Transcript Method 1: Riso

OPA277 Amplifier Stability Issue
capacitive loads
Iven Xu
12/14/2013
1
Capacitive Loads
Unity Gain Buffer Circuits
V- 12
-
Vout
+
T
U1 OPA277
Vin
V+ 12
Cload 200n
+
+
OUT
1.00
Vin
-1.00
1.76
Vout
-1.80
0.00
1.50m
Time (s)
3.00m
2
Capacitive Loads – Unity Gain Buffers - Results
NG = 1V/V = 0dB
3
METHOD#1 FOR CLOAD=200NF
1.00
T
500.00m
VOUT
0.00
-500.00m
-1.00
1.00
Vin
-1.00
0.00
4/8/2015
500.00u
1.00m
Time (s)
1.50m
2.00m
4
METHOD#2 FOR CLOAD=200NF
1.01
T
VOUT
-1.01
1.00
Vin
-1.00
0.00
4/8/2015
1.00m
Time (s)
2.00m
5
Back up
6
Method 1: Riso
V-
Vo
-
Riso 6
VLoad
+
+
VG1
+
U1 OPA627E
CLoad 1u
V+
7
Method 1: Riso - Results
Theory: Adds a zero to the Loaded AOL response to cancel the pole
120
100
100.00
Vo
Vin
V-
L1 1T
-
C1 1T
+
+
Riso 5
+
CLoad 1u
U1 OPA627E
V+
80.00
80
60.00
60
40.00
40
AOL + AOL*B
1/B
20
0.00
0
-20
-20.00
-40
-40.00
Rate of Closure
= 20dB/decade
Pole in AOL
Ro 54
20.00
Zero in AOL
Loaded AOL
+
GainGain(dB)
(dB)
T 120.00
AOL
Riso 5
180.00
180
Phase
(degrees)
Phase [deg]
VG1
CLoad 1u
Phase Margin
= 87.5degrees!
135.00
135
90.00
90
45.00
45
-
0 1.00
+ 1
-
0.00
+
C1
VG1
AOL*B
Phase
+
AOL 1M
Ro
54
10.00
10
100.00
100
1.00k
10.00k
100.00k
1k
10k (Hz) 100k
Frequency
Loaded AOL
Frequency (Hz)
Riso 5
CLoad 1u
1.00M
1M
10.00M
10M
100.00M
100M
8
Method 1: Riso - Results
When to use: Works well when DC accuracy is not important, or when
loads are very light
T
20.37m
V1
Vo
V-
Vo
-
Riso 6
VLoad
+
+
VG1
(V)
0.00
20.00m
+
U1 OPA627E
V+
CLoad 1u
V2 (V)
Vload
0.00
20.00m
VG1 (V)
Vin
0.00
0.00
125.00u
Time (s)
250.00u
9
Method 1: Riso - Theory
Vo
Vin
Ro 54
V-
L1 1T
-
+
+
Loaded AOL
+
C1 1T
Riso 5
+
CLoad 1u
U1 OPA627E
AOL
Riso 5
V+
VG1
CLoad 1u
-
-
+
+
+
C1
VG1
Ro 54
Loaded AOL
AOL 1M
Riso 5
CLoad 1u
10
Method 1: Riso - Theory
Ro 54Ohm
T
+
Loaded AOL
Vin
1+CLoad Riso s
Loaded AOL(s)=
1+(Ro+Riso) CLoad s
Pole Equation:
1
2 pi (Ro+Riso) CLoad s
Zero Equation:
1
f(zero)=
2 pi Riso CLoad s
-20.00
-20
-40
-40.00
0
0.00
Phase
(degrees)
Phase [deg]
Transfer function:
GainGain(dB)
(dB)
Riso 5Ohm
CLoad 1uF
f(pole)=
0
0.00
-45.00
-45
-90.00
-90
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
11
Method 1: Riso - Theory
120
100
100.00
T 120.00
T
80
60
40
40.00
0
0.00
60.00
GainGain(dB)
(dB)
GainGain(dB)
(dB)
80.00
-20.00
-20
20
0
0.00
-20
-20.00
-40
-40.00
20.00
-40
-40.00
0
0.00
X
135.00
135
90
90.00
-45.00
-45
45
45.00
0 1.00
1
Phase
(degrees)
Phase [deg]
Phase
(degrees)
Phase [deg]
180.00
180
-90.00
-90
1.00
1
0.00
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency
(Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
120
100.00
100
T 120.00
80
60
40.00
40
GainGain(dB)
(dB)
80.00
=
60.00
20
0.00
0
-20.00
-20
-40.00
-40
20.00
180.00
180
PhasePhase(degrees)
[deg]
10.00
10
135.00
135
90.00
90
45.00
45
0.00
0
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
10k
Frequency (Hz)
100.00k
100k
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
12
100.00M
100M
Method 1: Riso - Design
Ensure Good Phase Margin:
1.) Find: fcl and f(AOL = 20dB)
Transfer function:
2.) Set Riso to create AOL zero:
1+C
Good: f(zero) = Fcl
for Load
PM R
≈ iso
45 sdegrees.
Loaded
Better:AOL(s)=
f(zero) =1+(R
F(AOL
= 20dB)
) C willsyield slightly less than 90 degrees phase margin
+R
o
Load
iso
120
T
120.00
Pole Equation:
fcl = 222.74kHz
100
100.00
1
o
iso
Zero Equation:
1
f(zero)=
2 pi Riso CLoad s
s
80
80.00
Load
Gain (dB)(dB)
Gain
f(pole)=
f(AOL
= 20dB)
= 70.41kHz
+R ) C
2 pi (R
60
60.00
40
40.00
20
20.00
f(AOL = 20dB)
0
0.00
fcl
-20
-20.00
-40
-40.00
1.00
1
10.00
10
100.00
100
1.00k
10.00k
100.00k
1k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
13
Zero Equation:
Method 1: Riso - Design
1
f(zero)=
2 pi Riso CLoad s
Ensure Good Phase Margin: Test
f(AOL = 20dB) = 70.41kHz
fcl = 222.74kHz
→ Riso = 2.26Ohms
→ Riso = 0.715Ohms
Vin
Vin
Vo
L1 1T
L1 1T
2.26R
-
+
+
+
U1 OPA627E
V+
VG1
+
1uF
120
100
100.00
120
100
100.00
80.00
80
60.00
60
40.00
40
Riso 714m
CLoad 1u
GainGain(dB)
(dB)
GainGain(dB)
(dB)
1uF
U1 OPA627E
T 120.00
F(zero) =
70.41kHz
20.00
20
0.00
0
-20
-20.00
-40
-40.00
180.00
180
80.00
80
60.00
60
40.00
40
Riso 714m
CLoad 1u
F(zero) =
222.74kHz
20.00
20
0.00
0
-20
-20.00
-40
-40.00
180.00
180
135.00
135
Phase
(degrees)
Phase [deg]
Phase
(degrees)
Phase [deg]
+
V+
VG1
T 120.00
135.00
135
PM = 84°
90
90.00
45.00
45
0
0.715R
-
C1 1T
+
C1 1T
Vo
V-
V-
PM = 52°
90.00
90
45.00
45
0
0.00
0.00
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency
(Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
100.00M
10M
100M
14
Method 1: Riso - Design
Prevent Phase Dip:
Place the zero less than 1 decade from the pole, no more than 1.5 decades away
Good: 1.5 Decades: F(zero) ≤ 35*F(pole) → Riso ≥ Ro/34 → 70° Phase Shift
Better: 1 Decade:
F(zero) ≤ 10*F(pole) → Riso ≥ Ro/9
→ 55° Phase Shift
Riso = Ro/34
Riso = Ro/9
120
100
100.00
120
100
100.00
T 120.00
80.00
80
60.00
60
40.00
40
F(zero) =
26.5kHz
F(pole) =
2.65kHz
GainGain(dB)
(dB)
GainGain(dB)
(dB)
T 120.00
20.00
20
0.00
0
-20
-20.00
-40
-40.00
135.00
135
F(zero) =
100.2kHz
20.00
20
0.00
0
-20
-20.00
-40
-40.00
PM_min = 20°
135.00
135
PM_min = 35°
90.00
90
45.00
45
0
F(pole) =
2.86kHz
180.00
180
Phase
(degrees)
Phase [deg]
Phase
(degrees)
Phase [deg]
180.00
180
80.00
80
60.00
60
40.00
40
0.00
90.00
90
45.00
45
0
0.00
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency
(Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
15
100.00M
100M
Method 1: Riso - Design
Vin
Vo
V-
Prevent Phase Dip: Ratio of Riso to Ro
L1 1T
If Riso ≥ 2*Ro → F(zero) = 1.5*F(pole) → ~10° Phase Shift
**Almost completely cancels the pole.
C1 1T
108R
+
U1 OPA627E
+
+
VG1
V+
1uF
Riso = Ro*2
Riso = Ro*2
120
100
100.00
Phase Shift vs. Riso/Ro
Riso 714m
80.00
80
60.00
60
40.00
40
20.00
20
0.00
0
-20
-20.00
-40
-40.00
Phase (degrees)
Phase
(degrees)
Phase [deg]
180.00
180
PM_min = 80°
135
135.00
90.00
90
45.00
45
0 1.00
1
CLoad 1u
Gain (dB)
GainGain(dB)
(dB)
T 120.00
0.00
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
16
Method 1: Riso – Design Summary
Summary:
1.) Ensure stability by placing Fzero ≤ F(AOL=20dB)
2.) If Fzero is > 1.5 decades from F(pole) then increase Riso up to at least Ro/34
3.) If loads are very light consider increasing Riso > Ro for stability across all loads
120
100
100.00
T 120.00
Vin
Vo
V-
L1 1T
6R
+
U1 OPA627E
+
+
VG1
V+
1uF
80.00
80
60.00
60
40.00
40
20.00
20
0.00
0
-20
-20.00
-40
-40.00
180.00
180
Phase
(degrees)
Phase [deg]
C1 1T
GainGain(dB)
(dB)
Final Circuit
PM = 87.5°
135
Riso 714m
135.00
PM_min = 35°
CLoad 1u
90.00
90
45.00
45
0
0.00
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
17
Method 1: Riso - Disadvantage
Disadvantage:
Voltage drop across Riso may not be acceptable
T
20.19m
20.09m
Vo
Vo
+
+
VG1
+
+
V+
U1 OPA627E
25R
Riso 6
Vload
1uF
Voltage (V)
6R-
-
Voltage (V)
V-
VLoad
Riso Voltage Drop
RLoad 25
CLoad 1u
0.000
0.00
0
125.00u
125u
Time (s)
Time (seconds)
250.00
250u
18
Method 2:
Riso + Dual Feedback
Rf 49k
Cf 100n
V-
Vo
-
Riso 6
+
VLoad
VG1
+
+
U1 OPA627E
CLoad 1u
V+
19
Method 2: Riso + Dual Feedback
Theory: Features a low-frequency feedback to cancel the Riso drop
and a high-frequency feedback to create the AOL pole and zero.
Vfb
120
100
100.00
T 120.00
Rf 49k
VVo
L1 1T
-
C1 1T
+
+
VG1
Riso 6
+
U1 OPA627E
V+
80.00
80
60.00
60
40.00
40
AOL + AOL*B
1/B
Rate of Closure
= 20dB/decade
Pole in AOL
20
0.00
0
-20
-20.00
-40.00
CLoad 1u -40
180.00
180
Phase
(degrees)
Phase [deg]
Vin
GainGain(dB)
(dB)
Cf 100n
20.00
Zero in AOL
Phase Margin
= 87.5degrees!
135.00
135
AOL*B
Phase
90.00
90
45.00
45
0
0.00
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
100.00M
10M
100M
20
Method 2: Riso + Dual Feedback
When to Use: Only practical solution for very large capacitive loads ≥
10uF
When DC accuracy must be preserved across different
current loads
T
20.27m
20.3m
V1
Vo(V)
R2 49k
C1 100n
0
0.00
20.00m
20m
Vo
V-
V2
VLoad(V)
-
Riso 5
Vload
+
+
VG1
+
U1 OPA627E
CLoad 1u
0
0.00
20.00m
20m
V+
VG1
Vin(V)
0.00
0
0.00
0
150.00u
150u
Time (s)
Time (seconds)
300.00u
300u
21
Method 2: Riso + Dual Feedback - Design
Ensure Good Phase Margin:
1.) Find: fcl and f(AOL = 20dB)
2.) Set Riso to create AOL zero:
Good: f(zero) = Fcl for PM ≈ 45 degrees.
Transfer
Better: function:
f(zero) = F(AOL = 20dB) will yield slightly less than 90 degrees phase margin
1+CLoad Riso s
3.) Set Rf so Rf >>Riso
Loaded
Rf ≥AOL(s)=
(Riso * 100)
1+(Ro+Riso) CLoad s
4.) Set Cf ≥ (200*Riso*Cload)/Rf
120
T
120.00
Pole Equation:
100
100.00
1
fcl
= 222.74kHz
f(pole)=
80
2 pi (R +R ) C
Load
iso
o
f(AOL = 20dB) = 70.41kHz
Zero Equation:
1
f(zero)=
2 pi Riso CLoad s
s
Gain (dB)(dB)
Gain
80.00
60
60.00
40
40.00
20
20.00
f(AOL = 20dB)
0
0.00
fcl
-20
-20.00
-40
-40.00
1.00
1
10.00
10
100.00
100
1.00k
10.00k
100.00k
1k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
10M
100.00M
100M
22
Method 2: Riso + Dual Feedback - Summary
Ensure Good Phase Margin (Same as “Riso” Method):
1.) Set Riso so f(zero) = F(AOL = 20dB)
2.) Set Rf: Rf ≥ (Riso * 100)
3.) Set Cf: Cf ≥ (200*Riso*Cload)/Rf
120
100
100.00
T 120.00
R2 49k
GainGain(dB)
(dB)
C1 100n
Vo
V-
Riso 5
Vload
VG1
U1 OPA627E
V+
CLoad 1u
20.00
20
0.00
0
-20
-20.00
-40
-40.00
180.00
180
Phase
(degrees)
Phase [deg]
+
+
+
80.00
80
60.00
60
40.00
40
Phase Margin
= 87.5degrees!
135.00
135
90.00
90
45.00
45
0
0.00
1.00
1
10.00
10
100.00
100
1.00k
1k
10.00k
100.00k
10k
100k
Frequency (Hz)
Frequency (Hz)
1.00M
1M
10.00M
100.00M
10M
100M
23
Thank you!!
24