AN-1681| Application Note

Design Migration from the LM1881 to the LMH1980

National Semiconductor Application Note 1681 Alan Ocampo August 2007

Introduction
The LMH1980 is a SD/HD/PC video sync separator intended to replace the LM1881 sync separator in consumer, security and surveillance, and industrial video applications where high-definition compatibility is needed. This application note discusses the LMH1980's advantages over the LM1881 and considerations for designing the LMH1980 into current LM1881 sockets. In broadcast and professional video systems where timing jitter is a critical system parameter, another sync separator, the LMH1981, offers the best jitter performance while supporting all SD & HD analog video standards.

Component Video (YPBPR/GBR) --SDTV: SMPTE 125M, SMPTE 267M, ITU-R BT.601 (480I, 576I) --EDTV: ITU-R BT.1358 (480P, 576P) --HDTV: SMPTE 296M (720P), SMPTE 274M (1080I/P), SMPTE RP 211 (1080PsF) PC Graphics (RGsB) --VESA Monitor Timing Standards and Guidelines Version 1.0, Revision 0.8 Non-Standard Video --Composite NTSC & PAL (or Component 480I & 576I) without vertical serration and equalization pulses

Advantages of the LMH1980
The LMH1980 offers several advantages over the LM1881, including: Lower operating supply of 3.3V Wide temperature range of -40 C to +85 C Auto-detection of the input video format using a fixedvalue resistor biasing architecture Support for HD tri-level sync separation HSync output and HD Detect Flag output Smaller package

Additional Outputs
The LMH1980 provides a true Horizontal Sync signal (HSync), which was not available in the LM1881. Unlike the Composite Sync signal of the LM1881, the HSync signal can be directly input to a PLL to generate a synchronous linelocked clock without the complication of managing double frequency serration pulses in the vertical interval (e.g.: PLL coast mode). The LMH1980 also features an HD Detect Flag which outputs a logic low signal when a valid HD video input with tri-level sync is detected; otherwise, it outputs logic high. This can be useful in multiple format applications where the external chroma low-pass filter needs to be disabled when an HD video signal is applied; otherwise, the chroma filter can distort the HD tri-level sync pulses, which may lead to increased output propagation delay and jitter. An implementation of a switchcontrolled chroma filter is shown and described in the LMH1980 datasheet.

Improved Operating Range
The LM1881 required a supply voltage from 5V to 12V, whereas the LMH1980 operates with a supply voltage as low as 3.3V. This allows the LMH1980 to be used in applications where a lower supply voltage between 3.3V to 5V is available. Additionally, the improved operating temperature range makes it better suited for applications where reliability is needed at cold temperatures as low as -40 C.

Package & PCB Considerations
The LMH1980 is offered in a space-saving 10-lead MSOP, which reduces the area needed on the PCB. Because of the smaller package and additional outputs of the LMH1980, it is not a drop-in replacement for the LM1881, and the circuit schematic and PCB layout need to be slightly modified. As a comparison, the typical application circuit schematics are shown for the LM1881 and the LMH1980 in Figure 1 and Figure 2, respectively. Although omitted from the LM1881 circuit, the LMH1980 circuit shows the recommended supply bypass capacitors, chroma (RC low-pass) filter components, and output protection resistors. Please refer to the PCB Layout Considerations section in the LMH1980 datasheet and application note AN-1618 for more information about component placement and trace routing.

Video Auto-Detection and Formats
The LMH1980 accepts an analog video input with either bilevel sync or tri-level sync and automatically detects the video format, thus eliminating the need for external RSET adjustment required by the older LM1881 to support different line rates. Only a fixed 10 k 1% external resistor is needed to sustain the internal biasing architecture of the LMH1980, which adapts to the input line rate without physical or electrical intervention. The outputs are accurate after a sufficient start-up time is met, typically within one to two fields of video. The LMH1980 supports sync separation for analog CVBS, Y (luma) from Y/C and YPBPR, and G (Sync on Green) from GBR/RGsB, as specified in the following video standards. Composite Video (CVBS) and S-Video (Y/C) --SMPTE 170M (NTSC), ITU-R BT.470 (PAL)

AN-1681

2007 National Semiconductor Corporation

300299

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