LMH0001_10

Replacing the CLC001 Cable Driver with the LMH0001 Replacing the CLC001 Cable Driver with the LMH0001 National Semiconductor Application Note 2059 Gary Melchior May 11, 2010 Introduction The LMH0001 SD-SDI cable driver can replace the CLC001 cable driver in many applications. The LMH0001 and CLC001 are both cable drivers designed to drive 75Ω coaxial cable, primarily for the SMPTE 259M interface. The supported data rates are similar between the two devices. Both cable drivers are powered from a 3.3V supply and rated for industrial temperature range operation (-40°C to +85°C). The LMH0001, the newer generation SDI cable driver, has improved performance and is pin compatible with the LMH0002 HD/SD-SDI cable driver and the LMH0302 3G/HD/ SD-SDI cable driver. This allows a forward migration path from SD to HD to 3G. The LMH0001 provides improved output return loss along with a 46% power savings over the CLC001, with typical power of 125 mW in comparison with 231 mW for the CLC001. Table 1 shows the key differences between the CLC001 and LMH0001. TABLE 1. CLC001 and LMH0001 Key Differences CLC001 Supply Current (ICC) (Typical) Package Data Rates Output Rise/Fall Time (Typical) ESD Rating Input Interface Output Interface RREF Resistor (800 mVP-P Output) Output Return Loss 70 mA 8-pin SOIC DC to 622 Mbps 400 ps LMH0001 38 mA 16-pin LLP DC to 540 Mbps 560 ps ≥±7 kV HBM 0.05V to 3.25V common mode Requires external 75Ω resistor to GND 1.91 kΩ to ground Compliant to SMPTE spec 3. ≥±5 kV HBM 1.6V + VSDI/2 to VCC - VSDI/2 common mode (self biased) Requires external 75Ω resistor to VCC 750Ω to VCC Meets SMPTE spec with margin using recommended network How To Replace the CLC001 with the LMH0001 Replacing the CLC001 with the LMH0001 requires a few simple steps. The device packages and pinouts are different so this change requires a new PCB layout. To replace the CLC001 with the LMH0001, follow these steps: 1. Check the output common mode voltage of the driving device to determine if it is compatible with the LMH0001 input common mode range—if so, it is preferred to DC couple the inputs, and if not, add AC coupling capacitors in series with the inputs, prior to the input termination. When AC coupling, remove any bias applied to the inputs as this is not required for the LMH0001 since the inputs are self-biased. 2. Replace the 75Ω resistors to ground on the output with 75Ω resistors to VCC. These pullup resistors should be placed as close as possible to the LMH0001 output pins. In order to meet the SMPTE return loss specification, add a return loss network in series with the LMH0001 outputs. This network typically consists of a 5.6 nH inductor in parallel with a 75Ω resistor, immediately following the 75Ω pullup. If only one output is used, the return loss network is not necessary on the unused output. 4. For SMPTE applications, change the output coupling capacitor from 1 µF to 4.7 µF. Using this larger AC coupling capacitor value is good practice when dealing with the large DC shifts associated with the SMPTE pathological signals. 5. For 800 mVP-P output, change the RREF resistor from 1.91 kΩ connected to ground to 750Ω connected to VCC. For 1.0 VP-P output, change the RREF resistor from 1.5 kΩ connected to ground to 590Ω connected to VCC. Figure 1 shows the typical application for the CLC001, and Figure 2 shows the typical application for the LMH0001. AN-2059 © 2010 National Semiconductor Corporation 301239 www.national.com AN-2059 30123901 FIGURE 1. CLC001 Typical Application 30123902 FIGURE 2. LMH0001 Typical Application INPUT INTERFACE The input common mode voltage range of the CLC001 is between 0.05V and 3.25V, and the minimum differential input amplitude is 100 mV. The CLC001 inputs require a DC bias voltage to be applied when the inputs are AC coupled, and the VBB pin may be used for this. When driven differentially, the CLC001 inputs are typically terminated with a 100Ω resistor placed across the input pins, or two 50Ω resistors with a center tap capacitor to ground. The LMH0001 input interface is similar to that of the CLC001, but it does not support rail-to-rail inputs like the CLC001. The input common mode voltage range of the LMH0001 is from 1.6V + VSDI/2 to VCC - VSDI/2, with the same minimum differwww.national.com 2 ential input amplitude as the CLC001. The LMH0001 inputs are self-biased, so no DC bias voltage is necessary when the inputs are AC coupled. When driven differentially, the LMH0001 inputs are typically terminated with a 100Ω resistor or two 50Ω resistor like the CLC001. OUTPUT INTERFACE The CLC001 outputs are designed to drive 75Ω AC-coupled coaxial cable. They are current mode and ground referenced, requiring 75Ω resistors to ground to generate the output voltage. The output level is controlled by the RREF resistor. The LMH0001 outputs are also current mode and designed to drive 75Ω AC-coupled coaxial cable. But the LMH0001 AN-2059 outputs are positive supply referenced and require 75Ω pullups to VCC to generate the output voltage. Like the CLC001, the LMH0001 output level is controlled by the RREF resistor. OUTPUT LEVEL CONTROL (RREF RESISTOR) The RREF resistor controls the output driver amplitude. The output amplitude for both the CLC001 and LMH0001 is adjustable from below 800 mVP-P to above 1.0 VP-P. For the CLC001, the RREF resistor is connected to ground, and for the LMH0001, the RREF resistor is connected to VCC. The output amplitude is inversely proportional to the value of RREF and approximately linear. For the CLC001, RREF is 1.91 kΩ for 800 mVP-P output and 1.5 kΩ for 1.0 VP-P output. For the LMH0001, RREF is 750Ω for 800 mVP-P output and 590Ω for 1.0 VP-P output. OUTPUT RETURN LOSS The LMH0001 output structure provides improved return loss over the output structure used for the CLC001. This allows the LMH0001 to exceed the SMPTE return loss specification on the SD001SQ evaluation board with the recommended return loss network consisting of a 5.6 nH inductor in parallel with a 75Ω resistor. Return loss is dependent on board design so this return loss network may need to be optimized depending on the specific PCB design and BNC connector used. LMH0001 Enhancements over the CLC001 The LMH0001 is a solid upgrade and good replacement for the CLC001. It is designed in a newer, more advanced process. The LMH0001 offers much lower power and better output return loss. The LMH0001’s smaller, space-saving package allows for more compact designs. 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