LT1497 - Dual 125mA, 50MHz Current
LT1497
Dual 125mA, 50MHz
Current Feedback Amplifier
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DESCRIPTION
FEATURES
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Minimum Output Current: ±125mA
Maximum Supply Current per Amp: 7mA, VS = ±5V
Bandwidth: 50MHz, VS = ± 15V
Slew Rate: 900V/µs, VS = ±15V
Wide Supply Range: VS = ±2.5V to ±15V
(Enhanced θJA 16-Pin SO Package)
Enhanced θJA SO-8 Package for ±5V Operation
0.02% Differential Gain: AV = 2, RL = 150Ω
0.015° Differential Phase: AV = 2, RL = 150Ω
±13V Output Swing: IL = 100mA, VS = ±15V
±3.1V Output Swing: IL = 100mA, VS = ±5V
55ns Settling Time to 0.1%, 10V Step
Thermal Shutdown Protection
The LT®1497 dual current feedback amplifier features low
power, high output drive, excellent video characteristics
and outstanding distortion performance. From a low 7mA
maximum supply current per amplifier, the LT1497 drives
±100mA with only 1.9V of headroom. Twisted pairs can be
driven differentially with – 70dBc distortion up to 1MHz for
±40mA peak signals.
The LT1497 is available in a low thermal resistance 16-pin
SO package for operation with supplies up to ±15V. For
±5V operation the device is also available in a low thermal
resistance SO-8 package. The device has thermal and
current limit circuits that protect against fault conditions.
The LT1497 is manufactured on Linear Technology’s
complementary bipolar process. The device has characteristics that bridge the performance between the LT1229
and LT1207 dual current feedback amplifiers. The LT1229
has 30mA output drive, 100MHz bandwidth and 12mA
supply current. The LT1207 has 250mA output drive,
60MHz bandwidth and 40mA supply current.
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APPLICATIONS
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Twisted-Pair Drivers
Video Amplifiers
Cable Drivers
Test Equipment Amplifiers
Buffers
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATION
HDSL2 Single Pair Line Driver
2nd and 3rd Harmonic Distortion of
HDSL2 Single Pair Line Driver
560Ω
560Ω
– 40
–
– 50
VS = ± 5V
VIN = ±1.25V
VOUT = ± 2.5V
VIN
+
1:1*
560Ω
135Ω
560Ω
DISTORTION (dBc)
68.1Ω
1/2 LT1497
– 60
– 70
2ND
3RD
– 80
–
68.1Ω
*MIDCOM 671-7807
1/2 LT1497
– 90
100k
+
1M
2M
FREQUENCY (Hz)
1419 TA01
1497 TA02
1
LT1497
W W
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ABSOLUTE MAXIMUM RATINGS
Total Supply Voltage (V + to V –)
LT1497CS8.......................................................... 14V
LT1497CS............................................................ 36V
Noninverting Input Current ................................... ±2mA
Output Short-Circuit Duration (Note 1) .......... Continuous
Operating Temperature Range (Note 2) ... – 40°C to 85°C
Specified Temperature Range ...................... 0°C to 70°C
Maximum Junction Temperature (See Below) ....... 150°C
Storage Temperature Range .................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
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PACKAGE/ORDER INFORMATION
ORDER PART
NUMBER
1
16 V –
NC 2
15 NC
OUT A 3
14 V +
TOP VIEW
OUT A
LT1497CS8
8 V+
1
–IN A 2
7 OUT B
–IN A 4
A
+IN A 3
V– 4
B
6 –IN B
+IN A 5
5 +IN B
V– 6
S8 PACKAGE
8-LEAD PLASTIC SO
S8 PART MARKING
TJMAX = 150°C, θJA = 80°C/ W (NOTE 3)
ORDER PART
NUMBER
TOP VIEW
V–
1497
LT1497CS
13 OUT B
A
B
12 –IN B
11 +IN B
NC 7
10 NC
V– 8
9
V–
S PACKAGE
16-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 40°C/ W (NOTE 3)
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
VCM = 0V, ±2.5V ≤ VS ≤ ±15V (LT1497CS), ±2.5V ≤ VS ≤ ±5V (LT1497CS8), pulse tested unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VOS
Input Offset Voltage
TA = 25°C
MIN
TYP
MAX
UNITS
±3
±10
±15
mV
mV
±1
±3.5
±5.0
mV
mV
●
Input Offset Voltage Matching
TA = 25°C
●
Input Offset Voltage Drift
IIN+
Noninverting Input Current
TA = 25°C
±1
±3
±10
µA
µA
±0.3
±1.0
±1.5
µA
µA
±7
±20
±40
µA
µA
±3
±10
±15
µA
µA
●
Noninverting Input Current Matching
TA = 25°C
●
IIN–
Inverting Input Current
TA = 25°C
●
Inverting Input Current Matching
µV/°C
10
●
TA = 25°C
●
en
Input Noise Voltage Density
f = 1kHz, RF = 1k, RG = 10Ω, RS = 0Ω
3
nV/√Hz
+ in
Noninverting Input Noise Current Density
f = 1kHz, RF = 1k, RG = 10Ω, RS = 10k
2
pA/√Hz
– in
Inverting Input Noise Current Density
f = 1kHz, RF = 1k, RG = 10Ω, RS = 10k
20
pA/√Hz
RIN
Input Resistance
VIN = ±13V, VS = ±15V
VIN = ±3V, VS = ±5V
VIN = ±0.5V, VS = ±2.5V
10
8
8
MΩ
MΩ
MΩ
CIN
Input Capacitance
3
pF
2
●
●
●
1.5
1.5
1.5
LT1497
ELECTRICAL CHARACTERISTICS
VCM = 0V, ±2.5V ≤ VS ≤ ±15V (LT1497CS), ±2.5V ≤ VS ≤ ±5V (LT1497CS8), pulse tested unless otherwise noted.
SYMBOL
CMRR
PARAMETER
CONDITIONS
Input Voltage Range
VS = ±15V
VS = ±5V
VS = ±2.5V
Common Mode Rejection Ratio
VS = ±15V, VCM = ±13V, TA = 25°C
MIN
TYP
●
●
●
±13
±3.0
±0.5
±14
±4.0
±1.5
V
V
V
55
53
62
●
dB
dB
54
52
60
●
dB
dB
52
50
56
●
dB
dB
VS = ±5V, VCM = ±3V, TA = 25°C
VS = ±2.5V, VCM = ±0.5V, TA = 25°C
PSRR
Inverting Input Current
Common Mode Rejection
VS = ±15V, VCM = ±13V
VS = ±5V, VCM = ±3V
VS = ±2.5V, VCM = ±0.5V
Power Supply Rejection Ratio
VS = ±2V to ±15V, TA = 25°C
2.0
2.5
3.0
●
●
●
MAX
10
10
10
UNITS
µA/V
µA/V
µA/V
66
63
76
●
dB
dB
66
63
76
●
dB
dB
VS = ±2V to ±5V, TA = 25°C
Noninverting Input Current
Power Supply Rejection
VS = ±2V to ±15V
VS = ±2V to ±5V
●
●
5
5
50
50
nA/V
nA/V
Inverting Input Current
Power Supply Rejection
VS = ±2V to ±15V
VS = ±2V to ±5V
●
●
0.1
0.1
2
2
µA/V
µA/V
AVOL
Large-Signal Voltage Gain
VS = ± 15V, VOUT = ±10V, RL = 150Ω
VS = ±5V, VOUT = ±2.5V, RL = 50Ω
VS = ±2.5V, VOUT = ±0.5V, RL = 50Ω
●
●
●
66
66
66
80
80
80
dB
dB
dB
ROL
Transresistance, ∆VOUT/∆IIN–
VS = ±15V, VOUT = ±10V, RL = 150Ω
VS = ±5V, VOUT = ±2.5V, RL = 50Ω
VS = ±2.5V, VOUT = ±0.5V, RL = 50Ω
●
●
●
100
100
100
500
500
300
kΩ
kΩ
kΩ
VOUT
Maximum Output Swing
VS = ±15V, RL = 150Ω, TA = 25°C
±12.80
±12.60
±13.15
●
V
V
±12.65
±12.55
±13.0
●
V
V
±3.20
±3.10
±3.45
●
V
V
±2.75
±2.65
±3.10
●
V
V
±1.25
±1.15
±1.45
●
V
V
±1.00
±0.90
±1.15
●
V
V
●
●
± 125
± 125
± 220
± 220
± 140
mA
mA
mA
VS = ±15V, IL = ±100mA, TA = 25°C
VS = ±5V, RL = 50Ω, TA = 25°C
VS = ±5V, IL = ±100mA, TA = 25°C
VS = ±2.5V, RL = 50Ω, TA = 25°C
VS = ±2.5V, IL = ±50mA, TA = 25°C
IOUT
Maximum Output Current
RL = 1Ω, VS = ±15V
RL = 1Ω, VS = ±5V
RL = 1Ω, VS = ±2.5V
IS
Supply Current per Amplifier
VS = ±2.5V to ±5V, TA = 25°C
6.0
7.0
8.0
mA
mA
7.0
9.0
10.5
mA
mA
●
VS = ±15V, TA = 25°C
●
Channel Separation
VS = ±15V, VOUT = ±10V, RL = 150Ω
VS = ±5V, VOUT = ±2.5V, RL = 50Ω
●
●
100
100
120
115
dB
dB
3
LT1497
ELECTRICAL CHARACTERISTICS
VCM = 0V, ±2.5V ≤ VS ≤ ±15V (LT1497CS), ±2.5V ≤ VS ≤ ±5V (LT1497CS8), pulse tested unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
SR
Slew Rate
VS = ±15V, TA = 25°C (Note 4)
900
●
500
400
V/µs
V/µs
200
150
350
●
V/µs
V/µs
VS = ±5V, TA = 25°C (Note 4)
MAX
UNITS
BW
Small-Signal Bandwidth
VS = ±15V, RF = RG = 560Ω, RL = 100Ω
VS = ±5V, RF = RG = 560Ω, RL = 100Ω
VS = ±2.5V, RF = RG = 560Ω, RL = 100Ω
50
35
30
MHz
MHz
MHz
tr
Small-Signal Rise Time
VS = ±15V, RF = RG = 560Ω, RL = 100Ω
VS = ±5V, RF = RG = 560Ω, RL = 100Ω
VS = ±2.5V, RF = RG = 560Ω, RL = 100Ω
7.5
9.5
11
ns
ns
ns
Overshoot
VS = ±15V, RF = RG = 560Ω, RL = 100Ω
VS = ±5V, RF = RG = 560Ω, RL = 100Ω
VS = ±2.5V, RF = RG = 560Ω, RL = 100Ω
15
12
10
%
%
%
Propagation Delay
VS = ±15V, RF = RG = 560Ω, RL = 100Ω
VS = ±5V, RF = RG = 560Ω, RL = 100Ω
VS = ±2.5V, RF = RG = 560Ω, RL = 100Ω
6.8
8.4
9.7
ns
ns
ns
Settling Time
VS = ±15V, 10V Step, 0.1%, AV = – 1
VS = ±5V, 5V Step, 0.1%, AV = – 1
55
50
ns
ns
Differential Gain (Note 5)
VS = ±15V, RF = RG = 510Ω, RL = 150Ω
VS = ±15V, RF = RG = 510Ω, RL = 50Ω
VS = ±5V, RF = RG = 510Ω, RL = 150Ω
VS = ±5V, RF = RG = 510Ω, RL = 50Ω
0.02
0.19
0.08
0.41
%
%
%
%
Differential Phase (Note 5)
VS = ±15V, RF = RG = 510Ω, RL = 150Ω
VS = ±15V, RF = RG = 510Ω, RL = 50Ω
VS = ±5V, RF = RG = 510Ω, RL = 150Ω
VS = ±5V, RF = RG = 510Ω, RL = 50Ω
0.015
0.235
0.045
0.310
Deg
Deg
Deg
Deg
ts
The ● denotes specifications which apply over the full operating
temperature range.
Note 1: Applies to short circuits to ground only. A short circuit between the
output and either supply may damage the part when operated on supplies
greater than ±10V
Note 2: The LT1497 is designed, characterized and expected to operate
over the temperature range of – 40°C to 85°C, but is not tested at – 40°C
and 85°C. Guaranteed industrial grade parts are available, consult factory.
Note 3: Thermal resistance varies depending upon the amount of PC board
metal attached to the device. θJA is specified for a 2500mm2 test board
covered with 2oz copper on both sides.
4
Note 4: Slew rate is measured between ±5V on a ±10V output signal while
operating on ±15V supplies with RF = 453Ω, RG = 49.9Ω and RL = 150Ω.
On ±5V supplies slew rate is measured between ±1V on a ±3V output
signal. The slew rate is much higher when the input is overdriven and
when the amplifier is operated inverting. See the Applications Information
section.
Note 5: NTSC composite video with an amplifier output level of 2V peak.
LT1497
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SMALL-SIGNAL BANDWIDTH
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VS = ±15V, Peaking ≤ 1dB
AV
–1
RL
150
50
20
150
50
20
150
50
20
150
50
20
1
2
10
VS = ±5V, Peaking ≤ 1dB
RF
560
560
620
560
560
560
510
560
620
270
270
270
RG
560
560
620
–
–
–
510
560
620
30
30
30
– 3dB BW (MHz)
59.2
43.1
30.0
57.0
42.7
30.3
59.1
41.7
20.7
43.4
30.9
19.0
AV
–1
RL
150
50
20
150
50
20
150
50
20
150
50
20
1
2
10
RF
510
560
560
510
560
560
510
560
560
270
270
270
RG
510
560
560
–
–
–
510
560
560
30
30
30
– 3dB BW (MHz)
45.0
32.0
23.2
44.3
31.7
22.9
41.7
30.4
21.9
28.1
21.9
14.6
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TYPICAL PERFORMANCE CHARACTERISTICS
– 3dB Bandwidth
vs Supply Voltage
Voltage Gain and Phase
vs Frequency, Gain = 6dB
0
90
45
80
8
±15V
± 5V
135
5
180
4
225
± 5V
3
270
±15V
2
1
0
– 3dB BANDWIDTH (MHz)
90
GAIN
6
PHASE SHIFT (DEG)
VOLTAGE GAIN (dB)
7
RL = 100Ω
RF = RG = 560Ω
–1
0.1
PEAKING ≤ 1dB
PEAKING ≤ 5dB
90
GAIN = 2
RL = 1k
70
RF = 470Ω
60
RF = 560Ω
50
40
RF = 750Ω
30
RF = 1k
20
0
2
4
6
8 10 12 14
SUPPLY VOLTAGE (± V)
1497 G01
0
±15V
24
90
22
135
GAIN
20
180
18
225
16
12
10
8
0.1
270
± 5V
14
±15V
RL = 100Ω
RF = 270Ω
RG = 30Ω
PEAKING ≤ 1dB
PEAKING ≤ 5dB
80
PHASE SHIFT (DEG)
VOLTAGE GAIN (dB)
± 5V
90
45
– 3dB BANDWIDTH (MHz)
PHASE
16
100
1497 G04
RF = 560Ω
40
30
RF = 750Ω
20
RF = 1k
0
18
2
4
6
8 10 12 14
SUPPLY VOLTAGE (± V)
60
RF = 560Ω
90
PEAKING ≤ 1dB
PEAKING ≤ 5dB
80
RF = 430Ω
40
30
18
1497 G03
GAIN = 10
RL = 1k
RF = 270Ω
50
16
– 3dB Bandwidth
vs Supply Voltage
70
RF = 750Ω
20
RF = 1k
10
1
10
FREQUENCY (MHz)
50
– 3dB Bandwidth
vs Supply Voltage
28
RF = 470Ω
60
1497 G02
Voltage Gain and Phase
vs Frequency, Gain = 20dB
26
70
0
0
100
GAIN = 2
RL = 100Ω
10
10
1
10
FREQUENCY (MHz)
PEAKING ≤ 1dB
PEAKING ≤ 5dB
80
– 3dB BANDWIDTH (MHz)
PHASE
– 3dB BANDWIDTH (MHz)
9
– 3dB Bandwidth
vs Supply Voltage
70
60
50
RF = 430Ω
40
RF = 270Ω
RF = 560Ω
30
20
10
0
GAIN = 10
RL = 100Ω
RF = 750Ω
RF = 1k
0
0
2
4
6
8 10 12 14
SUPPLY VOLTAGE (± V)
16
18
1497 G05
0
2
4
6
8 10 12 14
SUPPLY VOLTAGE (± V)
16
18
1497 G06
5
LT1497
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TYPICAL PERFORMANCE CHARACTERISTICS
Differential Phase
vs Supply Voltage
Differential Gain
vs Supply Voltage
0.4
RL = 50Ω
0.3
0.2
0.1
0.3
RL = 50Ω
0.2
0.1
7
5
RL = 1k
RL = 1k
RL = 150Ω
11
13
9
SUPPLY VOLTAGE (± V)
0
15
–2
11
13
9
SUPPLY VOLTAGE (± V)
–3
IL = 100mA
3
2
IL = 125mA
1
IL = 100mA
IL = 50mA
V–
– 50 – 25
IL = 75mA
50
25
75
0
TEMPERATURE (°C)
100
IL = 50mA
–1
IL = 75mA
–2
IL = 125mA
–3
IL = 100mA
3
2
IL = 125mA
1
IL = 100mA
IL = 50mA
IL = 75mA
50
25
75
0
TEMPERATURE (°C)
V+
8.0
– 0.5
COMMON MODE RANGE (V)
SUPPLY CURRENT PER AMPLIFIER (mA)
8.5
6.5
VS = ± 5V
VS = ± 2.5V
5.5
5.0
– 50 – 25
100
125
1497 G13
6
100
3
2
IL = 75mA
1
V–
– 50 – 25
125
IL = 75mA
–3
IL = 25mA
IL = 50mA
50
25
75
0
TEMPERATURE (°C)
100
1497 G12
350
V + = 2V TO 18V
–1.0
–1.5
1.5
1.0
V–
– 50 – 25
V – = – 2V TO –18V
50
25
75
0
TEMPERATURE (°C)
125
Output Short-Circuit Current
vs Junction Temperature
0.5
50
25
75
0
TEMPERATURE (°C)
IL = 25mA
IL = 50mA
–2
Input Common Mode Limit
vs Junction Temperature
7.0
VS = ±2.5V
–1
1497 G11
Supply Current
vs Ambient Temperature
6.0
Output Saturation Voltage
vs Junction Temperature, ±2.5V
V+
1497 G10
VS = ±15V
3
1497 G09
VS = ± 5V
V–
– 50 – 25
125
7.5
1
2
FEEDBACK RESISTOR (kΩ)
0
15
OUTPUT SHORT-CIRCUIT CURRENT (mA)
IL = 125mA
10
1
7
5
OUTPUT SATURATION VOLTAGE (V)
OUTPUT SATURATION VOLTAGE (V)
OUTPUT SATURATION VOLTAGE (V)
V+
IL = 75mA
VS = ±15V
100
Output Saturation Voltage
vs Junction Temperature, ±5V
VS = ± 15V
IL = 50mA
VS = ± 5V
1497 G08
Output Saturation Voltage
vs Junction Temperature, ±15V
–1
1000
RL = 150Ω
1497 G07
V+
RL = 1k
AV = 2
PEAKING ≤ 5dB
RF = RG = 510Ω
AV = 2
AMPLIFIER OUTPUT = 2V PEAK
CAPACITIVE LOAD (pF)
0.4
0
10000
0.5
RF = RG = 510Ω
AV = 2
AMPLIFIER OUTPUT = 2V PEAK
DIFFERENTIAL GAIN (%)
DIFFERENTIAL PHASE (DEG)
0.5
Maximum Capacitive Load
vs Feedback Resistor
100
125
1497 G14
VS = ±15V
RL = 1Ω
300
250
SINKING
200
SOURCING
150
100
50
0
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
1497 G15
LT1497
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TYPICAL PERFORMANCE CHARACTERISTICS
10
AV = –1
AV = 1
6
4
2
0
–2
–4
AV = –1
–6
20
0
AV = –1
2
0
–2
–4
AV = –1
AV = 1
60
40
SETTLING TIME (ns)
80
100
25 50 75 100 125 150 175 200 225 250
SETTLING TIME (ns)
0
VOUT = 7VRMS
VOUT = 2VRMS
– 60
AV = 1
2ND
– 70
AV = –1
2ND
– 80
AV = 1
3RD
AV = –1
3RD
0.1
100k
1
FREQUENCY (MHz)
1497 G19
80
POWER SUPPLY REJECTION (dB)
10
1
RF = RG = 1.5k
0.1
RF = RG = 560Ω
100k
1M
10M
FREQUENCY (Hz)
30
25
20
15
100M
1497 G22
0
10
15
20
FREQUENCY (MHz)
5
60
50
NEGATIVE
POSITIVE
40
30
20
10
0
10k
30
Amplifier Crosstalk vs Frequency
VS = ±15V
RL = 50Ω
RF = RG = 560Ω
70
25
1497 G21
Power Supply Rejection
vs Frequency
VS = ±15V
0.01
10k
35
10
10
VS = ±15V
RL = 50Ω
RF = 270Ω
RG = 30Ω
PO1 = PO2 = 4dBm
1497 G20
Output Impedance vs Frequency
OUTPUT IMPEDANCE (Ω)
1497 G18
–100
1k
10k
FREQUENCY (Hz)
100k
3rd Order Intercept vs Frequency
– 50
– 90
100
1k
10k
FREQUENCY (Hz)
100
40
VS = ±15V
VOUT = 5VP-P
RL = 50Ω
RF = 560Ω
– 30
DISTORTION (dBc)
TOTAL HARMONIC DISTORTION (%)
– 20
– 40
100
1
10
2nd and 3rd Harmonic Distortion
vs Frequency
VS = ± 15V
RL = 100Ω
RF = RG = 560Ω
0.001
10
en
1497 G17
Total Harmonic Distortion
vs Frequency
0.01
10
+ in
1497 G16
0.10
– in
–8
–10
– 10
OUTPUT TO INPUT CROSSTALK (dB)
–10
AV = 1
4
–6
AV = 1
–8
VS = ±15V
RF = 560Ω
8
OUTPUT STEP (V)
OUTPUT STEP (V)
6
100
10
VS = ±15V
RF = 560Ω
Spot Noise Voltage and Current
vs Frequency
3RD ORDER INTERCEPT (dBm)
8
Settling Time to 1mV
vs Output Step
SPOT NOISE (nV/√Hz OR pA/√Hz)
Settling Time to 10mV
vs Output Step
– 20
– 30
– 40
VS = ±15V
AV = 10
RL = 100Ω
RF = 560Ω
RG = 62Ω
– 50
– 60
– 70
– 80
– 90
–100
100k
1M
10M
FREQUENCY (Hz)
100M
1497 G23
–110
10k
100k
1M
10M
FREQUENCY (Hz)
100M
1497 G24
7
LT1497
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APPLICATIONS INFORMATION
The LT1497 is a dual current feedback amplifier with high
output current drive capability. Bandwidth is maintained
over a wide range of voltage gains by the appropriate
choice of feedback resistor. These amplifiers will drive low
impedance loads such as cables with excellent linearity at
high frequencies.
Feedback Resistor Selection
The optimum value for the feedback resistor is a function
of the operating conditions of the device, the load impedance and the desired flatness of frequency response. The
Small-Signal Bandwidth table gives the values which
result in the highest bandwidth with less than 1dB of
peaking for various gains, loads and supply voltages. If
this level of flatness is not required, a higher bandwidth
can be obtained by use of a lower feedback resistor. The
characteristic curves of Bandwidth vs Supply Voltage
indicate feedback resistors for peaking up to 5dB. These
curves use a solid line when the response has less than
1dB of peaking and a dashed line when the response has
1dB to 5dB of peaking. Note that in a gain of 10 peaking is
always under 1dB for the resistor ranges shown. Reducing
the feedback resistor further than 270Ω in a gain of 10 will
increase the bandwidth, but it also loads the amplifier and
reduces the maximum current available to drive the load.
Capacitive Loads
The LT1497 can drive capacitive loads directly when the
proper value of feedback resistor is used. The graph of
Maximum Capacitive Load vs Feedback Resistor should
be used to select the appropriate value. The graph shows
feedback resistor values for 5dB frequency peaking when
driving a 1k load at a gain of 2. This is a worst-case
condition. The amplifier is more stable at higher gains and
driving heavier loads (smaller load resistors). Alternatively, a small resistor (10Ω to 20Ω) can be put in series
with the output to isolate the capacitive load from the
amplifier output. This has the advantage in that the amplifier bandwidth is only reduced when the capacitive load is
present, and the disadvantage that the gain is a function of
the load resistance.
8
Capacitance on the Inverting Input
Current feedback amplifiers require resistive feedback
from the output to the inverting input for stable operation.
Take care to minimize the stray capacitance between the
output and the inverting input. Capacitance on the inverting input to ground will cause peaking in the frequency
response (and overshoot in the transient response), but it
does not degrade the stability of the amplifier.
Power Supplies
The LT1497 will operate on single or split supplies from
±2V (4V total) to ±15V (30V total). It is not necessary to
use equal value split supplies, however, the offset voltage
and inverting input bias current will change. The offset
voltage changes about 1mV per volt of supply mismatch.
The inverting bias current can change as much as 10µA
per volt of supply mismatch, though typically the change
is less than 2.5µA per volt.
Thermal Considerations
The LT1497 contains a thermal shutdown feature that
protects against excessive internal (junction) temperature. If the junction temperature of the device exceeds the
protection threshold, the device will begin cycling
between normal operation and an off state. The cycling is
not harmful to the part. The thermal cycling occurs at a
slow rate, typically 10ms to several seconds, depending
upon the power dissipation and the thermal time constants of the package and the amount of copper on the
board under the package. Raising the ambient temperature until the device begins thermal shutdown gives a
good indication of how much margin there is in the
thermal design.
For surface mount devices heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Experiments have shown that the
heat spreading copper layer does not need to be electrically connected to the leads of the device. The PCB
material can be very effective at transmitting heat between
the pad area attached to V – pins of the device and a ground
LT1497
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APPLICATIONS INFORMATION
or power plane layer either inside or on the opposite side
of the board. Copper board stiffeners and plated throughholes can also be used to spread the heat generated by the
device. Table 1 lists the thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with 2oz copper.
This data can be used as a rough guideline in estimating
thermal resistance. The thermal resistance for each application will be affected by thermal interactions with other
components as well as board size and shape.
Table 1. Fused 16-lead and 8-lead SO Packages
COPPER AREA (2oz)
TOPSIDE
BACKSIDE
TOTAL
COPPER AREA
θJA
(16-LEAD)
θJA
(8-LEAD)
2500mm2
2500mm2
5000mm2
40°C/W
80°C/W
1000mm2
2500mm2
3500mm2
46°C/W
92°C/W
600mm2
2500mm2
3100mm2
48°C/W
96°C/W
180mm2
2500mm2
2680mm2
49°C/W
98°C/W
180mm2
1000mm2
1180mm2
56°C/W
112°C/W
180mm2
600mm2
780mm2
58°C/W
116°C/W
180mm2
300mm2
480mm2
59°C/W
118°C/W
180mm2
100mm2
280mm2
60°C/W
120°C/W
180mm2
0mm2
180mm2
61°C/W
122°C/W
thermal resistance is 40°C/W. The junction temperature
TJ is:
TJ = (1.24W)(40°C/W) + 85°C = 135°C
The maximum junction temperature for the LT1497 is
150°C, so the heat sinking capability of the board is
adequate for the application.
If the copper area on the PC board is reduced to 180mm2
the thermal resistance increases to 61°C/W and the junction temperature becomes:
TJ = (1.24W)(61°C/W) + 85°C = 161°C
which is above the maximum junction temperature indicating that the heat sinking capability of the board is
inadequate and should be increased.
560Ω
560Ω
A
–
15V
86.4mA
+
200Ω
10V
560Ω
–10V
560Ω
–
f = 2MHz
Calculating Junction Temperature
+
The junction temperature can be calculated from the
equation:
– 15V
TJ = (PD)(θJA) + TA
TJ = Junction Temperature
TA = Ambient Temperature
PD = Power Dissipation
θJA = Thermal Resistance (Junction-to-Ambient)
As an example, calculate the junction temperature for the
circuit in Figure 1 assuming an 85°C ambient temperature.
The device dissipation can be found by measuring the
supply currents, calculating the total dissipation and then
subtracting the dissipation in the load and feedback network. Both amplifiers are in a gain of –1.
The dissipation for each amplifier is:
PD = (1/2)(86.4mA)(30V) – (10V)2/(200||560) = 0.62W
The total dissipation is 1.24W. When a 2500mm2 PC
board with 2oz copper on top and bottom is used, the
200Ω
1497 F01
Figure 1. Thermal Calculation Example
Slew Rate
Unlike a traditional op amp, the slew rate of a current
feedback amplifier is not independent of the amplifier gain
configuration. There are slew rate limitations in both the
input stage and the output stage. In the inverting mode and
for higher gains in the noninverting mode, the signal
amplitude on the input pins is small and the overall slew
rate is that of the output stage. The input stage slew rate
is related to the quiescent current in the input devices.
Referring to the Simplified Schematic, for noninverting
applications the two current sources in the input stage
slew the parasitic internal capacitances at the bases of Q3
and Q4. Consider a positive going input at the base of Q1
and Q2. If the input slew rate exceeds the internal slew rate,
9
LT1497
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APPLICATIONS INFORMATION
the normally active emitter of Q2 will turn off as the entire
current available from the current source is used to slew
the base of Q3. The base of Q4 is driven by Q1 without slew
limitation. When the differential input voltage exceeds two
diode drops (about 1.4V) the extra clamp emitter on Q1
turns on and drives the base of Q3 directly. Once the base
of Q3 has been driven within 1.4V of its final value, the
clamp emitter of Q1 turns off and the node must finish
slewing using the current source.
This effect can be seen in Figure 2 which shows the large
signal behavior in a gain of 1 on ±15V supplies. The
clamping action enhances the slew rate beyond the input
limitation, but always leads to slew overshoot after the
clamps turn off. Figure 3 shows that for higher gain
1497 F02
RF = 560Ω
AV = 1
VS = ±15V RL = 100Ω
RF = 560Ω
AV = 10
VS = ±15V RL = 100Ω
Figure 2. Large-Signal Response
configurations there is much less slew rate enhancement
because the input only moves 2V, barely enough to turn on
the input clamps. In inverting configurations as shown in
Figure 4 the noninverting input does not move so there is
no input slew rate limitation. Slew overshoot is due to
capacitance on the inverting input and can be reduced with
a larger feedback resistor.
The output slew rate is set by the value of the feedback
resistors and the internal capacitance. Larger feedback
resistors will reduce the slew rate as will lower supply
voltages, similar to the way the bandwidth is reduced.
The larger feedback resistors will also cut back on slew
overshoot.
1497 F03
RG = 62Ω
RF = RG = 560Ω
AV = – 1
VS = ±15V RL = 100Ω
Figure 3. Large-Signal Response
Figure 4. Large-Signal Response
W
W
SI PLIFIED SCHE ATIC
One Amplifier
V+
Q6
Q5
Q7
Q13
Q3
Q8
Q2
– IN
+IN
VOUT
Q1
Q9
Q4
Q14
Q10
Q11
Q12
V–
1497 SS
10
1497 F04
LT1497
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TYPICAL APPLICATIONS
Differential Input/Differential Output Power Amp (AV = 2)
Paralleling Both Amplifiers for Guaranteed 250mA Output Drive
+
VIN
+
VIN
VOUT
1/2 LT1497
3Ω
VOUT
1/2 LT1497
–
–
560Ω
560Ω
560Ω
1.1k
560Ω
+
–
– VOUT
1/2 LT1497
560Ω
+
– VIN
3Ω
1/2 LT1497
–
1497 TA03
560Ω
1497 TA04
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PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.053 – 0.069
(1.346 – 1.752)
0.008 – 0.010
(0.203 – 0.254)
7
8
0.004 – 0.010
(0.101 – 0.254)
5
6
0°– 8° TYP
0.016 – 0.050
0.406 – 1.270
0.014 – 0.019
(0.355 – 0.483)
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
0.050
(1.270)
TYP
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
SO8 0996
1
3
2
4
S Package
16-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.386 – 0.394*
(9.804 – 10.008)
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
0.004 – 0.010
(0.101 – 0.254)
0.053 – 0.069
(1.346 – 1.752)
16
15
14
13
12
11
10
9
0° – 8° TYP
0.016 – 0.050
0.406 – 1.270
0.014 – 0.019
(0.355 – 0.483)
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
0.050
(1.270)
TYP
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
S16 0695
1
2
3
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
4
5
6
7
8
11
LT1497
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TYPICAL APPLICATION
±4A Current Boosted Power Amp (AV = 10)
Frequency Response of Current Boosted Power Amp
22
15V
21
+
VIN
200Ω
0.033Ω
20
Q1
D45VH4
19
0.01µF
V+
3Ω
VOUT
1/2 LT1497
–
1.8K
VOLTAGE GAIN (dB)
6.2Ω
200Ω
17
16
15
14
VS = ±15V
AV = 10
RF = 1.8k
RG = 200Ω
VOUT = 6VP-P
12
10k
3Ω
1/2 LT1497
–
RL = 2.5Ω
18
13
+
RL = 50Ω
100k
1M
FREQUENCY (Hz)
10M
1497 TA06
V–
1.8k
Q2
D44VH4
0.01µF
6.2Ω
0.033Ω
1497 TA05
– 15V
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1206
Single 250mA, 60MHz Current Feedback Amplifier
Shutdown Function, Stable with CL = 10,000pF,
900V/µs Slew Rate
LT1207
Dual 250mA, 60MHz Current Feedback Amplifier
Dual Version of LT1206
LT1210
Single 1A, 30MHz Current Feedback Amplifier
Higher Output Version of LT1206
LT1229/LT1230
Dual/Quad 100MHz Current Feedback Amplifiers
30mA Output Current, 1000V/µs Slew Rate
LT1363/LT1364/LT1365
Single/Dual/Quad 70MHz, 1000V/µs, C-LoadTM Amplifiers
50mA Output Current, 1.5mV Max VOS, 2µA Max IB
C-Load is a trademark of Linear Technology Corporation.
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 ● (408) 432-1900
FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com
1497f LT/TP 1097 4K • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 1997
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