april 2007 rev 1 1/10 AN2526 application note stmav340 evaluation board & pcb layout recommendation introduction stmav340 has 4 channels and a 2 to 1 video switch that is well suited for switching analog video signals on a tv or monitor set. below is a diagram of stmav340 typical application configuration. as illustrated on the picture below, the 3 channels of the switch can be used to switch two complete sets of rgb and ypbpr to a single rgb/ypbpr output. one additional channel can be used to switch 2 cbvs inputs to a single output that ca n be directly connected to a single cvbs receiver input. without stmav340 integrated in the system, the video receiver needs to have more input pins to accommodate the many video inputs at the back panel. with the introduction of stmav340, less io pins are required at the video receiver. together with the optimized pin arrangement, the result is a straightforward system design and overall cost reduction. stmav340 in a tv application stmav340 video receiver r y g pb b pr cvbs1/2 h/v sync id/vesa r/y g/pb b/p r cvbs www.st.com
contents AN2526 2/10 contents 1 evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 pcb layout recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
AN2526 evaluation board 3/10 1 evaluation board the evaluation board was designed with trouble-free customer evaluation in mind. instead of having two sets of rgb (vga connector) and ypbpr (rca connector) as inputs and branching the output to the vga and rca connectors, two vga rgb inputs are connected and the outputs go directly to single vga connector to a display unit. for the evaluation, the customer can connect the two sets of inputs to two vga port of pc, while the output can be directly connected to the vga port of the display unit. power supply the board can be powered either directly from a dc power supply or from a dc adaptor. if it is powered from dc power supply, 5vdc is connected to the power socket labeled v cc 5v and the ground is connected to the power socket labeled gnd. the board can also be powered from a dc adaptor having output voltage of 7 v to 15 v. the dc adaptor voltage is regulated to 5 v by the on board regulator. the dc adaptor can be center positive or negative and it should be connected to the dc jack. to prevent the digital lines switching from co upling to the analog lines, the power distribution is separated from digital and analog sections. although stmav340 has also two digital lines, which are oe and sel, it should still be powered from the anal og supply because its main function is to switch analog signals and those two digital lines are mostly static during operation. hence the analog lines are not disturbed by the digital part and, as such, there is no need to have a separate analog and digital supply for stmav340. the analog and digital sections on the board are isolated by ferrite beads. note: external power can be applied to either v cc 5v/gnd or dc jack, but not to both. video input & output ports there are four video input ports: vga1, vga2, cvbs1, and cvbs2. stmav340 performs the switching of these video inputs. depending on the sel control line, either vga1 or vga2 input is directed to vga_out and either cvbs1 or cvbs2 is directed to cvbs_out. since stmav340 does not perform the switching of vga synchronization signals, two stg719 are incorporated to switch both horizontal and vertical synchronization. vga port also has some id signals. to preven t clashing between id signals from vga input 1 and 2, the id lines are isolated by jumpers. if vga1 is active and to be directed to vga_out, jp1 to jp4 should be closed whereas jp5 to jp8 should be opened. if vga2 is active and to be directed to vga_out, jp5 to jp8 should be closed whereas jp1 to jp4 should be opened. table 1. stmav340 demo board power supply designator notes v cc 5v connect to 5 v of external power supply. gnd connect to ground of external power supply. dc jack connect to dc adaptor output if t he board is to be powered from dc adaptor. input voltage: 7 to 15 v
evaluation board AN2526 4/10 video inputs conditioning the blanking level might vary between different video transmitters. this stmav340 evaluation board can handle video signals with blanking level of 0 v to 2.5 v. if video signals blanking level at outside the above range, some input conditioning should be done to translate the blanking level to the range specified. digital control signals table 2. video ports on the stmav340 demo board designator notes vga1 ? connect to vga source from pc. ? when sel ?0? this port is active ? jp1 to jp4 closed. ? jp5 to jp8 open. vga2 ? connect to vga source from pc ? when sel ?1? this port is active ? jp1 to jp4 open. ? jp5 to jp8 closed. cvbs1 ? connect to cvbs source ? when sel ?0? this port is active cvbs2 ? connect to cvbs source ? when sel ?1? this port is active vga_out active vga input is directed to this port. connect to a display unit. cvbs_out active cvbs input is directed to this port. connect to a display unit. table 3. control signals on the stmav340 demo board signal function notes oe enable or disable the output. jp9 close = low (enable) jp9 open = high (disable) sel select which switch is activated. jp10 close = low (vga1, cvba1) jp10 open = high (vga2, cvbs2)
AN2526 evaluation board 5/10 figure 1. stmav340 evaluation board schematic table 4. bill of materials (bom) no component designator package manufacturer 1 stmav340 u1 tssop16 stmicroelectronics 2 stg719str u2, u3 sot23-6l stmicroelectronics 3 vga female con vga1, vga2, vga_out 4 rca con cvbs1, cvbs2, cvbs_out 5 ld1117v50 u5 to-220 stmicroelectronics 6 df04m rectifier u4 dfm general semi 7 dc jack dc jack 8 power socket v cc 5v, gnd 9 ferrite bead l1, l2 1206 10 100 nf cap c2,c3, c5 0603 11 10 f electrolytic cap c1, c4 cap7.62 mm
pcb layout recommendation AN2526 6/10 2 pcb layout recommendation for the purpose of maintaining 75 ? track for all the analog lines, a 4-layer structure is recommended. all the analog tracks are routed on the top layer of the board. the middle two layers are used for both analog and digital ground. the middle two layers are identical and separated into two sections, analog and digital grounds. they are separated by a slot area and are only connected at the top layer through ferrite bead. this ensures that the digital switching noise does not get coupled to the analog portion. all the analog tracks are routed on top of the analog ground and all the digital tracks are routed on top of digital ground. there is one signal, the sel line, which needs to cross from analog to the digital portion. the crossing is done at the bottom layer exactly below where the ferrite bead is used to bridge the analog and digital ground. since the sel digital line is mostly static during operation, it does not cause any disruption to the analog portion. stmav340 supply is connected to v cc 5v analog and gnd analog. to further guarantee the stability of stmav340 supply, a 100 nf capacitor is used to decouple the v cc pin. the stability of the analog supply lines is of sign ificant importance since the bias level of stmav340 is referenced to it. any instability on the analog supply affects the switch characteristics. each set of video signals, rgb1, rgb2, and rgb_out are routed with the same track length. this guarantees minimum channel-to-channel delay. to guarantee the impedance match on the analog lines, no via is used for routing these signals. 12 jumper jp1, jp2, jp3, jp4, jp5, jp6, jp7, jp8, jp9, jp10. hdr 2x1 13 10k resistor r1, r2 0603 table 4. bill of materials (bom) (continued) no component designator package manufacturer
AN2526 pcb layout recommendation 7/10 figure 2. pcb top layer
pcb layout recommendation AN2526 8/10 figure 3. pcb middle layer
AN2526 revision history 9/10 figure 4. pcb bottom layer 3 revision history table 5. revision history date revision changes 30-apr-2007 1 first issue
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