing the
current even further will not work since the opamp has no more room to
move.
My question, is there any way I can get this to work for a Vreg of
about 0.6V? I have seen some variants on this circuit with a JFET
instead of a MOSFEt? Would that help? If I used a dual supply Opamp so
the output can go negative, would taht help?
Thanks
Page 21, Figure 69:
http://www.analog.com/static/imported-files/data_sheets/AD8551_8552_8554.pdf
The problem I am having is that as I lower the voltage at the node
Rsense/R1 are connected to (lets called that Vreg), and increase the
load current past a certain point, the output voltage Vout eventually
maxes out and stays constant regardless of the load current (IL). This
is because the voltage at the source is probably low enough that the
MOSFET can't operate in saturation anymore.
To give some numbers... at about Vreg of 0.8V, I start having problems
when Vsense (drop across Rsense) is 100mV. This would put the Voltage
at Opamp Pin 2 at 0.7V, and the Vgs of the MOSFET is now very close to
the threshold since the Opamp output is near zero. Increasing the
current even further will not work since the opamp has no more room to
move.
My question, is there any way I can get this to work for a Vreg of
about 0.6V? I have seen some variants on this circuit with a JFET
instead of a MOSFEt? Would that help? If I used a dual supply Opamp so
the output can go negative, would taht help?
Thanks