1

LM675Improved Howland Current Pump

Stability Analysis

Tim Green

Senior Analog Applications Engineers

Precision Analog Linear Applications

July 18, 2013

2

Analysis Summary

1) Look at New Circuit and move snubber with values shown directly on output of LM675. Snubber is for LM675 output stage stability when driving reactive loads and not overall loop compensation.

2) Macromodel is for AC use only and small signal transient to check for stability. There are no large signal parameters modeled. It is basically an Aol curve. Aol curve is basedon information extracted indirectly from data sheet since there is no Aol curve in data sheet.

3) Key to accuracy of stability analysis is based on modeling of the load. Double check load DC resistance value with a 4-wire ohmmeter check. If an impedance analyzer is used detailed curves of load Z might help in double check of the load model.

4) These type of V-I circuits can get in trouble if you try to out-run the laws of physics based on the input signal. Remember V=L*dI/dt. Do not command faster current than supply Limitations, LM675 slew rate, and closed loop bandwidth will allow. If the system can demandA step input out of this circuit put a slew rate limit on a previous stage.

5) New circuit compensation has Iout/Vin small signal bandwidth at about 541Hz.

3

Original Circuit

-

+

IOP1 LM675_Aol

R1 5kOhm

R2 5kOhm

R3 2.5kOhm

R4 2.5kOhm

R5 500mOhm

R6

98O

hmR

7 50

0mO

hm

L1 4

1.8m

H

C1

530n

F

A+

AM1

Vin 2V

R8

10O

hmC

3 47

0nF

LM675 Basic Ckt.TSC

4

Original Circuit Aol, 1/Beta, Loop Gain

J1

J1

-

+

IOP1 LM675_Aol

R1 5kOhm

R2 5kOhm

R3 2.5kOhm

R4 2.5kOhm

R5 500mOhm

R6

98O

hmR

7 50

0mO

hm

L1 4

1.8m

H

C1

530n

F

A+

AM1

Vin 200mV

-

+

-

+

VCVS1 1 Ro 1.62Ohm

L2 1TH

Vo

+

VG1

C2 1TF

V+

Vfb

VL

Voa

R8

10O

hmC

3 47

0nF

Aol = Vo / Vfb

1/Beta = 1 / Vfb

Loop Gain = Vo

LM675 Basic Loop.TSC

5

Original Circuit 1/Beta and Aol

T

Frequency (Hz)

1 10 100 1k 10k 100k 1M 10M

Vo

ltag

e (

V)

-60

-40

-20

0

20

40

60

80

100

Rate of closurefirst look is possiblemarginal stability

Peaking in 1/Beta inidcates complex poles/zerosin loop gain

aol A:(124.216155k; 20.783297) b1 A:(124.216155k; 20.629828)

fcl

Original CktAol and 1/Beta

a

6

Original Circuit Loop Gain

T

Vo

-80

-60

-40

-20

0

20

40

60

Frequency (Hz)

1 10 100 1k 10k 100k 1M 10M

Vo

0

45

90

135

180

Dip in Loop gain inidcatespossible resonance

Rapid phase shift indicates possible resonance

Rapid phase fall at fcl46.7 degrees typicalOther real world parasitics may erode phase margin

fcl

Vo: Vo A:(125.644166k; -8.798517f)

Vo: Vo A:(125.644166k; 46.792622)

Original CircuitLoop Gain

a

7

Original Circuit Transient

-

+

IOP1 LM675_Aol

R1 5kOhm

R2 5kOhm

R3 2.5kOhm

R4 2.5kOhm

R5 500mOhm

R6

98O

hmR

7 50

0mO

hm

L1 4

1.8m

H

C1

530n

F

A+

AM1

R8

10O

hmC

3 47

0nF

+ VG1

Voa

LM675 BasicTran.TSC

8

Original Circuit TransientT

Time (s)

94.58m 0.10 111.12m

AM1

-1.39m

1.39m

VG1

-1.00m

1.00m

Voa

-331.08m

331.08m

Original Transient

9

New Circuit No Comp & New 1/Beta

J1

J1

-

+

IOP1 LM675_Aol

R1 5kOhm

R2 5kOhm

R3 2.5kOhm

R4 2.5kOhm

R5 500mOhm

R6

98O

hmR

7 50

0mO

hm

L1 4

1.8m

H

C1

530n

F

A+

AM1

Vin 200mV

-

+

-

+

VCVS1 1 Ro 1.62Ohm

L2 1TH

Vo

+

VG1

C2 1TF

V+

Vfb

VL

Voa

Rsn 1Ohm

Csn 220nF

Aol = Vo / Vfb

1/Beta = 1 / Vfb

Loop Gain = Vo

Note:

Rsn and Csn are NOT for Loop Compensation

They are for Output Stage Compensation of LM675

w hen driving reactive loads.

LM675 New Loop No Comp.TSC

10

New Circuit No Comp & New 1/BetaT

Aol

Frequency (Hz)

1 10 100 1k 10k 100k 1M 10M

Vo

ltag

e (

V)

-60

-40

-20

0

20

40

60

80

100

fcl

Aol

1/BetaNo Compensation

Modified 1/Beta

B+ FB#2 toModify 1/Beta

New CircuitNo Compensation

11

New Circuit Loop Gain, Aol, 1/Beta

J1

J1

-

+

IOP1 LM675_Aol

R1 5kOhm

R2 5kOhm

R3 2.5kOhm

R4 2.5kOhm

R5 500mOhm

R6

98O

hmR

7 50

0mO

hm

L1 4

1.8m

H

C1

530n

F

A+

AM1

Vin 200mV

-

+

-

+

VCVS1 1 Ro 1.62Ohm

L2 1TH

Vo

+

VG1

C2 1TF

V+

Vfb

VL

Voa

Rsn 1Ohm

Csn 220nF

R8 620kOhmC3 513pF

Aol = Vo / Vfb

1/Beta = 1 / Vfb

Loop Gain = Vo

Note:

Rsn and Csn are NOT for Loop Compensation

They are for Output Stage Compensation of LM675

w hen driving reactive loads.

LM675 New Loop New Comp.TSC

12

New Circuit Aol and 1/BetaT

aol

1/Beta

Frequency (Hz)

1 10 100 1k 10k 100k 1M 10M

Vo

ltag

e (

V)

-60

-40

-20

0

20

40

60

80

100

New CircuitAol and 1/Beta

1/Beta

aol

13

New Circuit Loop GainT

Vo

-80

-60

-40

-20

0

20

40

60

Frequency (Hz)

1 10 100 1k 10k 100k 1M 10M

Vo

-20

0

20

40

60

80

100

120

140

160

180

New Circuit Loop

fcl

a

14

New Circuit Transient

-

+

IOP1 LM675_Aol

R1 5kOhm

R2 5kOhm

R3 2.5kOhm

R4 2.5kOhm

R5 500mOhm

R6

98O

hmR

7 50

0mO

hm

L1 4

1.8m

H

C1

530n

F

A+

AM1

-

+

-

+

VCVS1 1 Ro 1.62Ohm

Vo VL

Voa

Rsn 1Ohm

Csn 220nF

R8 619kOhmC3 510pF

+ VG2

Note:

Rsn and Csn are NOT for Loop Compensation

They are for Output Stage Compensation of LM675

w hen driving reactive loads.

LM675 New Comp Tran.TSC

15

New Circuit TransientT

Time (s)

86.03m 106.30m 126.56m

AM1

-1.29m

1.15m

VG2

-1.00m

1.00m

VL

-197.30m

197.29m

Vo

-197.64m

197.63m

Voa

-176.32m

197.37m

New Circuit Transient

16

New Circuit AC Transfer – Iout/VinT

Frequency (Hz)

1.00 1.00k 1.00M

AM1

-40

-20

0

20

New CircuitIout / Vin AC Transfer

AM1 A:(60.000395; -192.751567m) B:(541.09919; -3.191541)

a b

17

TINA Plots

Plots.tdr