Silicon Laboratories SI2494/39, SI2493/57/34/15/04 manual Layout Guidelines

AN93

4.4. Layout Guidelines

The key to a good layout is proper placement of the components. It is best to copy the placement shown in Figure 20. Alternatively, follow the following steps, referring to the schematics and Figure 21. It is strongly recommended to complete the checklist in Table 34 on page 51 while reviewing the final layout.

1.All traces, open pad sites, and vias connected to the following components are considered to be in the DAA section and must be physically separated from non-DAA circuits by 5 mm to achieve the best possible surge performance: R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R15, R16, U2, Z1, D1, FB1, FB2, RJ11, Q1, Q2, Q3, Q4, Q5, C3, C4, C5, C6, C7, C8, C9, C10, RV1, C1 pin 2 only, C2 pin 2 only, C8 pin 2 only, and C9 pin 2 only.

2.The isolation capacitors, C1, C2, C8 and C9, are the only components permitted to straddle between the DAA section and non-DAA section components and traces. This means that for each of these capacitors, one of the terminals is on the DAA side, and the other is not. Maximize the spacing between the terminals (between pin 1 and pin 2) of each of these capacitors.

3.Place and group the following components: U1, U2, R12*, R13*, C1, C2.

*Note: Do not use ferrite beads in place of R12 and R13.

a.U1 and U2 are placed so that the right side of U1 faces the left side of U2. b.C1 and C2 are placed directly between U1 and U2.

c.Keep R12 and R13 close to U1.

d.Place U1, U2, C1, and C2 so that the minimum creepage distance for the target application is met.

e.Place C1 and C2 so that traces connected to U2 pin 5 (C1B) and U2 pin 6 (C2B) are physically separated from traces connected to:

i.C8, R15, FB1 ii.C9, R16, FB2 iii.U2 pin 8, R7 iv.U2 pin 9, R9

4.Place and group the following components around U2: C4, R9, C7, R2, C5, C6, R7, R8. These components should form the critical “inner circle” of components around U2.

a.Place C4 close to U2 pin 3. This is best achieved by placing C4 northwest of U2.

b.Place R9 close to U2 pin 4. This is best achieved by placing R9 horizontally, directly to the north of U2. c.Place C7 close to U2 pin 15. This is best achieved by placing C7 next to R9.

d.Place R2 next to U2 pin 16. This is best achieved by placing R2 northeast of U2. e.Place C6 close to U2 pin 10. This is best achieved by placing C6 southeast of U2.

f.Place R7 and R8 close to U2. This is best achieved by placing these components to the south of U2. g.Place C5 close to U2 pin 7. This is best achieved by placing C5 southwest of U2.

5.Place Q5 next to R2 so that the base of Q5 can be connected to R2 directly.

6.Place Q4 so that the base of Q4 can be routed to pin 13 of U2 easily and the emitter of Q4 can be routed to U2 pin 12 easily. Route these two traces next to each other so that the loop area formed by these two traces is minimized.

7.Place and group the following components around the RJ11 jack: FB1, FB2, RV1, R15, R16, C8, and C9. a.Use 20-mil-wide traces on this grouping to minimize impedance.

b.Place C8 and C9 close to the RJ11 jack, recognizing that a GND trace will be routed between C8 and C9 back to the Si24xx GND pin through a 20-mil-wide trace. The GND trace from C8 and C9 must be isolated from the rest of the Si3018/10 traces.

c.The trace from C8 to GND and the trace from C9 to GND must be short and of equal lengths.

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