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FEATURES
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LT1795 Dual 500mA/50MHz Current Feedback Line Driver Amplifier DESCRIPTIO
The LT(R)1795 is a dual current feedback amplifier with high 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 device is not in use. For lower bandwidth applications, the supply current can be reduced with a single external resistor. The LT1795 comes in the very small, thermally enhanced, 20-lead TSSOP package for maximum port density in line driver applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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500mA Output Drive Current 50MHz Bandwidth, AV = 2, RL = 25 900V/s Slew Rate, AV = 2, RL = 25 Low Distortion: -75dBc at 1MHz High Input Impedance, 10M Wide Supply Range, 5V to 15V Full Rate, Downstream ADSL Supported Power Enhanced Small Footprint Packages TSSOP-20, S0-20 Wide Low Power Shutdown Mode Power Saving Adjustable Supply Current Stable with CL = 10,000pF
APPLICATIO S
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ADSL HDSL2, G.lite Drivers Buffers Test Equipment Amplifiers Video Amplifiers Cable Drivers
TYPICAL APPLICATION
Low Loss, High Power Central Office ADSL Line Driver
V+ +IN
+
1/2 LT1795
12.5
-
1k 1:2* 165 1k 100
-
1/2 LT1795 -IN
12.5
+
V-
1795 TA01
* MIDCOM 50215 OR EQUIVALENT
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LT1795
ABSOLUTE
AXI U RATI GS
Supply Voltage ...................................................... 18V Input Current ...................................................... 15mA Output Short-Circuit Duration (Note 2) ............ Indefinite Operating Temperature Range ................ - 40C to 85C
PACKAGE/ORDER I FOR ATIO
TOP VIEW V- NC -IN +IN SHDN SHDNREF +IN -IN NC 1 2 3 4 5 6 7 8 9 20 V- 19 NC 18 OUT 17 V+
ORDER PART NUMBER LT1795CFE LT1795IFE
16 COMP 15 COMP 14 V + 13 OUT 12 NC 11 V -
V - 10
FE PACKAGE 20-LEAD PLASTIC TSSOP TJMAX = 150 C, JA = 40C/W (Note 4)
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The q denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25C. VCM = 0V, 5V VS 15V, pulse tested, VSHDN = 2.5V, VSHDNREF = 0V unless otherwise noted. (Note 3)
SYMBOL VOS PARAMETER Input Offset Voltage
q
CONDITIONS
Input Offset Voltage Matching
q
Input Offset Voltage Drift IIN
+
Noninverting Input Current
q
Noninverting Input Current Matching
q
IIN-
Inverting Input Current
q
Inverting Input Current Matching
q
en + in - in
Input Noise Voltage Density Input Noise Current Density Input Noise Current Density
f = 10kHz, RF =1k, RG = 10, RS = 0 f = 10kHz, RF =1k, RG = 10, RS = 10k f = 10kHz, RF =1k, RG = 10, RS = 10k
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(Note 1)
Specified Temperature Range (Note 3) ... - 40C to 85C Junction Temperature ........................................... 150C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
TOP VIEW COMP 1 V+ 2 OUT 3 V- V- V- 4 V- 5 6 7 20 COMP 19 V + 18 OUT 17 V- 16 V - 15 V - 14 V - 13 -IN 12 +IN 11 SHDNREF
ORDER PART NUMBER LT1795CSW LT1795ISW
-IN 8 +IN 9 SHDN 10
S PACKAGE 20-LEAD PLASTIC SW TJMAX = 150 C, JA 40C/W (Note 4)
MIN
TYP 3 4.5 1 1.5 10 2 8 0.5 1.5 10 20 10 20 3.6 2 30
MAX 13 17 3.5 5.0 5 20 2 7 70 100 30 50
UNITS mV mV mV mV V/C A A A A A A A A nV/Hz pA/Hz pA/Hz
q
LT1795
ELECTRICAL CHARACTERISTICS
The q denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25C. VCM = 0V, 5V VS 15V, pulse tested, VSHDN = 2.5V, VSHDNREF = 0V unless otherwise noted. (Note 3)
SYMBOL RIN
+
PARAMETER Input Resistance Input Capacitance Input Voltage Range (Note 5)
CONDITIONS VIN = 12V, VS = 15V V = 2V, VS = 5V VIN = 15V VS = 15V VS = 5V VS = 15V, VCM = 12V VS = 5V, VCM = 2V VS = 15V, VCM = 12V VS = 5V, VCM = 2V VS = 5V to 15V VS = 5V to 15V VS = 5V to 15V VS = 15V, VOUT = 10V, RL = 25 VS = 5V, VOUT = 2V, RL = 12 VS = 15V, VOUT = 10V, RL = 25 VS = 5V, VOUT = 2V, RL = 12 VS = 15V, RL = 25
q q q q q q q q q q q q q q q q
MIN 1.5 0.5 12 2 55 50
TYP 10 5 2 13.5 3.5 62 60 1 1
MAX
UNITS M M pF V V dB dB
CIN+
CMRR
Common Mode Rejection Ratio Inverting Input Current Common Mode Rejection
10 10 500 5
A/V A/V dB nA/V A/V dB dB k k V V V V A
PSRR
Power Supply Rejection Ratio Noninverting Input Current Power Supply Rejection Inverting Input Current Power Supply Rejection
60
77 30 1
AV ROL VOUT
Large-Signal Voltage Gain Transresistance, VOUT/IIN - Maximum Output Voltage Swing
55 55 75 75 11.5 10.0 2.5 2.0 0.5
68 68 200 200 12.5 11.5 3 3 1 29 34 42 20 25 200 10
VS = 5V, RL = 12
q
IOUT IS
Maximum Output Current Supply Current Per Amplifier Supply Current Per Amplifier, RSHDN = 51k, (Note 6) Positive Supply Current, Shutdown Output Leakage Current, Shutdown Channel Separation
VS = 15V, RL = 1 VS = 15V, VSHDN = 2.5V
q q
mA mA mA mA A A dB dBc V/s V/s MHz MHz
VS = 15V
q
15
q
VS = 15V, VSHDN = 0.4V VS = 15V, VSHDN = 0.4V VS = 15V, VOUT = 10V, RL = 25 f = 1MHz, VO = 20VP-P, RL = 50, AV = 2 AV = 4, RL = 400 AV = 4, RL = 25 AV = 2, VS = 15V, Peaking 1.5dB RF = RG = 910, RL = 100 AV = 2, VS = 15V, Peaking 1.5dB RF = RG = 820, RL = 25
1 1 80 110 -75 400 900 900 65 50
HD2, HD3 SR BW
2nd and 3rd Harmonic Distortion Differential Mode Slew Rate (Note 7) Slew Rate Small-Signal BW
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 3: The LT1795C is guaranteed to meet specified performance from 0C to 70C and is designed, characterized and expected to meet these extended temperature limits, but is not tested at - 40C and 85C. The LT1795I is guaranteed to meet the extended temperature limits.
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: RSHDN is connected between the SHDN pin and V +. Note 7: Slew rate is measured at 5V on a 10V output signal while operating on 15V supplies with RF = 1k, RG = 333 (AV = +4) and RL = 400.
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LT1795
SMALL-SIGNAL BANDWIDTH
RSD = 0, IS = 30mA per Amplifer, VS = 15V, Peaking 1dB, RL = 25
AV -1 1 2 10 RF 976 1.15k 976 649 RG 976 -- 976 72 -3dB BW (MHz) 44 53 48 46
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Ambient Temperature
40
V+
OUTPUT SATURATION VOLTAGE (V)
SUPPLY CURRENT PER AMPLIFIER (mA)
35 30 25 20 15 10 5
-1 -2 -3 -4
OUTPUT SHORT-CIRCUIT CURRENT (A)
VS = 15V AV = 1 RL = RSD = 0
RSD = 51k
0 -50 -25
50 25 75 0 TEMPERATURE (C)
SHDN Pin Current vs Voltage
0.6 VS = 15V VSHDNREF = 0V
-40 -50 DISTORTION (dBc) -60 -70
CURRENT INTO SHDN PIN (mA)
0.5 0.4 0.3 0.2 0.1 0
DISTORTION (dBc)
0
1 2 3 4 VOLTAGE APPLIED AT SHDN PIN (V)
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RSD = 51k, IS = 15mA per Amplifer, VS = 15V, Peaking 1dB, RL = 25
AV -1 1 2 10 RF 976 1.15k 976 649 RG 976 -- 976 72 -3dB BW (MHz) 30 32 32 27
Output Saturation Voltage vs Junction Temperature
VS = 15V
Output Short-Circuit Current vs Junction Temperature
2.0 VS = 15V 1.8 1.6 1.4 SOURCING 1.2 SINKING 1.0 0.8 0.6 -50 -25
RL = 2k RL = 25
4 3 2 1 RL = 2k 25 50 0 75 TEMPERATURE (C) 100 125 RL = 25
100
125
V- -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
LT1795 G01
LT1795 G02
LT1795 G03
Second Harmonic Distortion vs Frequency
-40
AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA IQ = 10mA -80 -90 -100 10k IQ = 15mA IQ = 20mA 100k FREQUENCY (Hz) 1M
LT1795 G05
Third Harmonic Distortion vs Frequency
AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER
-50 -60 -70 -80 -90
IQ = 5mA
IQ = 10mA IQ = 20mA
IQ = 15mA -100 10k 100k FREQUENCY (Hz) 1M
LT1795 G06
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1795 G04
LT1795 TYPICAL PERFOR A CE CHARACTERISTICS
Second Harmonic Distortion vs Frequency
-40 -50 AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER IQ = 20mA -40 -50 DISTORTION (dBc) -60 -70 -80 -90 -100 10k AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA IQ = 10mA IQ = 20mA IQ = 15mA -100 10k 100k FREQUENCY (Hz) 1M
LT1795 G07
DISTORTION (dBc)
DISTORTION (dBc)
-60 -70 -80
IQ = 15mA -90 IQ = 5mA
Third Harmonic Distortion vs Frequency
-40 -50
DISTORTION (dBc)
-40
-60 -70
DISTORTION (dBc)
DISTORTION (dBc)
AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA IQ = 20mA
-80 IQ = 15mA -90 IQ = 10mA -100 10k 100k FREQUENCY (Hz) 1M
LT1795 G10
Second Harmonic Distortion vs Frequency
-40 -50 DISTORTION (dBc) -60 -70 IQ = 5mA -80 -90 -100 10k IQ = 20mA IQ = 10mA IQ = 15mA AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER
-40 -50
DISTORTION (dBc)
-60 -70 -80 -90
DISTORTION (dBc)
100k FREQUENCY (Hz)
UW
IQ = 10mA
LT1795 G13
Third Harmonic Distortion vs Frequency
-40 -50 -60 -70
Second Harmonic Distortion vs Frequency
AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA IQ = 10mA -80 -90 -100 10k IQ = 15mA IQ = 20mA 100k FREQUENCY (Hz) 1M
LT1795 G09
100k FREQUENCY (Hz)
1M
LT1795 G08
Second Harmonic Distortion vs Frequency
-40
AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER
Third Harmonic Distortion vs Frequency
AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA -70 -80 -90 -100 10k IQ = 10mA IQ = 15mA 100k FREQUENCY (Hz) 1M
LT1795 G12
-50 -60 -70 -80 -90
-50 -60
IQ = 20mA IQ = 10mA IQ = 15mA IQ = 5mA
IQ = 20mA
-100 10k
100k FREQUENCY (Hz)
1M
LT1795 G11
Third Harmonic Distortion vs Frequency
-40
AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER
Second Harmonic Distortion vs Frequency
AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER IQ = 10mA IQ = 5mA IQ = 20mA IQ = 15mA
IQ = 5mA
-50 -60 -70 -80 -90
IQ = 10mA
IQ = 15mA
IQ = 20mA
1M
-100 10k
100k FREQUENCY (Hz)
1M
LT1795 G14
-100 10k
100k FREQUENCY (Hz)
1M
LT1795 G15
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LT1795 TYPICAL PERFOR A CE CHARACTERISTICS
Third Harmonic Distortion vs Frequency
-40 -50
DISTORTION (dBc)
-40
DISTORTION (dBc)
-60 -70 -80 -90 -100 10k
DISTORTION (dBc)
-60 -70 -80 -90 -100
AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER
IQ = 5mA
IQ = 10mA IQ = 15mA
IQ = 20mA
-110 10k
100k FREQUENCY (Hz)
Second Harmonic Distortion vs Frequency
-40 -50
DISTORTION (dBc)
DISTORTION (dBc)
-60 -70 -80 -90
AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER IQ = 20mA IQ = 15mA IQ = 10mA
IQ = 5mA
-100 10k
Slew Rate vs Supply Current
1200 1000 800 FALLING 600 400 200 0 -3dB BANDWIDTH (MHz) RISING SLEW RATE (V/s) 50
25 15 20 30 7.5 10 SUPPLY CURRENT PER AMPLIFIER (mA)
1795 * G21
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LT1795 G16
Second Harmonic Distortion vs Frequency
-40
AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER -50
Third Harmonic Distortion vs Frequency
-50 -60 -70 -80 -90 -100 10k IQ = 15mA IQ = 20mA AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER
IQ = 5mA
IQ = 10mA
IQ = 5mA IQ = 10mA IQ = 20mA IQ = 15mA
1M
100k FREQUENCY (Hz)
1M
LT1795 G17
100k FREQUENCY (Hz)
1M
LT1795 G18
Third Harmonic Distortion vs Frequency
-40 -50 -60 -70 -80 -90 -100 10k IQ = 15mA AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA
IQ = 10mA
IQ = 20mA 100k FREQUENCY (Hz) 1M
LT1795 G20
100k FREQUENCY (Hz)
1M
LT1795 G19
-3dB Bandwidth vs Supply Current
45
40
35 VS = 15V TA =25C AV = 4 RLOAD = 25 RF = 1k
VS = 15V TA =25C AV = 4 RLOAD = 25 RF = 1k
30
25 25 15 7.5 10 20 30 SUPPLY CURRENT PER AMPLIFIER (mA)
1795 * G22
LT1795
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 transmission lines with excellent linearity. SHUTDOWN/CURRENT SET 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 200A, or to control the quiescent current in normal operation.
V+ RSHDN 10 SHDN 11 SHDNREF
1795 F01
Figure 1. RSHDN Connected Between V + and SHDN (Pin 10); SHDNREF (Pin 11) = GND. See Figure 2
80 70
AMPLIFIER SUPPLY CURRENT, ISY - mA (BOTH AMPLIFIERS)
TA = 25C VS = 15V
AMPLIFIER SUPPLY CURRENT, ISY - mA (BOTH AMPLIFIERS)
60 50 40 30 20 10 0 0 25 50 75 100 125 150 175 200 225 RSHDN (k)
1795 F02
Figure 2. LT1795 Amplifier Supply Current vs RSHDN. RSHDN Connected Between V+ and SHDN, SHDNREF = GND (See Figure 1)
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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 1.5s. 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
V+ 10 SHDN 11 SHDNREF RSHDNREF
1795 F03
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Figure 3. RSHDNREF Connected Between SHDNREF (Pin 11) and GND; SHDN (Pin 10) = V +. See Figure 4
80 70 60 50 40 30 20 10 0 50 100 150 200 250 300 350 400 450 500 RSHDNREF (k)
1795 F04
TA = 25C VS = 15V
Figure 4. LT1795 Amplifier Supply Current vs RSHDNREF. RSHDNREF Connected Between SHDNREF and GND, SHDN = V+ (See Figure 3)
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LT1795
APPLICATIO S I FOR ATIO
quiescent current to 33mA with VS = 15V. If ON/OFF control is desired in addition to reduced quiescent current, then the circuits in Figures 5 to 7 can be employed.
V+ RSHDN OFF ON (0V) (3.3V/5V) Q1: 2N3904 OR EQUIVALENT
1795 F05
RB 10k Q1
10 SHDN 11 SHDNREF
INTERNAL LOGIC THRESHOLD ~1.4V
Figure 5. Setting Amplifier Supply Current Level with ON/OFF Control, Version 1
V+ 10 SHDN RPULLUP >500k 11 SHDNREF RSHDN1 ON OFF (0V) (3.3V/5V) RB1 10k Q1A ON OFF (0V) (3.3V/5V) RSHDN2 RB2 10k Q1B
Q1A, Q1B: ROHM IMX1 or FMG4A (W/INTERNAL RB)
Figure 6. Setting Multiple Amplifier Supply Current Levels with ON/OFF Control, Version 2
ON
REXT
OFF (0V) IPROG (3.3V/5V) IPROG 0.5mA FOR REXT = 0 (SEE SHDN PIN CURRENT vs VOLTAGE CHARACTERISTIC)
SHDN 10 ISY CONTROL SHDNREF 11
1795 F07
INTERNAL LOGIC THRESHOLD ~ 1.4V
Figure 7. Setting Amplifier Supply Current Level with ON/OFF Control, Version 3
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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 termination. The design equations are:
R1 = 115 * VH
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(IS )ON - (IS )OFF (VSHDN / VH ) * (IS )ON - (IS )OFF + (IS )OFF
115 * VCC - VSHDN
R2 =
(
)
where VH = Logic High Level (IS)ON = Supply Current Fully On (IS)OFF = Supply Current Partially On VSHDN = Shutdown Pin Voltage 1.4V VCC = Positive Supply Voltage
VCC R2
1795 F06
ON OFF (0V) (3.3V/5V)
R1 10 SHDN INTERNAL LOGIC THRESHOLD ISY ~ 1.4V CONTROL
11 SHDNREF
1795 F08
Figure 8. Partial Shutdown
THERMAL CONSIDERATIONS The LT1795 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, which depends on the power dissipation and the thermal time constants of the package and heat sinking. Raising the ambient tempera-
LT1795
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. 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 the total copper area connected to the device. For example, the thermal resistance of the LT1795 connected to a 2 x 2 inch, double sided 2 oz copper plane is 40C/W. CALCULATING JUNCTION TEMPERATURE The junction temperature can be calculated from the equation: TJ = (PD)(JA) + TA where TJ = Junction Temperature TA = Ambient Temperature PD = Device Dissipation JA = Thermal Resistance (Junction-to-Ambient) 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. POWER SUPPLY BYPASSING To obtain the maximum output and the minimum distortion from the LT1795, the power supply rails should be well bypassed. For example, with the output stage supply-
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ing 0.5A current peaks into the load, a 1 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 1F 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 4.7F, are recommended in place of the 1F unit mentioned above. Inadequate bypassing is evidenced by reduced output swing and "distorted" clipping effects when the output is driven to the rails. If this is observed, check the supply pins of the device for ripple directly related to the output waveform. Significant supply modulation indicates poor bypassing. 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. Feedback Resistor Selection The optimum value for the feedback resistors is a function of the operating conditions of the device, the load impedance 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 operating conditions. If this level of flatness is not required, a higher bandwidth can be obtained by use of a lower feedback resistor. For resistive loads, the COMP pin should be left open (see Capacitive Loads section). Capacitive Loads The LT1795 includes an optional compensation network for driving capacitive loads. This network eliminates most of the output stage peaking associated with capacitive loads, allowing the frequency response to be flattened.
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LT1795
APPLICATIONS INFORMATION
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 0.01F bypass capacitor between the output and the COMP pins connects the compensation
14 12 10 VS = 15V CL = 200pF RF = 1k COMPENSATION RF = 3.4k NO COMPENSATION
VOLTAGE GAIN (dB)
8 6 4 2 0 -2 -4 -6 1
RF = 3.4k COMPENSATION 10 FREQUENCY (MHz) 100
1795 F09
Figure 9.
PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted. SW Package 20-Lead Plastic Small Outline (Wide 0.300)
(LTC DWG # 05-08-1620)
0.496 - 0.512* (12.598 - 13.005) 20 19 18 17 16 15 14 13 12 11
0.291 - 0.299** (7.391 - 7.595) 0.010 - 0.029 x 45 (0.254 - 0.737)
0.009 - 0.013 (0.229 - 0.330)
NOTE 1 0.016 - 0.050 (0.406 - 1.270)
NOTE: 1. 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 *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
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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. 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 25 load, the bandwidth drops from 48MHz to 32MHz when the compensation is connected. Hence, the compensation was made optional. To disconnect the optional compensation, leave the COMP pin open. DEMO BOARD 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 applications. For details, consult your local sales representative.
NOTE 1
0.394 - 0.419 (10.007 - 10.643)
1 0.093 - 0.104 (2.362 - 2.642)
2
3
4
5
6
7
8
9
10 0.037 - 0.045 (0.940 - 1.143)
0 - 8 TYP
0.050 (1.270) TYP 0.014 - 0.019 (0.356 - 0.482) TYP
0.004 - 0.012 (0.102 - 0.305)
S20 (WIDE) 0396
LT1795
PACKAGE DESCRIPTIO U
Dimensions in millimeters (inches) unless otherwise noted.
FE Package 20-Lead Plastic TSSOP (4.4mm)
(LTC DWG # 05-08-1663)
6.40 - 6.60* (0.252 - 0.260) 20 19 18 17 16 15 14 13 12 11
3.0 (0.118)
6.25 - 6.50 (0.246 - 0.256)
4.30 - 4.48** (0.169 - 0.176) 0 - 8
1 2 3 4 5 6 7 8 9 10 5.12 (0.202)
1.15 (0.453) MAX
0.09 - 0.18 (0.0035 - 0.0071)
0.50 - 0.70 (0.020 - 0.028)
0.65 (0.0256) BSC
NOTE: DIMENSIONS ARE IN MILLIMETERS *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.152mm (0.006") PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.254mm (0.010") PER SIDE
0.18 - 0.30 (0.0071 - 0.0118)
0.05 - 0.15 (0.002 - 0.006)
FE20 TSSOP 0200
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.
11
LT1795
SI PLIFIED SCHEMATIC
SHDN
RELATED PARTS
PART NUMBER LT1497 LT1207 LT1886 DESCRIPTION Dual 125mA, 50MHz Current Feedback Amplifier Dual 250mA, 60MHz Current Feedback Amplifier Dual 200mA, 700MHz Voltage Feedback Amplifier COMMENTS 900V/s Slew Rate Shutdown/Current Set Function Low Distortion: -72dBc at 200kHz
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
W
+IN
W
SHDNREF
TO ALL CURRENT SOURCES
V+
Q5 Q2 Q1 Q6 Q9 V- RC D1 Q15
Q10 Q11
V- -IN CC
50 COMP OUTPUT
V+ V+ Q12 Q8 Q4 Q7 D2 Q13 Q16 Q14
Q3
V-
1795 SS
1795f LT/TP 4K 0200 * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1999

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