34708 datasheet(149/159 Pages) FREESCALE

149 / 159 page

Analog Integrated Circuit Device Data

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149

Freescale Semiconductor

MC34708

Typical Applications

Figure 39. Recommended Shielding for Critical Signals.

• These signals can be placed on an outer layer of the board to reduce their capacitance in respect to the ground plane.

• The crystal connected to the XTAL1 and XTAL2 pins must not have a ground plane directly below.

• The following are clock signals: CLK, CLK32K, CLK32KMCU, XTAL1, and XTAL2. These signals must not run parallel to

each other, or in the same routing layer. If it is necessary to run clock signals parallel to each other, or parallel to any other

signal, then follow a MAX PARALLEL rule as follows:

• Up to 1 inch parallel length – 25 mil minimum separation

• Up to 2 inch parallel length – 50 mil minimum separation

• Up to 3 inch parallel length – 100 mil minimum separation

• Up to 4 inch parallel length – 250 mil minimum separation

• Care must be taken with these signals not to contaminate analog signals, as they are high frequency signals.

Another good practice is to trace them perpendicularly on different layers, so there is a minimum area of

proximity between signals.

2. The traces BATTISNSN and BATTISNSP that go to the R1 resistor must run in parallel.

8.3.5

Differential Routing

1. DP and DM traces should be routed as 90 ohm differential signals.

2. DPLUS and DMINUS traces should be routed as 90 ohm differential signals.

8.3.6

Switching Regulator Layout Recommendations

1. Per design, the MC34708 is designed to operate with only 1 input bulk capacitor. However, it is recommended to add a

high frequency filter input capacitor (CIN_hf), to filter out any noise at the regulator input. This capacitor should be in the

range of 100 nF and should be placed right next to or under the IC, closest to the IC pins.

2. Make high-current ripple traces low inductance (short, high W/L ratio).

3. Make high-current traces wide or copper islands.

4. Make high-current traces SYMETRICAL for dual–phase regulators (SW1, SW4).

34708 Datasheet(PDF) 149 Page - Freescale Semiconductor, Inc