DESIGN FEATURES Triple and Quad RGB Amplifiers Deliver Full Performance on 3.3V by Jon Munson and Raj Ramchandani Introduction Amplifier Characteristics The block diagrams in Figure 1 show the differences between the LT6550 and LT6551. The LT6551 quad is designed for single supply operation with the feedback returned to ground. The LT6550 triple has a separate VEE pin and can be used on either single or split supplies. These devices feature internal feedback resistors and a flow-thru pin out, Linear Technology Magazine • March 2003 IN1 450Ω 450Ω 450Ω IN1 VCC IN2 450Ω 450Ω – OA + OUT1 450Ω IN2 – OA + IN3 450Ω 450Ω 450Ω VCC – OA + OUT1 450Ω – OA + OUT2 450Ω IN3 – OA + OUT2 450Ω – OA + OUT3 GND 450Ω IN4 OUT3 450Ω – OA + VEE N/C GND OUT4 Figure 1a. LT6550 block diagram Figure 1b. LT6551 block diagram which simplifies PC board layout and enhances performance by minimizing input to output stray capacitance. The amplifiers feature a rail-to-rail output and an input common mode range which includes ground. Figure 2 shows the output swing driving a 150Ω load vs supply voltage. On a single 3.3V supply, the input voltage range extends from ground to 1.55V and the output typically swings to within 400mV of the supply voltage while driving a 150Ω load. Table 1 summarizes the major performance specifications. Figure 3 shows a simplified schematic of one channel of the LT6551. Resistors RF and RG provide an internal gain of 2. (The LT6550 triple is a slight variation with the gain setting resistor, RG, connected to a separate ground pin). The input stage consists of transistors Q1 to Q8 and resistor R1. This topology allows for high slew rates at low supply voltages. Transistors Q3 and Q4 are class AB biased as are transistors Q5 and Q6. The input stage transconductance is derived from 1/gm of these transistors and resistor R1. The input stage drives the folded cascode degeneration resistors of PNP and NPN current mirrors, Q9 to Q12, which convert the differential signals into a single-ended output. There are back-to-back series diodes, D1 to D4, across the plus and minus inputs of each amplifier to limit the differential input voltage to ±1.4V. RIN limits the current through these VCC VIN(OFFSET) = VCC/2 VOUT 150Ω 10 RL = 150Ω GND = 0V VIN(OFFSET) = VCC/2 9 8 7 VOUT (V) The LT6550 and LT6551 3.3V triple and quad high speed amplifiers make it possible to create compact solutions for driving RGB and component video cables. These voltage feedback amplifiers drive either 50Ω or 75Ω double terminated cables and are preconfigured for a fixed gain of two, thus eliminating six or eight external gain setting resistors. The industry trend of using lower supply voltages increases the demands placed on analog signal handling characteristics. For example, a 3.3V video amplifier not only requires high slew rates and fast settling times but must also have wide input and output voltage swing ranges to avoid clipping any portion of the video waveform. Current feedback amplifiers cannot be used because they lack sufficient signal swing at low supplies and they require input signal above ground. The LT6550 and LT6551 are true voltage feedback amplifiers featuring 110MHz (–3dB) bandwidth, 340V/µs slew rate, and fast settling time, making them ideal for low voltage, high resolution, RGB Video Processing. The LT6550 and LT6551 operate from 3V to 12.6V and are fully specified on single 3.3V and 5V supplies, the LT6550 is also fully specified on ±5V supplies. Both parts are available in compact 10-pin MSOP packages and performance is guaranteed over the industrial temperature range. 6 5 4 3 2 1 0 0 1 2 3 4 5 6 VCC (V) 7 8 9 10 Figure 2. Output swing high vs supply voltage 13 DESIGN FEATURES V+ RF 450Ω I1 I2 R2 I3 R3 Q13 Q2 V+ DESD1 Q7 Q5 Q9 Q10 CM V+ R1 RIN 225Ω IN Q3 DESD2 GND + Q1 D1 D3 D2 D4 Q4 Q8 Q6 DESD3 – COMPLEMENTARY DRIVE GENERATOR OUT DESD4 Q11 GND Q12 Q14 RG 450Ω I4 R4 R5 GND Figure 3. Simplified schematic diodes if the input differential voltage exceeds ±1.4V. The complementary drive generator supplies current to the output transistors that swing from rail-to-rail. (lowest output for all three colors) to white (highest) voltage range for each LT6551 450Ω RIN 3.3V 75Ω RGB Video Applications RGB (Red, Green, and Blue) video format requires three signals that represent the amplitudes of the respective colors plus timing signals (sync) that are sometimes combined with the green component. With video amplifiers driving double terminated 50Ω or 75Ω cables, the video output taken from the far end of the cable is 6dB lower then the output of the amplifier. For this reason these video amplifiers are configured for a closed loop gain of +2. The black 450Ω – OA + 450Ω GIN 75Ω 450Ω BIN 75Ω SYNCIN 75Ω GOUT 75Ω 450Ω – OA + 450Ω ROUT 75Ω 450Ω – OA + BOUT 75Ω 450Ω – OA + GND VOUT 75Ω 75Ω 75Ω of the respective RGB channels is approximately 700mV, sync pulses are typically 300mV lower then the black level resulting in a total voltage range of 1.0V. This means that for DC- 0V 75Ω SYNCOUT Figure 4a. 3.3.V single supply RGB plus SYNC cable driver 0V VIN 75Ω VS = 3.3V VIN = 0.5V TO 1.25V f = 10MHz Figure 4b. Output step response Table 1. Typical performance specifications (TA = 25°C) Parameter Conditions Typical [email protected]/0V Typical Values@ 5V/0V –3dB Bandwidth RL = 150Ω 90MHz 110MHz 0.25dB Gain Flatness RL = 150Ω 30MHz 30MHz Output Voltage Swing High RL = 150Ω 2.5V Minimum 3.5V Minimum Output Voltage Swing Low ISINK= 10mA 200mV Maximum 200mV Maximum Slew Rate RL = 150Ω 250V/µs 340V/µs Settling Time to 3% VOUT = 1.5V step, RL = 150Ω 20ns 20ns Channel Separation Between all Channels at 10MHz –60dB –60dB Differential Gain RL = 150 0.09% 0.05% Differential Phase RL = 150 0.09° 0.05° 8.5mA 9.5mA Supply Current per Channel 14 Linear Technology Magazine • March 2003 DESIGN FEATURES 3.9k 470µF + LUMINANCE 75Ω LT6551 450Ω 1 1k 450Ω 3.9k + 75Ω 450Ω 3 1k 450Ω LUMINANCE OUT1 8 75Ω 450Ω S-VIDEO CONNECTOR CHROMA OUT1 7 75Ω Buffered RGB to ColorDifference Matrix OUT1 450Ω – OA + 5 LUMINANCE OUT2 450Ω – OA + 4 VCC = 5V 9 75Ω – OA + VCC = 5V VIDEO CHROMA 10 – OA + 2 470µF 450Ω Figure 5 shows an AC-coupled luma and chroma channel video cable driver that provides dual Y and C output ports. Operating from a single 5V supply, the LT6551 provides a guaranteed output swing of 3.3V, with the bias point established by the input resistor network shown. The chroma signal is a color subcarrier signal with no picture content offset, so it is readily accommodated with the same biasing scheme. S-VIDEO CHROMA CONNECTOR OUT2 6 75Ω OUT2 Figure 5. S-video splitter coupled applications, the output of the amplifier needs to swing at least 2.0V while driving a 150Ω load and the input range should be greater then 1.0V. The LT6550 and LT6551 were designed to meet these requirements. Figure 4 shows a 3.3V powered RGB cable-driver application that could handle additional sync information on any channel. Using DC-coupled inputs allows precise control of the signal swing within the guaranteed 2.3V available output range. For applications that require a separate sync output, the fourth channel of the LT6551 can be utilized as shown in Figure 4. AC Coupled S-Video Splitter The S-video format separates the luma signal (Y) from the chroma subcarrier signal (C), and is usually AC-coupled. AC-coupled applications require the design to accommodate picture-content offset in the signal by allowing output swing of 3.2V for composite or 2.5V for sync-stripped video. 3.3V LT6550 3.3V LT6550 10 450Ω 450Ω 450Ω 9 1070Ω – 1 R + 75Ω 450Ω 8 549Ω 75Ω 450Ω 75Ω 7 2940Ω 4 5 + 5 –3.3V PR = 0.713(R – Y) f3dB ≈ 44MHz Figure 6. Buffered RGB to color-difference matrix Linear Technology Magazine • March 2003 75Ω 7 133Ω 174Ω 4 –3.3V Y = 0.299R + 0.587G + 0.114B PB = 0.565(B – Y) 8 Y 450Ω – 3 PR 261Ω + 450Ω + 105Ω 450Ω – 2 450Ω – 3 B 9 + 450Ω + 450Ω – 1 450Ω – 2 G 10 PB High performance consumer products require generation of YPbPr luminance and chrominance component signals, often from standard RGB source content. The YPbPr format has a luma signal and two weighted color difference signals at baseband. Even with their fixed internal gain resistors, two LT6550s connected as shown in Figure 6 easily implement the required conversion matrix equations (also shown in Figure 6). To perform the conversion, the input to the Y channel of the second LT6550 is a simple weighted sum of the 2× amplified RGB signals from the first LT6550, creating a signal of 2Y. The Y channel output in the second LT6550 is fed back to its feedback resistor common pin. This configuration implicitly performs the required Y subtraction function for both of the color difference channels and sets the Y channel output stage to the required unity gain. The necessary scaling of the color-difference signals is performed passively by their respective output termination resistor networks. Since this circuit naturally produces bi-polar color difference signals (±0.35V at the cable load), the simplest implementation is to power the circuit with ±3.3V split supplies. With an available output swing of about 5.6V for this supply configuration, the circuit handles video with composite syncs and various DC offsets without difficulty. Since the Y channel normally needs to incorporate sync, either all of the RGB signals can have sync included or a 1.8mA gated current-sink can be continued on page 18 15 DESIGN FEATURES 4.5V TO 44V 113k 1% 11.8k 1% 3V 4 10M 1% RS – 5 CMPD6263 1 LT1716 3 10.7k 1% LOAD than 0.4mA to the “on” state current of the relay coil. 0.1µF LT1634-1.25 + 2 10M 1% OVERCURRENT IMAX = 0.1 RS Figure 4. Overcurrent indicator overvoltage at the logic load and to properly set the hysteresis. Voltage-Sensing Relay Trigger Figure 5 shows a circuit that creates a precision voltage-level actuated coil-drive trigger for a miniature relay (or large relay with an additional transistor). With an output capability of sinking more than 10mA, an LT1716 can directly drive low-coilcurrent relays and provide simple resistor programmable make or break thresholds. This basic circuit offers a convenient solution for providing alarm annunciation or load-protection + D1 49.9k 4 + 5 LT1716 3 – 2 switching related to DC bus voltage monitoring. The threshold reference is established by the 1.25V drop of the LT1634-1.25, which is biased by the LT1716 supply current. The resistor divider at the non-inverting input sets the trip-point as a multiplier of the reference. The 10MΩ positive-feedback resistor sets the LT1716 input hysteresis at about 0.5% of the trip voltage for clean state changes and noise rejection. The relay should have guaranteed pull-in capability somewhat below the desired trip-on voltage to allow for the VOL drop and thus ensure that the comparator has full control of the contact state. The Schottky diode at the output provides fast clamping of the relay turn-off transient. The entire circuit uses less than 0.1mA in the “off” state and adds less D2 1 5V TO 44V RELAY** 10M LAMP ON/OFF 100k – * R1 = 39.7k(VRELAYON – 1.25V) VRELAYON ≈ VRELAYON – (VRELAYON2/300) ** COTO 2211-12 (401) 943-2686 D1: LINEAR TECHNOLOGY LT1634-1.25 (408) 432-1900 D2: CENTRAL SEMICONDUCTOR CMPD6263 (631) 435-1110 Figure 5. Voltage-sensing relay trigger 4 RS 0.15Ω 1W 0.25A TO 2.5A 0.1µF CMPD6001 5.1k R1 348k* 3V 1M – 5 LT1716 3 RS ≥ + 1 LAMP GOOD 2 0.04 IL Figure 6. Lamp integrity monitor LT6550 and LT6551, continued from page 15 Conclusion introduced to the Y signal summing node to add sync. The LT6550 and LT6551 triple and quad voltage feedback amplifiers are well suited for use in a variety of video applications. Their high slew Lamp Integrity Monitor Even with their limited lifetimes, incandescent lamps are still widely used as low-cost hi-level illumination in many products like automobiles and aircraft. With the trend in products to provide more self-diagnostic information, it is optimal to have a circuit that provides a full-time status of the lamp load, whether it is activated or not. Figure 6 shows a circuit using the LT1716 for monitoring a typical automotive lamp-load. The LT1716 is shown powered from a logic-voltage supply of 3V, while it monitors a lamp powered from a battery system supply like 14V or 28V in vehicles. When the lamp is on, a voltage drop exists across the sense resistor that exceeds the bias-current induced drop on the 5.1kΩ resistor, thereby detecting that a suitable load current is flowing. When the lamp is off, the filament will pull-down through the 100kΩ and the low-leakage diode/ 1MΩ will cause a slight voltage rise across the 5.1kΩ, signifying to the comparator that the lamp load is intact. Conclusion The LT1716 provides the designer with the most flexible power supply and output interfacing options possible in that it has the unique ability to precisely monitor signals that may be completely unrelated to the logic voltage involved. This feature, plus its micropower performance and its easy-to-use SOT-23 footprint, make the LT1716 an ideal choice for integrated system monitoring applications. rates, fast settling time, and wide input and output ranges make them an excellent choice for 3.3V RGB applications. For more information on parts featured in this issue, see http://www.linear.com/go/ltmag 18 Linear Technology Magazine • March 2003