LT1795CFE#TR [Linear]
LT1795 - Dual 500mA/50MHz Current Feedback Line Driver Amplifier; Package: TSSOP; Pins: 20; Temperature Range: 0°C to 70°C;
| 型号： | LT1795CFE#TR |
| 厂家： | 
Linear |
| 描述： | LT1795 - Dual 500mA/50MHz Current Feedback Line Driver Amplifier; Package: TSSOP; Pins: 20; Temperature Range: 0°C to 70°C 放大器 光电二极管 |
| 文件： | 总12页 (文件大小：154K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT1795
Dual 500mA/50MHz
Current Feedback Line Driver
Amplifier
U
FEATURES
DESCRIPTIO
TheLT®1795isadualcurrentfeedbackamplifierwithhigh
output current and excellent large signal characteristics.
The combination of high slew rate, 500mA output drive
and up to ±15V operation enables the device to deliver
significant power at frequencies in the 1MHz to 2MHz
range. Short-circuit protection and thermal shutdown
insure the device’s ruggedness. The LT1795 is stable with
large capacitive loads and can easily supply the large
currents required by the capacitive loading. A shutdown
feature switches the device into a high impedance, low
current mode, reducing power dissipation when the de-
vice is not in use. For lower bandwidth applications, the
supply current can be reduced with a single external
resistor.
■
500mA Output Drive Current
■
50MHz Bandwidth, AV = 2, RL = 25
Ω
■
900V/µs Slew Rate, AV = 2, RL = 25Ω
■
■
■
■
■
■
■
■
Low Distortion: –75dBc at 1MHz
High Input Impedance, 10MΩ
Wide Supply Range, ±5V to ±15V
Full Rate, Downstream ADSL Supported
Low Power Shutdown Mode
Power Saving Adjustable Supply Current
Stable with CL = 10,000pF
Power Enhanced Small Footprint Packages
TSSOP-20, S0-20 Wide
Available in a 20-Lead TSSOP Package
■
U
APPLICATIO S
The LT1795 comes in the very small, thermally enhanced,
20-lead TSSOP package for maximum port density in line
driver applications.
■
ADSL HDSL2, G.lite Drivers
■
Buffers
■
Test Equipment Amplifiers
Video Amplifiers
Cable Drivers
, LTC and LT are registered trademarks of Linear Technology Corporation.
■
■
U
TYPICAL APPLICATION
Low Loss, High Power Central Office ADSL Line Driver
+
V
+IN
+
12.5Ω
1/2
LT1795
–
1k
1k
1:2*
165Ω
100Ω
–
12.5Ω
1/2
LT1795
–IN
+
–
V
1795 TA01
* MIDCOM 50215 OR EQUIVALENT
1795fa
1
LT1795
W W
U W
ABSOLUTE AXI U RATI GS
(Note 1)
Specified Temperature Range (Note 3)... –40°C to 85°C
Junction Temperature........................................... 150°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
Supply Voltage ...................................................... ±18V
Input Current ...................................................... ±15mA
Output Short-Circuit Duration (Note 2)............ Indefinite
Operating Temperature Range ................ –40°C to 85°C
U
W
U
PACKAGE/ORDER I FOR ATIO
TOP VIEW
TOP VIEW
ORDER PART
NUMBER
ORDER PART
–
–
V
1
2
20
V
COMP
1
2
3
4
5
6
7
8
9
20 COMP
+
NUMBER
+
NC
–IN
19 NC
V
19
V
LT1795CFE
LT1795IFE
LT1795CSW
LT1795ISW
3
18 OUT
+
OUT
18 OUT
–
–
+IN
4
17
V
V
17
16
15
14
V
V
V
V
–
–
–
–
SHDN
SHDNREF
+IN
5
16 COMP
V
–
6
15 COMP
+
V
–
7
14
V
V
–IN
8
13 OUT
–IN
+IN
13 –IN
NC
9
12 NC
–
12 +IN
–
V
10
11
V
SHDN 10
11 SHDNREF
FE PACKAGE
20-LEAD PLASTIC TSSOP
S PACKAGE
20-LEAD PLASTIC SW
TJMAX = 150° C, θJA = 40°C/W (Note 4)
UNDERSIDE METAL INTERNALLY CONNECTED TO V–
(PCB CONNECTION OPTIONAL)
TJMAX = 150° C, θJA ≈ 40°C/W (Note 4)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25°C.
VCM = 0V, ±5V ≤ VS ≤ ±15V, pulse tested, VSHDN = 2.5V, VSHDNREF = 0V unless otherwise noted. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
Input Offset Voltage
±3
±4.5
±13
±17
mV
mV
OS
●
Input Offset Voltage Matching
±1
±3.5
±5.0
mV
mV
●
●
±1.5
Input Offset Voltage Drift
Noninverting Input Current
10
µV/°C
+
I
I
±2
±8
±5
±20
µA
µA
IN
●
●
●
●
Noninverting Input Current Matching
Inverting Input Current
±0.5
±1.5
±2
±7
µA
µA
–
±10
±20
±70
±100
µA
µA
IN
Inverting Input Current Matching
±10
±20
±30
±50
µA
µA
e
Input Noise Voltage Density
Input Noise Current Density
Input Noise Current Density
f = 10kHz, R =1k, R = 10Ω, R = 0Ω
3.6
2
nV/√Hz
pA/√Hz
pA/√Hz
n
F
G
S
+i
–i
f = 10kHz, R =1k, R = 10Ω, R = 10kΩ
F G S
n
f = 10kHz, R =1k, R = 10Ω, R = 10kΩ
30
n
F
G
S
1795fa
2
LT1795
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25°C.
VCM = 0V, ±5V ≤ VS ≤ ±15V, pulse tested, VSHDN = 2.5V, VSHDNREF = 0V unless otherwise noted. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
= ±12V, V = ±15V
MIN
TYP
MAX
UNITS
+
R
IN
Input Resistance
V
●
●
1.5
0.5
10
5
MΩ
MΩ
IN
S
V = ±2V, V = ±5V
S
+
C
Input Capacitance
V
= ±15V
IN
2
pF
IN
Input Voltage Range (Note 5)
V = ±15V
S
●
●
±12
±2
±13.5
±3.5
V
V
S
V = ±5V
CMRR
PSRR
Common Mode Rejection Ratio
V = ±15V, V = ±12V
●
●
55
50
62
60
dB
dB
S
CM
V = ±5V, V = ±2V
S
CM
Inverting Input Current
Common Mode Rejection
V = ±15V, V = ±12V
●
●
1
1
10
10
µA/V
µA/V
S
CM
V = ±5V, V = ±2V
S CM
Power Supply Rejection Ratio
V = ±5V to ±15V
S
●
●
60
77
30
dB
Noninverting Input Current
Power Supply Rejection
V = ±5V to ±15V
S
500
5
nA/V
Inverting Input Current
Power Supply Rejection
V = ±5V to ±15V
S
●
1
µA/V
A
Large-Signal Voltage Gain
V = ±15V, V
= ±10V, R = 25Ω
●
●
55
55
68
68
dB
dB
V
S
OUT
L
V = ±5V, V
= ±2V, R = 12Ω
L
S
OUT
–
R
Transresistance, ∆V /∆I
V = ±15V, V
= ±10V, R = 25Ω
●
●
75
75
200
200
kΩ
kΩ
OL
OUT IN
S
OUT
L
V = ±5V, V
= ±2V, R = 12Ω
S
OUT
L
V
Maximum Output Voltage Swing
V = ±15V, R = 25Ω
±11.5
±10.0
±12.5
±11.5
V
V
OUT
S
L
●
V = ±5V, R = 12Ω
±2.5
±2.0
±3
±3
V
V
S
L
●
●
I
I
Maximum Output Current
V = ±15V, R = 1Ω
0.5
1
A
OUT
S
S
L
Supply Current Per Amplifier
V = ±15V, V
= 2.5V
29
34
42
mA
mA
S
SHDN
●
Supply Current Per Amplifier,
SHDN
V = ±15V
15
20
25
mA
mA
S
R
= 51k, (Note 6)
●
●
Positive Supply Current, Shutdown
Output Leakage Current, Shutdown
Channel Separation
V = ±15V, V
S
= 0.4V
= 0.4V
1
200
200
µA
µA
SHDN
V = ±15V, V
S
1
SHDN
V = ±15V, V
S
= ±10V, R = 25Ω
80
110
–75
dB
OUT
L
HD , HD
2nd and 3rd Harmonic Distortion
Differential Mode
f = 1MHz, V = 20V , R = 50, A = 2
dBc
2
3
O
P-P
L
V
SR
Slew Rate (Note 7)
Slew Rate
A = 4, R = 400Ω
400
900
900
65
V/µs
V/µs
MHz
V
L
A = 4, R = 25Ω
V
L
BW
Small-Signal BW
A = 2, V = ±15V, Peaking ≤ 1.5dB
V S
R = R = 910Ω, R = 100Ω
F
G
L
A = 2, V = ±15V, Peaking ≤ 1.5dB
50
MHz
V
S
R = R = 820Ω, R = 25Ω
F
G
L
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Applies to short-circuits to ground only. A short-circuit between
the output and either supply may permanently damage the part when
operated on supplies greater than ±10V.
Note 4: Thermal resistance varies depending upon the amount of PC board
metal attached to the device. If the maximum dissipation of the package is
exceeded, the device will go into thermal shutdown and be protected.
Note 5: Guaranteed by the CMRR tests.
+
Note 6: R
is connected between the SHDN pin and V .
SHDN
Note 3: The LT1795C is guaranteed to meet specified performance from
0°C to 70°C and is designed, characterized and expected to meet these
extended temperature limits, but is not tested at –40°C and 85°C. The
LT1795I is guaranteed to meet the extended temperature limits.
Note 7: Slew rate is measured at ±5V on a ±10V output signal while
operating on ±15V supplies with R = 1k, R = 333Ω (A = +4) and
R = 400Ω.
L
F
G
V
1795fa
3
LT1795
W
U
U
SMALL-SIGNAL BANDWIDTH
RSD = 0Ω, IS = 30mA per Amplifer, VS = ±15V,
Peaking ≤ 1dB, RL = 25Ω
RSD = 51kΩ, IS = 15mA per Amplifer, VS = ±15V,
Peaking ≤ 1dB, RL = 25Ω
–3dB BW
(MHz)
–3dB BW
(MHz)
AV
–1
1
RF
976
RG
976
—
AV
–1
1
RF
976
RG
976
—
44
30
32
32
27
1.15k
976
53
1.15k
976
2
976
72
48
2
976
72
10
649
46
10
649
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Ambient
Temperature
Output Saturation Voltage vs
Junction Temperature
Output Short-Circuit Current vs
Junction Temperature
+
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
V
40
35
30
25
20
15
10
5
V
S
= ±15V
V
S
= ±15V
V
A
= ±15V
= 1
= ∞
S
V
L
R
L
= 2k
–1
–2
–3
–4
R
R
L
= 25Ω
R
SD
= 0Ω
SOURCING
4
3
2
1
SINKING
R
SD
= 51kΩ
R
L
= 25Ω
R
L
= 2k
–
V
0
25
50
TEMPERATURE (°C)
75
100 125
–50 –25
0
50
TEMPERATURE (°C)
100 125
50
125
–50 –25
0
25
75
–50
0
25
75
–25
100
TEMPERATURE (°C)
LT1795 G03
LT1795 G01
LT1795 G02
Second Harmonic Distortion vs
Frequency
Third Harmonic Distortion vs
Frequency
SHDN Pin Current vs Voltage
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
0.6
0.5
0.4
0.3
0.2
0.1
0
V
V
= ±15V
SHDNREF
A
V
V
= 2 DIFFERENTIAL
= 20V
S
V
OUT
S
R
A
V
V
= 2 DIFFERENTIAL
V
= 0V
P-P
= 20V
OUT
P-P
= ±15V
= ±15V
S
= 50Ω
LOAD
PER AMPLIFIER
R
I
= 50Ω
LOAD
I
Q
PER AMPLIFIER
I
Q
= 5mA
Q
I
Q
= 5mA
I
= 10mA
Q
I
Q
= 10mA
I
= 20mA
Q
I
Q
= 15mA
I
Q
= 15mA
I
Q
= 20mA
10k
100k
FREQUENCY (Hz)
1M
10k
100k
FREQUENCY (Hz)
1M
0
1
2
3
4
5
VOLTAGE APPLIED AT SHDN PIN (V)
LT1795 G05
LT1795 G06
1795 G04
1795fa
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LT1795
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Second Harmonic Distortion vs
Frequency
Third Harmonic Distortion vs
Frequency
Second Harmonic Distortion vs
Frequency
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
A
V
V
= 2 DIFFERENTIAL
A
V
V
= 10 DIFFERENTIAL
= 20V
A
V
V
= 10 DIFFERENTIAL
= 20V
V
V
OUT
S
V
OUT
= ±15V
S
= 20V
OUT
P-P
P-P
P-P
= ±12V
= ±15V
S
R
I
= 50Ω
R
I
= 50Ω
R
I
= 50Ω
LOAD
PER AMPLIFIER
LOAD
LOAD
PER AMPLIFIER
PER AMPLIFIER
Q
Q
Q
I
= 20mA
Q
I
Q
= 5mA
I
Q
= 5mA
I
I
= 10mA
Q
= 10mA
Q
I
Q
= 10mA
I
= 15mA
Q
I
Q
= 20mA
I
= 15mA
Q
I
= 5mA
Q
I
= 15mA
I
= 20mA
Q
Q
10k
100k
FREQUENCY (Hz)
1M
10k
100k
FREQUENCY (Hz)
1M
10k
100k
1M
FREQUENCY (Hz)
LT1795 G09
LT1795 G07
LT1795 G08
Third Harmonic Distortion vs
Frequency
Second Harmonic Distortion vs
Frequency
Third Harmonic Distortion vs
Frequency
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
A
V
V
= 2 DIFFERENTIAL
= 20V
A
V
V
= 10 DIFFERENTIAL
= 20V
A
V
V
= 10 DIFFERENTIAL
= 20V
V
OUT
S
V
OUT
S
V
OUT
S
P-P
P-P
P-P
= ±12V
= ±12V
= ±12V
R
I
= 50Ω
R
I
= 50Ω
LOAD
PER AMPLIFIER
R
I
= 50Ω
LOAD
LOAD
PER AMPLIFIER
PER AMPLIFIER
Q
Q
Q
I
Q
= 5mA
I
= 5mA
Q
I
= 20mA
Q
I
Q
= 20mA
I
= 10mA
Q
I
Q
= 15mA
I
Q
= 10mA
I
Q
= 20mA
I = 5mA
Q
I
= 15mA
Q
I
Q
= 10mA
I
= 15mA
Q
10k
100k
FREQUENCY (Hz)
1M
10k
100k
1M
10k
100k
FREQUENCY (Hz)
1M
FREQUENCY (Hz)
LT1795 G10
LT1795 G12
LT1795 G11
Second Harmonic Distortion vs
Frequency
Third Harmonic Distortion vs
Frequency
Second Harmonic Distortion vs
Frequency
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
A
V
V
= 2 DIFFERENTIAL
= 4V
A
V
V
= 10 DIFFERENTIAL
= 4V
A
V
V
= 2 DIFFERENTIAL
= 4V
V
OUT
S
V
OUT
S
V
OUT
S
P-P
P-P
P-P
= ±12V
= ±12V
= ±12V
I
Q
= 5mA
R
I
= 50Ω
R
I
= 50Ω
R
I
= 50Ω
LOAD
LOAD
LOAD
PER AMPLIFIER
PER AMPLIFIER
PER AMPLIFIER
Q
Q
Q
I
= 10mA
Q
I
= 10mA
Q
I
Q
= 5mA
I
= 10mA
I
Q
= 15mA
I
= 5mA
Q
Q
I
= 15mA
Q
I
Q
= 15mA
I
= 20mA
Q
I
Q
= 20mA
I
= 20mA
Q
10k
100k
1M
10k
100k
FREQUENCY (Hz)
1M
10k
100k
FREQUENCY (Hz)
1M
FREQUENCY (Hz)
LT1795 G13
LT1795 G14
LT1795 G15
1795fa
5
LT1795
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Third Harmonic Distortion vs
Frequency
Third Harmonic Distortion vs
Frequency
Second Harmonic Distortion vs
Frequency
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
–40
–50
A
V
V
= 2 DIFFERENTIAL
= 4V
A
V
V
= 10 DIFFERENTIAL
= 4V
V
OUT
S
V
OUT
S
A
V
V
= 2 DIFFERENTIAL
= 4V
V
OUT
S
R
P-P
P-P
P-P
I
= 5mA
Q
= ±5V
= ±12V
= ±5V
I
= 5mA
Q
R
I
= 50Ω
R
I
= 50Ω
LOAD
LOAD
= 50Ω
LOAD
–60
PER AMPLIFIER
PER AMPLIFIER
Q
Q
I
PER AMPLIFIER
Q
I
Q
= 10mA
–70
I
= 10mA
Q
I
= 15mA
Q
I
Q
= 5mA
–80
I
Q
= 10mA
I = 15mA
Q
–90
I
Q
= 20mA
I
Q
= 15mA
I
= 20mA
Q
I
Q
= 20mA
–100
–110
10k
100k
1M
10k
100k
FREQUENCY (Hz)
1M
10k
100k
FREQUENCY (Hz)
1M
FREQUENCY (Hz)
LT1795 G17
LT1795 G18
LT1795 G16
Second Harmonic Distortion vs
Frequency
Third Harmonic Distortion vs
Frequency
–40
–50
–60
–70
–80
–90
–100
–40
–50
–60
–70
–80
–90
–100
A
V
V
= 10 DIFFERENTIAL
= 4V
A
V
V
= 10 DIFFERENTIAL
= 4V
V
V
OUT
S
R
I
= 5mA
OUT
= ±5V
S
P-P
Q
P-P
= ±5V
R = 50Ω
LOAD
= 50Ω
LOAD
I PER AMPLIFIER
Q
I
Q
PER AMPLIFIER
I
= 10mA
Q
I
= 20mA
Q
I
= 15mA
Q
I
= 15mA
Q
I
= 10mA
Q
I
= 5mA
Q
I
= 20mA
Q
10k
100k
FREQUENCY (Hz)
1M
10k
100k
FREQUENCY (Hz)
1M
LT1795 G20
LT1795 G19
–3dB Bandwidth vs
Supply Current
Slew Rate vs Supply Current
50
45
1200
1000
800
600
400
200
0
RISING
40
FALLING
35
30
25
V
= ±15V
=25°C
= 4
S
A
V
= ±15V
=25°C
= 4
S
A
T
T
A
V
A
V
R
= 25Ω
LOAD
R
= 25Ω
LOAD
R = 1k
F
R = 1k
F
7.5 10
15
20
25
30
7.5 10
15
20
25
30
SUPPLY CURRENT PER AMPLIFIER (mA)
SUPPLY CURRENT PER AMPLIFIER (mA)
1795 • G22
1795 • G21
1795fa
6
LT1795
U
W U U
APPLICATIO S I FOR ATIO
The LT1795 is a dual current feedback amplifier with high
output current drive capability. The amplifier is designed
to drive low impedance loads such as twisted-pair trans-
mission lines with excellent linearity.
When VSHDN = VSHDNREF, the device is shut down. The
device will interface directly with 3V or 5V CMOS logic
when SHDNREF is grounded and the control signal is
applied to the SHDN pin. Switching time between the
active and shutdown states is about 1.5µs.
SHUTDOWN/CURRENT SET
Figures 1 to 4 illustrate how the SHDN and SHDNREF pins
can be used to reduce the amplifier quiescent current. In
both cases, an external resistor is used to set the current.
The two approaches are equivalent, however the required
resistor values are different. The quiescent current will be
approximately 115 times the current in the SHDN pin and
230 times the current in the SHDNREF pin. The voltage
across the resistor in either condition is V+ – 1.5V. For
example, a 50k resistor between V+ and SHDN will set the
If the shutdown/current set feature is not used, connect
SHDN to V+ and SHDNREF to ground.
The SHDN and SHDNREF pins control the biasing of the
two amplifiers. The pins can be used to either turn off the
amplifiers completely, reducing the quiescent current to
less then 200µA, or to control the quiescent current in
normal operation.
+
V
+
V
10 SHDN
R
SHDN
11 SHDNREF
10 SHDN
R
SHDNREF
11 SHDNREF
1795 F03
1795 F01
Figure 1. RSHDN Connected Between V+ and SHDN (Pin 10);
SHDNREF (Pin 11) = GND. See Figure 2
Figure 3. RSHDNREF Connected Between SHDNREF (Pin 11)
and GND; SHDN (Pin 10) = V+. See Figure 4
80
80
T
= 25°C
= ±15V
T
= 25°C
= ±15V
A
S
A
S
V
V
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
0
25 50 75 100 125 150 175 200 225
(kΩ)
50 100 150 200 250 300 350 400 450 500
(kΩ)
R
R
SHDNREF
SHDN
1795 F02
1795 F04
Figure 2. LT1795 Amplifier Supply Current vs RSHDN. RSHDN
Connected Between V+ and SHDN, SHDNREF = GND (See
Figure 1)
Figure 4. LT1795 Amplifier Supply Current vs RSHDNREF
RSHDNREF Connected Between SHDNREF and GND,
SHDN = V+ (See Figure 3)
.
1795fa
7
LT1795
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APPLICATIO S I FOR ATIO
Figure 8 illustrates a partial shutdown with direct logic
control. By keeping the output stage slightly biased on, the
output impedance remains low, preserving the line termi-
nation. The design equations are:
quiescent current to 33mA with VS = ±15V. If ON/OFF
controlisdesiredinadditiontoreducedquiescentcurrent,
then the circuits in Figures 5 to 7 can be employed.
+
V
115•VH
R
B
SHDN
R1=
R
10 SHDN
INTERNAL
LOGIC THRESHOLD
~1.4V
OFF
IS
– IS
( )ON ( )OFF
10k
Q1
11 SHDNREF
ON
(0V)
(3.3V/5V)
115• VCC – VSHDN
(
)
1795 F05
Q1: 2N3904 OR EQUIVALENT
R2 =
V
SHDN /VH • IS
– IS
+ IS
(
) (
)
( )OFF ( )OFF
ON
Figure 5. Setting Amplifier Supply Current
Level with ON/OFF Control, Version 1
where
VH = Logic High Level
+
V
10 SHDN
(IS)ON = Supply Current Fully On
(IS)OFF = Supply Current Partially On
VSHDN = Shutdown Pin Voltage ≈1.4V
VCC = Positive Supply Voltage
R
PULLUP
>500k
11 SHDNREF
R
R
SHDN1
SHDN2
R
B1
R
B2
ON
ON
10k
10k
Q1A
Q1B
V
CC
OFF
OFF
(0V)
(0V)
R2
(3.3V/5V)
(3.3V/5V)
ON
1795 F06
R1
Q1A, Q1B: ROHM IMX1 or FMG4A (W/INTERNAL R )
B
10
INTERNAL
OFF
LOGIC THRESHOLD
~ 1.4V
(0V)
SHDN
I
SY
(3.3V/5V)
CONTROL
Figure 6. Setting Multiple Amplifier Supply
Current Levels with ON/OFF Control, Version 2
11
SHDNREF
1795 F08
Figure 8. Partial Shutdown
ON
SHDN
10
R
EXT
INTERNAL
OFF
(0V)
LOGIC THRESHOLD
~ 1.4V
I
PROG
I
SY
THERMAL CONSIDERATIONS
(3.3V/5V)
CONTROL
I
0.5mA
= 0Ω
SHDNREF
11
PROG
The LT1795 contains a thermal shutdown feature that
protectsagainstexcessiveinternal(junction)temperature.
If the junction temperature of the device exceeds the
protectionthreshold, thedevicewillbegincyclingbetween
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, which depends on
thepowerdissipationandthethermaltimeconstantsofthe
package and heat sinking. Raising the ambient tempera-
FOR R
EXT
(SEE SHDN PIN
CURRENT vs
1795 F07
VOLTAGE
CHARACTERISTIC)
Figure 7. Setting Amplifier Supply Current Level
with ON/OFF Control, Version 3
1795fa
8
LT1795
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APPLICATIO S I FOR ATIO
ture until the device begins thermal shutdown gives a
good indication of how much margin there is in the
thermal design.
ing 0.5A current peaks into the load, a 1Ω power supply
impedance will cause a droop of 0.5V, reducing the
available output swing by that amount. Surface mount
tantalum and ceramic capacitors make excellent low ESR
bypass elements when placed close to the chip. For
frequencies above 100kHz, use 1µF and 100nF ceramic
capacitors. If significant power must be delivered below
100kHz, capacitive reactance becomes the limiting factor.
Larger ceramic or tantalum capacitors, such as 4.7µF, are
recommended in place of the 1µF unit mentioned above.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. For the TSSOP package, power is
dissipated through the exposed heatsink. For the SO
package, power is dissipated from the package primarily
through the V– pins (4 to 7 and 14 to 17). These pins
should have a good thermal connection to a copper plane,
either by direct contact or by plated through holes. The
copper plane may be an internal or external layer. The
thermal resistance, junction-to-ambient will depend on
thetotalcopperareaconnectedtothedevice.Forexample,
the thermal resistance of the LT1795 connected to a 2 × 2
inch, double sided 2 oz copper plane is 40°C/W.
Inadequate bypassing is evidenced by reduced output
swing and “distorted” clipping effects when the output is
driventotherails.Ifthisisobserved,checkthesupplypins
of the device for ripple directly related to the output
waveform. Significant supply modulation indicates poor
bypassing.
Capacitance on the Inverting Input
CALCULATING JUNCTION TEMPERATURE
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 invert-
ing 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.
The junction temperature can be calculated from the
equation:
TJ = (PD)(θJA) + TA
where
TJ = Junction Temperature
TA = Ambient Temperature
PD = Device Dissipation
θJA = Thermal Resistance (Junction-to-Ambient)
Feedback Resistor Selection
The optimum value for the feedback resistors is a function
of the operating conditions of the device, the load imped-
ance and the desired flatness of response. The Typical AC
Performance tables give the values which result in less
than 1dB of peaking for various resistive loads and oper-
ating conditions. If this level of flatness is not required, a
higher bandwidth can be obtained by use of a lower
feedback resistor.
Differential Input Signal Swing
The differential input swing is limited to about ±5V by an
ESD protection device connected between the inputs. In
normal operation, the differential voltage between the
input pins is small, so this clamp has no effect. However,
in the shutdown mode, the differential swing can be the
same as the input swing. The clamp voltage will then set
the maximum allowable input voltage.
For resistive loads, the COMP pin should be left open (see
Capacitive Loads section).
Capacitive Loads
POWER SUPPLY BYPASSING
The LT1795 includes an optional compensation network
for driving capacitive loads. This network eliminates most
of the output stage peaking associated with capacitive
To obtain the maximum output and the minimum distor-
tion from the LT1795, the power supply rails should be
well bypassed. For example, with the output stage supply-
loads, allowing the frequency response to be flattened.
1795fa
9
LT1795
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APPLICATIONS INFORMATION
and greatly reduces the peaking. A lower value feedback
resistor can now be used, resulting in a response which is
flat to ±1dB to 45MHz. The network has the greatest effect
for CL in the range of 0pF to 1000pF.
Figure 9 shows the effect of the network on a 200pF load.
Without the optional compensation, there is a 6dB peak at
85MHz caused by the effect of the capacitance on the
output stage. Adding a 0.01µF bypass capacitor between
theoutputandtheCOMPpinsconnectsthecompensation
Although the optional compensation works well with
capacitive loads, it simply reduces the bandwidth when it
is connected with resistive loads. For instance, with a 25Ω
load, the bandwidth drops from 48MHz to 32MHz when
thecompensationisconnected. Hence, thecompensation
wasmadeoptional.Todisconnecttheoptionalcompensa-
tion, leave the COMP pin open.
14
V
C
= ±15V
S
L
12
10
8
= 200pF
R = 3.4k
F
NO
R = 1k
F
COMPENSATION
COMPENSATION
6
4
2
0
DEMO BOARD
–2
–4
–6
R = 3.4k
F
COMPENSATION
A demo board (DC261A) is available for evaluating the
performence of the LT1795. The board is configured as a
differential line driver/receiver suitable for xDSL applica-
tions. For details, consult your local sales representative.
1
10
100
FREQUENCY (MHz)
1795 F09
Figure 9
U
PACKAGE DESCRIPTIO
SW Package
20-Lead Plastic Small Outline (Wide .300 Inch)
(Reference LTC DWG # 05-08-1620)
.050 BSC .045 ±.005
.030 ±.005
TYP
.496 – .512
(12.598 – 13.005)
NOTE 4
N
19 18
16
14 13 12 11
20
N
17
15
.325 ±.005
.420
MIN
.394 – .419
(10.007 – 10.643)
NOTE 3
1
2
3
N/2
N/2
RECOMMENDED SOLDER PAD LAYOUT
.291 – .299
(7.391 – 7.595)
NOTE 4
2
3
5
7
8
9
10
1
4
6
.037 – .045
.093 – .104
(2.362 – 2.642)
.010 – .029
(0.940 – 1.143)
× 45°
(0.254 – 0.737)
.005
(0.127)
RAD MIN
0° – 8° TYP
.050
(1.270)
BSC
.004 – .012
.009 – .013
(0.102 – 0.305)
NOTE 3
(0.229 – 0.330)
.014 – .019
.016 – .050
(0.406 – 1.270)
INCHES
(MILLIMETERS)
S20 (WIDE) 0502
(0.356 – 0.482)
TYP
NOTE:
1. DIMENSIONS IN
2. DRAWING NOT TO SCALE
3. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.
THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS
4. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
1795fa
10
LT1795
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PACKAGE DESCRIPTIO
FE Package
20-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation CA
6.40 – 6.60*
(.252 – .260)
4.95
(.195)
4.95
(.195)
20 1918 17 16 15 14 1312 11
6.60 ±0.10
2.74
(.108)
4.50 ±0.10
2.74
(.108)
6.40
BSC
SEE NOTE 4
0.45 ±0.05
1.05 ±0.10
0.65 BSC
5
7
8
1
2
3
4
6
9 10
RECOMMENDED SOLDER PAD LAYOUT
1.20
(.047)
MAX
4.30 – 4.50*
(.169 – .177)
0° – 8°
0.65
(.0256)
BSC
0.45 – 0.75
(.018 – .030)
0.09 – 0.20
(.0036 – .0079)
0.05 – 0.15
(.002 – .006)
FE20 (CA) TSSOP 0203
0.195 – 0.30
(.0077 – .0118)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
MILLIMETERS
(INCHES)
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
1795fa
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
LT1795
W
W
SI PLIFIED SCHEMATIC
SHDN
TO ALL
CURRENT
SOURCES
SHDNREF
+
V
Q5
Q10
Q2
D1
Q11
Q6
Q15
Q1
Q9
–
–
V
50Ω
COMP
V
C
C
+IN
–IN
R
C
OUTPUT
+
V
+
V
Q12
Q3
Q8
Q16
Q14
D2
Q4
Q13
Q7
–
V
1795 SS
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1497
Dual 125mA, 50MHz Current Feedback Amplifier
Dual 250mA, 60MHz Current Feedback Amplifier
Dual 200mA, 700MHz Voltage Feedback Amplifier
900V/µs Slew Rate
LT1207
Shutdown/Current Set Function
Low Distortion: –72dBc at 200kHz
LT1886
1795fa
LT/TP 0603 1K REVA • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
LINEAR TECHNOLOGY CORPORATION 1999
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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