The Improved Howland Current Pump
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Transcript The Improved Howland Current Pump
The Improved Howland Current Pump
By Tim Green – PA- Linear Applications
Engineering Manager
V-I Improved Howland Current Pump
Given:
Design Goal:
PPSL (Preferred Parts Selection List) Parts:
Vcc =+5V
OPA569 Power Op Amp
Vin = 0V to 500mV Iout = 0 to 500mA
OPA569 Pow er Op Amp:
Rload = 8 ohms, Grounded Load
2.7V to 5.5V supply
1% FSR accuracy in transfer function (Iout/Vin)
Design Considerations:
Need to Sense current in High Side
Need to drive high current -> Power Op Amp?
Need RRO Op Amp, Input CM to Ground
Input voltage range includes Vcc & Gnd
2A Output Current Capability
+/-300mV sw ing from rail at +/-2A out
Input Offset Voltage = +/-2mV max
Design Preliminary Analysis:
If there is voltage headroom then:
Power Op Amp offers single IC solution
Low drop across sense resistor -> maximize headroom & minimize power dissipation
V-I Improved Howland Current Pump
RT 0
RF 5k
RI 1k
VM 100m
X1G
-
-
+
+
VCV1
RS 5
VL
RZ 1k
+
A
Ideal
Op Amp
RX 5k
RX RF
RS
RF RX
VP
1
VM
RZ RI
RZ
RI RZ
IL
RS 1 RX RS RX RF RL
RL
RZ
RZ
RI
IL
RL 10
VP 200m
VO
1
RT0 can be used to increase the effective output impedance of the Current Source BUT
care must be taken to always ensure more feedback to the –input of the op amp than
to the +input of the op amp as net positive feedback = INSTABILITY !!
V-I Improved Howland Current Pump
Set RX=RF and RZ=RI and move all tolerances to worst case for accuracy analysis
RT
2.858407
0
2.858407
2.858407
2.858407
2.858407
2.858407
2.858407
RF
RX
5000 5000
5000 5000
5050 5000
5000 5050
5000 5000
5000 5000
5000 5000
5000 5000
0
2.858407
5050
5050
4950
4950
RI
RZ
RS
RL
1000 1000
5
10
1000 1000
5
10
1000 1000
5
10
1000 1000
5
10
1010 1000
5
10
1000 1010
5
10
1000 1000 5.05
10
1000 1000
5 10.1
990
990
1010
1010
4.95
4.95
IL
0.100000052
0.099866893
0.102371216
0.098700599
0.097727653
0.101353602
0.099009365
0.099999329
10 0.108995522
10 0.109152449
VL
1.000000100
0.998668931
1.023712000
0.987005991
0.977276527
1.013536000
0.990094651
1.009993000
VO
1.500667000
1.498669000
1.536255000
1.481159000
1.466563000
1.520981000
1.490756000
1.510665000
1.089955000 1.630222000
1.091524000 1.632570000
AM1 Sensitivity
(%)
0.000000000
0.133158931
-2.371162767
1.299452324
2.272397818
-1.353549296
0.990686485
0.000723
Comments
Rt adjusted for Ideal IL
Rt=0, Nominal Values
1% Resistor Changes
1% Resistor Changes
1% Resistor Changes
1% Resistor Changes
1% Resistor Changes
1% Resistor Changes
-8.995465322 1% Worst Case w/RT=0)
-9.152392241 1% Worst Case w/RT=Nom)
1% Resistors (w/RT=0) could yield 9% Accuracy at T=25°C
0.1% Resistors (w/RT=0) could yield 0.9% Accuracy at T=25°C
Still useful for V-I control in Motors, Valves, TECs, LEDs, etc
Other system feedback re-adjusts VM or VP for final results
V-I Improved Howland Current Pump
VP
RX
RF
1
RZ RI
RS 1 RX
RZ
RL
IL
VP 1
IL
Assume:
RF = RX
RI = RZ
RF>>RS
RF>>RL
RF
1
RZ
RI RZ
RS RX RF
RL
RZ RI
VM
1 VM
RI RF RI
RI
RL
RS 1
RF RF
RS
RS
RS
RX
RF
( VP VM )
IL
RF
RF
RI
Set RX=RF and RZ=RI
Input Voltage is Gained up
and Differentially forced
across RS
V-I Improved Howland Current Pump:
Vin = 500mV, Iout = 500mA
SW-SPST1
Ros 20M
Vm 3.421328V
Vcc
-
Iset
Imon
V_RS 100.180912mV
+
V
Voa 4.106949V
Iflag
RS 200m
VL
U1 OPA569
+ +
En
Vcc
Vin 500m
4.006768V
0.1W resistor
Tflag
R3 500k
RI 49.9k
0V < Vin < 500mV
R5 500k
RI 49.9k
R4 500k
Rset 5.76k
RF 10k
Rload 8
Vcc 5
+
A
Iout 500.846014mA
RF 10k
0V < Iout < 500mA
OPA569 Pow er Op Amp:
2.7V to 5.5V supply
Input voltage range includes Vcc & Gnd
2A Output Current Capability
+/-300mV sw ing from rail at +/-2A out
Input Offset Voltage = +/-2mV max
Always Check Loop Gain & Compensate for Stability
V-I Improved Howland Current Pump: Vin = 0V, Iout = 0A
SW-SPST1
Ros 20M
Vm 675.029386uV
Vcc
-
Iset
Imon
V_RS 18.56733uV
+
V
Voa 761.161426uV
Iflag
RS 200m
VL
U1 OPA569
+ +
Vcc
Vin 0
742.594096uV
0.1W resistor
Tflag
En
R3 500k
RI 49.9k
0V < Vin < 500mV
R5 500k
RI 49.9k
R4 500k
Rset 5.76k
RF 10k
Rload 8
Vcc 5
+
A
Iout 92.824262uA
RF 10k
0V < Iout < 500mA
OPA569 Pow er Op Amp:
2.7V to 5.5V supply
Input voltage range includes Vcc & Gnd
2A Output Current Capability
+/-300mV sw ing from rail at +/-2A out
Input Offset Voltage = +/-2mV max
Always Check Loop Gain & Compensate for Stability
V-I Improved Howland Current Pump :
Vin = 0V, Iout = 0A (modified)
SW-SPST1
Ros 20M
Add Ros to force Vm > 2mV (worst case Vos) to
drive Voa as low as possible when Vin = 0V
Vm 2.081893mV
Vcc
-
Iset
Imon
V_RS 376.332996pV
+
V
Voa 15.515232nV
Iflag
RS 200m
VL
U1 OPA569
+ +
Vcc
Vin 0
15.138899nV
0.1W resistor
Tflag
En
R3 500k
RI 49.9k
0V < Vin < 500mV
R5 500k
RI 49.9k
R4 500k
Rset 5.76k
RF 10k
Rload 8
Vcc 5
+
A
Iout 1.892362nA
RF 10k
0V < Iout < 500mA
OPA569 Pow er Op Amp:
2.7V to 5.5V supply
Input voltage range includes Vcc & Gnd
2A Output Current Capability
+/-300mV sw ing from rail at +/-2A out
Input Offset Voltage = +/-2mV max
Always Check Loop Gain & Compensate for Stability