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LT6556 750MHz Gain of 1 Triple 2:1Video Multiplexer FEATURES

DESCRIPTIO
750MHz -3dB Small Signal Bandwidth 450MHz -3dB 2VP-P Large-Signal Bandwidth 120MHz 0.1dB Bandwidth High Slew Rate: 2100V/s Fixed Gain of 1; No External Resistors Required 72dB Channel Separation at 10MHz 52dB Channel Separation at 100MHz -84dBc 2nd Harmonic Distortion at 10MHz, 2VP-P -87dBc 3rd Harmonic Distortion at 10MHz, 2VP-P Low Supply Current: 9.5mA per Amplifier 6.5ns 0.1% Settling Time for 2V Step ISS 330A per Amplifier When Disabled Differential Gain of 0.033%, Differential Phase of 0.022 Wide Supply Range: 2.25V (4.5V) to 6V (12V) Available in 24-Lead SSOP and 24-Lead QFN Packages
The LT(R)6556 is a high speed triple 2:1 video multiplexer with an internally fixed gain of 1. The individual buffers are optimized for performance with a 1k load and feature a 2VP-P -3dB bandwidth of 450MHz, making them ideal for driving very high resolution video signals. Separate power supply pins for each amplifier boost channel separation to 72dB, allowing the LT6556 to excel in many high speed applications. While the performance of the LT6556 is optimized for dual supply operation, it can also be operated with a single supply as low as 4.5V. Using dual 5V supplies, each amplifier draws only 9.5mA. When disabled, the amplifiers draw less than 330A and the outputs become high impedance. For applications requiring a fixed gain of 2, refer to the LT6555 datasheet. The LT6556 is available in 24-lead SSOP and ultra-compact 24-lead QFN packages.
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIO S

RGB Buffers UXGA Video Multiplexing LCD Projectors
TYPICAL APPLICATIO
RINA GINA BINA LT6556 75 x1
RGB Multiplexer and Line Driver
V+
Large-Signal Transient Response
1.5 ROUT 1k 1.0 0.5 OUTPUT (V) VIN = 2VP-P VS = 5V RL = 1k TA = 25C
75
75 AGND x1 1k RINB GINB BINB GOUT
0 -0.5 -1.0
75 -1.5 75 x1 SELECT A/B VREF 75 ENABLE DGND V
-
6556 TA01
0
2
BOUT 1k
U
4 6 8 10 12 14 16 18 20 TIME (ns)
6556 TA02
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U
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1
LT6556 ABSOLUTE AXI U RATI GS
.............................12.6V Input Current (Note 2) .........................................10mA Output Current (Continuous) ..............................70mA EN to DGND Voltage (Note 2) ..................................5.5V SEL to DGND Voltage (Note 2) ....................................8V Output Short-Circuit Duration (Note 3) ............ Indefinite Operating Temperature Range (Note 4) ... -40C to 85C Specified Temperature Range (Note 5) .... -40C to 85C Total Supply Voltage (V+ to V-)
PACKAGE/ORDER I FOR ATIO
TOP VIEW IN1A DGND IN2A VREF IN3A AGND1 IN1B AGND2 IN2B 1 2 3 4 5 6 7 8 9
G = +1 G = +1 G = +1
23 EN 22 SEL A/B 21 V+ 20 OUT1 19 V- 18 OUT2 17 V+ VREF 1 IN3A 2 AGND1 3 V- 4 IN1B 5 AGND2 6
24 23 22 21 20 19 18 V+ 17 OUT1 16 V- 15 OUT2 14 V+ 13 OUT3 7 IN2B 8 AGND3 9 10 11 12 V+ V+ IN3B V-
25
16 OUT3 15 V- 14 V+ 13 V+
AGND3 10 IN3B 11 V- 12 GN PACKAGE 24-LEAD PLASTIC SSOP
UF PACKAGE 24-LEAD (4mm x 4mm) PLASTIC QFN TJMAX = 125C, JA = 37C/W, JC = 2.6C/W EXPOSED PAD (PIN 25) IS V - MUST BE SOLDERED TO PCB
TJMAX = 150C, JA = 90C/W
ORDER PART NUMBER LT6556CGN LT6556IGN
GN PART MARKING LT6556CGN LT6556IGN
ORDER PART NUMBER LT6556CUF LT6556IUF
EN
V+
24
V+
UF PART MARKING* 6556
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VS = 5V, RL = 1k, CL = 1.5pF, VEN = 0.4V, VAGND, VDGND, VVREF = 0V.
SYMBOL VOS IIN RIN CIN PSRR PARAMETER Offset Voltage Input Current Input Resistance Input Capacitance Power Supply Rejection Ratio VIN = 1V f = 100kHz VS = 2.25V to 6V (Note 6) CONDITIONS VIN = 0V, VOS = VOUT

ELECTRICAL CHARACTERISTICS
MIN
SEL A/B
DGND
IN2A
IN1A
2
U
U
W
WW U
W
(Note 1)
Junction Temperature SSOP ................................................................ 150C QFN................................................................... 125C Storage Temperature Range SSOP ................................................. -65C to 150C QFN.................................................... -65C to 125C Soldering Temperature (10 sec) ............................ 300C
TOP VIEW
TYP 18 -12
MAX 67 75 45
UNITS mV mV A k pF dB
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100 51
500 1 62
LT6556
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VS = 5V, RL = 1k, CL = 1.5pF, VEN = 0.4V, VAGND, VDGND, VVREF = 0V.
SYMBOL IPSRR AV ERR AV MATCH VOUT IS PARAMETER Input Current Power Supply Rejection Gain Error Gain Matching Output Voltage Swing Supply Current, Per Amplifier Supply Current, Disabled, Per Amplifier IEN ISEL ISC SR -3dB BW 0.1dB BW FPBW Enable Pin Current Select Pin Current Output Short-Circuit Current Slew Rate Small-Signal -3dB Bandwidth Gain Flatness 0.1dB Bandwidth Full Power Bandwidth 2V Full Power Bandwidth 4V All-Hostile Crosstalk Selected Channel to Unselected Channel Crosstalk Channel Select Output Transient Channel-to-Channel Select Time tS tR, tF dG dP HD2 HD3 Settling Time Small-Signal Rise and Fall Time Differential Gain Differential Phase 2nd Harmonic Distortion 3rd Harmonic Distortion CONDITIONS VS = 2.25V to 6V (Note 6) VOUT = VREF = 2V, Nominal Gain 1V/V Any One Channel to Another (Note 7) RL = VEN = 4V, RL = VEN = Open, RL = VEN = 0.4V VEN = 4V VSEL = 0.4V VSEL = 4V RL = 0, VIN = 2V, VREF = 1V 1V on 2.2V Output Step (Note 8) VOUT = 200mVP-P VOUT = 200mVP-P VOUT = 2VP-P (Note 9) VOUT = 4VP-P (Note 9) f = 10MHz, VIN = 2VP-P f = 100MHz, VIN = 2VP-P f = 10MHz, VIN = 2VP-P f = 100MHz, VIN = 2VP-P INA = INB = 0V INA = -1V, INB = 1V from 50% SEL to VOUT = 0V 0.1% of VFINAL, VSTEP = 2V 10% to 90%, VOUT = 200mVP-P (Note 10) (Note 10) f = 10MHz, VOUT = 2VP-P f = 10MHz, VOUT = 2VP-P 190

ELECTRICAL CHARACTERISTICS
MIN -2.8 3.65
TYP 1 -1.15 0.05 3.85 9.5 47 42
MAX 3 0
UNITS A/V % % V
13 14.5 330 330
-200 -75 -50 -50 50 1200
-95 -21 -5 -1 105 2100 750 120 335 175 -72 -52 -85 -64 200 8 6.5 500 0.056 0.028 -84 -87
mA mA A A A A A A mA V/s MHz MHz MHz MHz dB dB dB dB mVP-P ns ns ps % Deg dBc dBc
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: This parameter is guaranteed to meet specified performance through design and characterization. It is not production tested. Note 3: As long as output current and junction temperature are kept below the Absolute Maximum Ratings, no damage to the part will occur. Depending on the supply voltage, a heat sink may be required. Note 4: The LT6556C is guaranteed functional over the operating temperature range of -40C to 85C. Note 5: The LT6556C is guaranteed to meet specified performance from 0C to 70C. The LT6556C is designed, characterized and expected to meet specified performance from -40C and 85C but is not tested or QA sampled at these temperatures. The LT6556I is guaranteed to meet specified performance from -40C to 85C. Note 6: In order to follow the constraints for 4.5V operation for PSRR and IPSRR testing at 2.25V, the DGND pin is set to V-, the EN pin is set
to V- + 0.4V, and the SEL pin is set to either V- + 0.4V or V- + 4V. At 6V and all other cases, DGND is set to ground and the EN and SEL pins are referenced from it. Note 7: The VREF pin is set to 3V when testing positive swing and -3V when testing negative swing to ensure that the internal input clamps do not limit the output swing. Note 8: Slew rate is 100% production tested using both inputs of channel 2. Slew rates of channels 1 and 3 are guaranteed through design and characterization. Note 9: Full power bandwidth is calculated from the slew rate: FPBW = SR/( * VP-P) Note 10: Differential gain and phase are measured using a Tektronix TSG120YC/NTSC signal generator and a Tektronix 1780R video measurement set. The resolution of this equipment is better than 0.05% and 0.05. Nine identical amplifier stages were cascaded giving an effective resolution of better than 0.0056% and 0.0056.
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LT6556 TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current per Amplifier vs Temperature
12 10 SUPPLY CURRENT (mA) VEN = 0.4V 8 6 4 2 VEN = 4V 0 -55 -35 -15 0 5 25 45 65 85 105 125 TEMPERATURE (C)
6556 G01
VEN = 0V
SUPPLY CURRENT (mA)
SUPPLY CURRENT (mA)
Offset Voltage vs Temperature
25 VS = 5V VIN = 0V INPUT BIAS CURRENT ( A) 0 -5
20 OFFSET VOLTAGE (mV)
EN PIN CURRENT (A)
15
10
5
0 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
6556 G04
Maximum Output Voltage Swing vs VREF Pin Voltage
5 4 3 OUTPUT VOLTAGE (V) 2 1 0 -1 -2 -3 -4 -5 TA = -55C LOW SWING -2 -1.5 -1 -0.5 0 0.5 1 VREF PIN VOLTAGE (V) 1.5 2 0 TA = 125C TA = 25C TA = 25C TA = 125C VS = 5V RL = 1k HIGH SWING OUTPUT VOLTAGE (V) 5 TA = -55C 4
OUTPUT VOLTAGE (V)
4
UW
VS = 5V RL = VIN = 0V
Supply Current per Amplifier vs Supply Voltage
12 VS = 5V VEN, VIN, VDGND, VSEL = 0V 10 TA = 25C 8 6 4 2 12 10
Supply Current per Amplifier vs EN Pin Voltage
VS = 5V RL = VIN = 0V TA = -55C 8 TA = 25C 6 TA = 125C 4 2 0
0
1
2
3 4 5 6 7 8 9 10 11 12 TOTAL SUPPLY VOLTAGE (V)
6556 G02
0
0.5
1.0 1.5 2.0 2.5 3.0 EN PIN VOLTAGE (V)
3.5
4.0
6556 G03
Input Bias Current vs Temperature
VS = 5V VIN = 1.5V -10 -15 VIN = 0V -20 VIN = -1.5V -25 -30 -35 -40 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
6556 G05
EN Pin Current vs EN Pin Voltage
0 -20 -40 -60 -80 TA = 25C -100 -120 -140 0 1 3 2 EN PIN VOLTAGE (V) 4 5
6556 G06
VS = 5V VDGND = 0V
TA = 125C TA = -55C
Output Voltage Swing vs ILOAD (Output High)
VS = 5V VIN = 4V VVREF = 3V TA = -55C TA = 125C 0
Output Voltage Swing vs ILOAD (Output Low)
VS = 5V VIN = -4V VVREF = -3V TA = 125C TA = -55C -2 TA = 25C -3
-1
3
2 TA = 25C
1
-4
0
10 20 30 40 50 60 70 80 90 100 SOURCE CURRENT (mA)
6556 G08
-5
0
10 20 30 40 50 60 70 80 90 100 SINK CURRENT (mA)
6556 G09
6556 G07
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LT6556 TYPICAL PERFOR A CE CHARACTERISTICS
Input Noise Spectral Density
1000 INPUT NOISE (nV/Hz OR pA/Hz) 1000 VS = 5V TA = 25C INPUT IMPEDANCE (k) 100
REJECTION RATIO (dB)
100
en
10
in
1 0.001 0.01 0.1 1 FREQUENCY (kHz) 10 100
6556 G10
Frequency Response vs Output Amplitude
3 2 1 0 GAIN (dB) GAIN (dB) -1 -2 -3 -4 -5 -6 0.1 1 10 100 FREQUENCY (MHz) 1000
6556 G13
VS = 5V RL = 1k TA = 25C
0.05 IN1A 0 VOUT = 200mVP-P VOUT = 2VP-P VOUT = 4VP-P -0.10 IN1B
IN2B IN3B
AMPLITUDE (dB)
Crosstalk vs Frequency
0 VS = 5V VOUT = 2VP-P -20 RL = 1k TA = 25C AMPLITUDE (dB) AMPLITUDE (dB) -40 -60 -80 DRIVE IN A; SELECT IN B DRIVE IN B; SELECT IN A 0
DISTORTION (dBc)
-100 -120 0.1
1
10 100 FREQUENCY (MHz)
UW
6556 G16
Input Impedance vs Frequency
VS = 5V VIN = 0V TA = 25C 70 60 50 40 30 20 10 0.1 0.01
PSRR vs Frequency
PSRR -PSRR +PSRR VS = 5V TA = 25C
10
1
0.1
1 10 FREQUENCY (MHz)
100
1000
6556 G11
0 0.001
0.01
0.1 1 FREQUENCY (MHz)
10
100
6556 G12
Gain Flatness vs Frequency
0.15 0.10 VS = 5V VOUT = 200mVP-P RL = 1k TA = 25C 9
Frequency Response with Capacitive Loads
VS = 5V VOUT = 200mVP-P RL = 1k TA = 25C CL = 10pF CL = 15pF CL = 6.8pF
6
3
0 CL = 3.3pF -3 CL = 0pF
-0.05 IN3A IN2A
-0.15 0.1
1
10 100 FREQUENCY (MHz)
1000
6556 G14
-6 0.1
1
10 100 FREQUENCY (MHz)
1000
6555 G15
Crosstalk vs Frequency
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -120 0.1 -100 -110 1000 1 10 100 FREQUENCY (MHz) 1000
6556 G17
Harmonic Distortion vs Frequency
VS = 5V VOUT = 2VP-P RL = 1k TA = 25C
VS = 5V VOUT = 2VP-P -20 RL = 1k TA = 25C -40 -60 -80
WORST ADJACENT
ALL CHANNELS DRIVEN
HD2 HD3
-120 0.01
0.1
1 10 FREQUENCY (MHz)
100
6556 G18
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LT6556 TYPICAL PERFOR A CE CHARACTERISTICS
Output Impedance vs Frequency
1000000 OUTPUT SERIES RESISTANCE ( ) 100000 OUTPUT IMPEDANCE () 10000 1000 100 10 1 0.1 0.01 ENABLED VEN = O.4V VS = 5V TA = 25C 100 1000
6556 G19
DISABLED VEN = 4V
20 15 10 5 0
OUTPUT (V)
0.1
1 10 FREQUENCY (MHz)
Video Amplitude Transient Response
0.9 0.8 0.7 0.6 OUTPUT (V) 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 0 2 4 6 8 10 12 14 16 18 20 TIME (ns)
6556 G22
PERCENT OF UNITS (%)
VIN = 700mVP-P VS = 5V RL = 1k TA = 25C OUTPUT (V)
Gain Error Matching Distribution
40 35 PERCENT OF UNITS (%) 30 25 20 15 SEL A/B (V) 10 5 0 -0.1 -0.075 -0.05 -0.025 0 0.025 0.05 0.075 0.1 GAIN ERROR--BETWEEN CHANNELS (%)
6556 G25
3 2 1 0 0 10 20 30 40 50 60 70 80 90 100 TIME (ns)
6556 G26
SEL A/B (V)
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UW
VS = 5V VOUT = 2V RL = 1k TA = 25C
Maximum Capacitive Load vs Output Series Resistor
35 30 25 0.20 VOUT = 2VP-P VS = 5V RL = 1k TA = 25C 0.15 0.10 0.05 0
Small-Signal Transient Response
AC PEAKING >3dB
-0.05 -0.10 -0.15 -0.20 VIN = 200mVP-P VS = 5V RL = 1k TA = 25C 0 2 4 6 8 10 12 14 16 18 20 TIME (ns)
6556 G21
1
10 100 CAPACITIVE LOAD (pF)
1000
6556 G20
Large-Signal Transient Response
2.5 2.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5 0 2 4 6 8 10 12 14 16 18 20 TIME (ns)
6556 G23
Gain Error Distribution
35 30 25 20 15 10 5 0 -1.3 -1.25 -1.2 -1.15 -1.1 -1.05 -1.0 -.95 -.90 GAIN ERROR--INDIVIDUAL CHANNEL (%)
6556 G24
VIN = 4VP-P VS = 5V RL = 1k TA = 25C
VS = 5V VOUT = 2V RL = 1k TA = 25C
Channel Switching Transient
0.15 OUTPUT (V) 0.10 0.05 0 VS = 5V RL = 1k 5 4 INA = INB = 0V TA = 25C
Channel Switching Transient
1.5 1.0 OUTPUT (V) 0.5 0 -0.5 -1.0 5 4 3 2 1 0 0 10 20 30 40 50 60 70 80 90 100 TIME (ns)
6556 G27
-0.05 -0.10 VS = 5V RL = 1k INB = 300MHz, 2VP-P SINE TA = 25C INA = 0V
-1.5
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LT6556 PI FU CTIO S
IN1A (Pin 1): Channel 1 Input A. This pin has a nominal impedance of 500k and does not have any internal termination resistor. DGND (Pin 2): Digital Ground Reference for Enable Pin. This pin is normally connected to ground. IN2A (Pin 3): Channel 2 Input A. This pin has a nominal impedance of 500k and does not have any internal termination resistor. VREF (Pin 4): Voltage Reference for Input Clamping. This is the tap to an internal voltage divider that defines midsupply. It is normally connected to ground in dual supply, DC coupled applications. IN3A (Pin 5): Channel 3 Input A. This pin has a nominal impedance of 500k and does not have any internal termination resistor. AGND (Pin 6): Analog Ground for Isolation between IN3A and IN1B. AGND pins have ESD protection and should not be connected to potentials outside the power supply range. IN1B (Pin 7): Channel 1 Input B. This pin has a nominal impedance of 500k and does not have any internal termination resistor. AGND (Pin 8): Analog Ground for Isolation between IN1B and IN2B. AGND pins have ESD protection and should not be connected to potentials outside the power supply range. IN2B (Pin 9): Channel 2 Input B. This pin has a nominal impedance of 500k and does not have any internal termination resistor. AGND (Pin 10): Analog Ground for Isolation between IN2B and IN3B. AGND pins have ESD protection and should not be connected to potentials outside the power supply range. IN3B (Pin 11): Channel 3 Input B. This pin has a nominal impedance of 500k and does not have any internal termination resistor. V- (Pin 12): Negative Supply Voltage. V- pins are not internally connected to each other and must all be connected externally. Proper supply bypassing is necessary for best performance. See the Applications Information section. V+ (Pins 13, 14, 24): Positive Supply Voltage. V+ pins are not internally connected to each other and must all
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(GN24 Package)
be connected externally. Proper supply bypassing is necessary for best performance. See the Applications Information section. V- (Pin 15): Negative Supply Voltage for Channel 3 Output Stage. V- pins are not internally connected to each other and must all be connected externally. Proper supply bypassing is necessary for best performance. See the Applications Information section. OUT3 (Pin 16): Channel 3 Output. It is the buffered output of the selected Channel 3 input. V+ (Pin 17): Positive Supply Voltage for Channels 2 and 3 Output Stages. V+ pins are not internally connected to each other and must all be connected externally. Proper supply bypassing is necessary for best performance. See the Applications Information section. OUT2 (Pin 18): Channel 2 Output. It is the buffered output of the selected Channel 2 input. V- (Pin 19): Negative Supply Voltage for Channels 1 and 2 Output Stages. V- pins are not internally connected to each other and must all be connected externally. Proper supply bypassing is necessary for best performance. See the Applications Information section. OUT1 (Pin 20): Channel 1 Output. It is the buffered output of the selected Channel 1 input. V+ (Pin 21): Positive Supply Voltage for Channel 1 Output Stage. V+ pins are not internally connected to each other and must all be connected externally. Proper supply bypassing is necessary for best performance. See the Applications Information section. SEL A/B (Pin 22): Select Pin. This high impedance pin selects which set of inputs are sent to the output pins. When the pin is pulled low, the A inputs are selected. When the pin is pulled high, the B inputs are selected. EN (Pin 23): Enable Control Pin. An internal pull-up resistor of 46k defines the pin's impedance and will turn the part off if the pin is unconnected. When the pin is pulled low, the amplifiers are enabled. Exposed Pad (Pin 25, QFN Only): The Exposed Pad is V- and must be soldered to the PCB. It is internally connected to the QFN Pin 4, V-.
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LT6556 APPLICATIO S I FOR ATIO
Power Supplies The LT6556 is optimized for 5V supplies but can be operated on as little as 2.25V or a single 4.5V supply and as much as 6V or a single 12V supply. Internally, each supply is independent to improve channel isolation. Do not leave any supply pins disconnected or the part may not function correctly! Enable/Shutdown The LT6556 has a shutdown mode controlled by the EN pin and referenced to the DGND pin. If the amplifier will be enabled at all times, the EN pin can be connected directly to DGND. If the enable function is desired, either driving the pin above 2V or allowing the internal 46k pull-up resistor to pull the EN pin to the top rail will disable the amplifier. When disabled, the output will become very high impedance. Supply current into the amplifier in the disabled state will be: IS = V + - VEN V + - V - + 46k 80k
It is important that the following constraints on the DGND, EN and SEL pins are always followed: V+ - VDGND 4.5V -0.5V VEN - VDGND 5.5V VSEL - VDGND 8V In dual supply cases where V+ is less than 4.5V, DGND should be connected to a potential below ground, such as V-. Since the EN and SEL pins are referenced to DGND, they may need to be pulled below ground in those cases. However, in order to protect the internal enable circuitry, the EN pin should not be forced more than 0.5V below DGND. In single supply applications above 5.5V, an additional resistor may be needed from the EN pin to DGND if the pin is ever allowed to float. For example, on a 12V single supply, a 33k resistor would protect the pin from floating too high while still allowing the internal pull-up resistor to disable the part. On dual 2.25V supplies, connecting the DGND pin to V- is the only way of ensuring that V+ - VDGND 4.5V.
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The enable/disable times of the LT6556 are fast when driven with a logic input. Turn on (from 50% EN input to 50% output) typically occurs in less than 50ns. Turn off is slower, but is typically below 500ns. Channel Select The SEL pin uses the same internal threshold as the EN pin and is also referenced to DGND. When the pin is logic low, the channel A inputs are passed to the output. When the pin is logic high, the channel B inputs are passed to the output. The pin should not be floated but can be tied to DGND to force the outputs to always be channel A or to V+ (when less than 8V) to force the outputs to always be channel B.
Truth Table
SEL A/B 0 1 X EN 0 0 1 OUT IN A IN B OFF
W
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Input Considerations The LT6556 uses input clamps referenced to the VREF pin to prevent damage to the input stage on the unselected channel. Three transistors in series limit the input voltage to within three diode drops () from VREF . VREF is nominally set to half of the sum of the supplies by the 40k resistors. A simplified schematic is shown in Figure 1.
V+
40k
IN
VREF
40k
V-
6556 F01
Figure 1. Simplified Schematic of VREF Pin and Input Clamping
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LT6556 APPLICATIO S I FOR ATIO
To improve clamping, the pin's DC impedance should be minimized by connecting the VREF pin directly to ground in the symmetric dual supply case with a common mode voltage of 0V. If the common mode voltage is not centered at ground or the input voltage exceeds plus or minus three diodes from ground, an external resistor to either supply can be added to shift the VREF voltage to the desired level. The only way to cover the full input voltage range of V- + 1V to V+ - 1V is to shift VREF up or down. The VREF pin can also be directly driven with a DC source. Figure 2 shows the effect of the clamp on input current when sweeping input voltage with various VREF pin voltages. Bypassing the VREF pin is not necessary.
250 200 150 INPUT CURRENT (A) 100 50 0 -50 -100 -150 -200 -250 -4 -3 -2 TA = 25C VS = 5V 0 1 -1 2 INPUT VOLTAGE (V) 3 4 VREF = -2V VREF = -1V VREF = 0V VREF = 1V VREF = 2V
6556 F02
Figure 2. Input Current vs Input Voltage at Different VREF Voltages
The inputs can be driven beyond the point at which the output clips so long as input currents are limited to less than 10mA. Continuing to drive the input beyond the output limit can result in increased current drive and slightly increased swing, but will also increase supply current and may result in delays in transient response at larger levels of overdrive.
AMPLITUDE (dB)
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Layout and Grounding It is imperative that care is taken in PCB layout in order to benefit from the very high speed and very low crosstalk of the LT6556. Separate power and ground planes are highly recommended and trace lengths should be kept as short as possible. If input traces must be run over a distance of several centimeters, they should use a controlled impedance with either series or shunt terminations (nominally 50 or 75) to maintain signal fidelity. Care should be taken to minimize capacitance on the LT6556's output traces by increasing spacing between traces and adjacent metal and by eliminating metal planes in underlying layers. To drive cable or traces longer than several centimeters, using the LT6555 with its fixed gain of+2 in conjunction with series and load termination resistors may provide better results. A plot of AC performance driving a 1k load with various trace lengths is shown in Figure 3. All data is from a 4-layer board with 2oz copper, 18mil of board layer thickness to the ground plane, a trace width of 12mils and spacing to adjacent metal of 18mils. The 0.2cm output trace places the 1k resistor as close to the part as possible, while the other curves show the load resistor consecutively further away. The worst case, 4cm, trace has almost 10pF of parasitic capacitance.
6 4 2 2cm TRACE 0 0.2cm TRACE -2 -4 -6 VS = 5V VOUT = 200mVP-P RL = 1k TA = 25C 4cm TRACE 0.1 1 10 100 FREQUENCY (MHz) 1000
6556 F03
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Figure 3. Response vs Output Trace Length
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LT6556 APPLICATIO S I FOR ATIO
In order to counteract any peaking in the frequency response from driving a capacitive load, a series resistance can be inserted in the line at the output of the part to flatten the response. Figure 4 shows the frequency response with the same 4cm trace from Figure 3, now with a 10 series resistor inserted near the output pin of the amplifier. Note that using a 10 series resistor with a 1k load only decreases the output amplitude by 0.1dB or 1% and has a minimal effect on the bandwidth of the system. See the graph labeled "Maximum Capacitive Load vs Output Series Resistor" in the Typical Performance Characteristics section for more information.
6 4 AMPLITUDE (dB) 2 0 -2 -4 -6 4cm TRACE RS, OUT = 10 VS = 5V VOUT = 200mVP-P RL = 1k TA = 25C
4cm TRACE
0.1
1
10 100 FREQUENCY (MHz)
1000
6556 F04
Figure 4. Response vs Series Output Resistance
VIN
While the AGND pins on the LT6556 are not connected to the amplifier circuitry, tying them to ground or another "quiet" node significantly increases channel isolation and is always recommended. The AGND pins do have ESD protection and therefore should not be connected to potentials outside the power supply range. Low ESL/ESR bypass capacitors should be placed as close to the positive and negative supply pins as possible. One 4700pF ceramic capacitor is recommended for both V+ and V- supply busses. Additional 470pF ceramic capacitors with minimal trace length on each supply pin will further improve AC and transient response as well as channel isolation. For high current drive and large-signal transient applications, additional 1F to 10F tantalums should be added on each supply. The smallest value capacitors should be placed closest to the package.
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To maintain the LT6556's channel isolation, it is beneficial to shield parallel input and parallel output traces using a ground plane or power supply traces. Vias between topside and backside metal may be required to maintain a low inductance ground near the part where numerous traces converge. See Figures 7 and 8 for photos of an optimized layout. Single Supply Operation Figure 5 illustrates how to use the LT6556 with a single supply ranging from 4.5V to 12V. Since the output range is comparable to the input range, the DC bias point at the input can be set anywhere between the supplies that will prevent the AC-coupled signal from running into the output range limits. As shown, the DC input level is mid-supply. The only additional power dissipation in the single supply configuration is through the resistor bias string at the input and through any load resistance at the output. In many cases, the output can be used to directly drive other single supply devices without additional coupling and without any resistive load.
4.5V TO 12V 22F 5k IN V+ 1/3 LT6556 V- OUT 5k AGND
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Figure 5. Single Supply Configuration, One Channel Shown
Input Expansion In applications with more than two inputs per channel, multiple LT6556s can be connected directly together at the outputs. Logic circuitry can be used to drive the EN pins of each LT6556 to ensure that only one set of channels is buffered at a time. See Figure 9 for a schematic. Since the output impedance of a disabled LT6556 is high, adding additional channels will not resistively load an
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LT6556 APPLICATIO S I FOR ATIO
enabled output. However, since the disabled LT6556 and its traces have around 6pF of capacitance, it may be desirable to resistively isolate the outputs of each channel to maintain flat frequency response as shown in the graph labeled "Maximum Capacitive Load vs Output Series Resistor" in the Typical Performance Characteristics section.
TYPICAL APPLICATIO
RGB Multiplexer Demo Board
The DC892A Demo Board illustrates optimal routing, bypassing and termination using the LT6556 as an RGB video multiplexer. The schematic is shown in Figure 6. All inputs and outputs are routed to have a characteristic impedance of 75 and 75 input shunt and output series terminations are connected as close to the part as possible. The board is fabricated with four layers with internal ground and power planes. While the 75 back termination resistors at the outputs of the LT6556 minimize signal reflections in the output
E1 EN J1 50 BNC 1 EN Z = 50 JP1 3 CONTROL 1 2 EXT ENABLE JP2 3 DGND 1 2 FLOAT AGND JP5 VREF E5 VREF E2 DGND R8 50 OPT
5 4 3 2 DGND
3 BNC x 6 5 JP12 1 L1 Z = 75 4 3 2 5 JP13 1 L1 Z = 75 4 3 2 5 JP14 1 L1 Z = 75 4 3 2 5 JP5 1 L1 Z = 75 4 3 2 5 JP6 1 L1 Z = 75 4 3 2 5 JP7 1 L1 Z = 75 4 3 2
1 C1 4700pF U1 LT6556CUF 22 21 V+ IN1A DGND 23 24 VREF 1 2 3 5 6 7 8 9 DGND IN2A VREF IN3A AGND1 IN1B AGND2 IN2B AGND3 IN3B V- V
-
2 EXT
GND
IN1A
IN2A
IN3A
IN1B
IN2B
IN3B
R10 75
R11 75
R12 75
R4 75 E3 AGND
R5 75
R6 75
4
J3 BANANA JACK AGND
SINGLE DUAL 2 1 JP3 3 SUPPLY NOTE: 470pF BYPASS CAPACITORS LOCATED AS CLOSE TO PINS AS POSSIBLE
Figure 6. Demo Board Schematic
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ESD Protection The LT6556 has reverse-biased ESD protection diodes on all pins. If any pins are forced a diode drop above the positive supply or a diode drop below the negative supply, large currents may flow through these diodes. If the current is kept below 10mA, no damage to the devices will occur. traces and isolate the part from any capacitive loading in those traces, they also contribute to gain error if the output is not terminated with high impedance. For example, if the output is terminated with a 1k load, the 75 back termination will cause a 7% gain error. Decreasing the value of the back termination resistors will decrease the signal attenuation but may compromise the AC response. However, connecting the LT6556 output pins to the output traces on the DC892A board without some series resistance is not recommended; 10 to 20 is generally sufficient. Figures 7 and 8 show the top and bottom side board layout and placement.
E4 SEL A/B Z = 50 R9 50 OPT 2 3A R7 B 1 20k J8 50 BNC 1 SEL A/B VCC DGND 5432 JP4 SEL C2 470pF C3 470pF C10 4700pF C4 10F 16V 1206 C7 0.33F 10V V J2 BANANA JACK VCC 3.3V TO 5V
CC
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EN SEL A/B
20 EN 19 SEL R1 75 R2 75 R3 75 BNC x 3 5 J9 4 3 2 J10 5 1 4 3 2 J11 5 1 4 3 2
18 V+ OUT1 V- OUT2 V+ OUT3 V- V+ V+ 25 C5 4700pF C6 470pF C9 10F 16V 1206 C8 0.33F 10V 17 16 15 14 13 12 11 10
Z = 75
L2
1
OUT1
Z = 75
L2
OUT2
Z = 75
L2
OUT3
J4 BANANA JACK VEE -3.3V TO -5V VEE
6556 F06
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11
LT6556 TYPICAL APPLICATIO U
Figure 7. Demo Board Topside (IC Removed for Clarity) Figure 8. Demo Board Bottom Side
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SI PLIFIED SCHE ATIC W
V+ OUT
V+
40k V REF 40k V- VREF 770 INA 100 V+ INB 100 V+ SEL 360 360
BIAS
TO OTHER OUTPUT STAGES
V+
(One channel shown)
EN
1k
46k
V-
V-
DGND VREF SELECT TO OTHER INPUT STAGES V-
6556 SS
VREF
V-
W LT6556
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LT6556 PACKAGE DESCRIPTIO U
GN Package 24-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.337 - .344* (8.560 - 8.738) 24 23 22 21 20 19 18 17 16 15 1413
.045 .005 .033 (0.838) REF
.229 - .244 (5.817 - 6.198)
.254 MIN .150 - .165
.150 - .157** (3.810 - 3.988)
1
.0165 .0015 RECOMMENDED SOLDER PAD LAYOUT .0250 BSC
23
4
56
7
8
9 10 11 12
.015 .004 x 45 (0.38 0.10)
.0075 - .0098 (0.19 - 0.25) 0 - 8 TYP
.0532 - .0688 (1.35 - 1.75)
.004 - .0098 (0.102 - 0.249)
.016 - .050 (0.406 - 1.270)
NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE *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
.008 - .012 (0.203 - 0.305) TYP
.0250 (0.635) BSC
GN24 (SSOP) 0204
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14
LT6556 PACKAGE DESCRIPTIO U
UF Package 24-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW--EXPOSED PAD 0.75 0.05 R = 0.115 TYP PIN 1 NOTCH R = 0.20 TYP OR 0.35 x 45 CHAMFER 23 24 0.40 0.10 1 2 2.45 0.10 (4-SIDES)
(UF24) QFN 0105
4.50 0.05 2.45 0.05 3.10 0.05 (4 SIDES)
4.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
0.200 REF 0.00 - 0.05 NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)--TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
0.25 0.05 0.50 BSC
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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.
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LT6556 TYPICAL APPLICATIO U
LT6556 #1 75 IN1A IN1B 75 x1 V+ 5V OUT1 75 RED 2 GREEN 2 BLUE 2 IN2A IN2B 75 x1 OUT2 75 IN3A IN3B 75 SEL EN x1 OUT3 AGND DGND VREF V
-
RED 1 GREEN 1 BLUE 1
ROUT
RED 3 GREEN 3 BLUE 3
-2V 5V LT6556 #2 75 IN1A IN1B 75 x1 OUT1 V+
GOUT
BOUT
75 RED 4 GREEN 4 BLUE 4
IN2A IN2B x1 OUT2
75
75
IN3A IN3B x1 OUT3 AGND DGND VREF V-
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75 SEL0 NC7SZ14 SEL1
SEL EN
SEL1 SEL0 OUTPUT 0 0 1 0 1 2 1 0 3 1 1 4
-2V
Figure 9. 4:1 RGB Multiplexer
RELATED PARTS
PART NUMBER LT1203 LT1399 LT1675 LT6550/LT6551 LT6553 LT6554 LT6555 DESCRIPTION 150MHz Single 2:1 Multiplexer 300MHz Triple Current Feedback Amplifier 250MHz Triple RGB Multiplexer 3.3V Triple and Quad Video Buffers 650MHz Gain of 2 Triple Video Amplifier 650MHz Gain of 1 Triple Video Amplifier 650MHz Gain of 2 Triple Video Multiplexer COMMENTS Single SPDT Video Switch 0.1dB Gain Flatness to 150MHz, Shutdown 100MHz Pixel Switching, 1100V/s Slew Rate, 16-Lead SSOP 110MHz Gain of 2 Buffers in MS Package Same Pinout as the LT6554 but Optimized for Driving 75 Cables Performance Similar to the LT6556 with One Set of Inputs, 16-Lead SSOP Same Pinout as the LT6556 but Optimized for Driving 75 Cables
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16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT/TP 0805 500 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005

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