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LTM8020MPV Datasheet(PDF) 9 Page - Linear Technology |
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LTM8020MPV Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page LTM8020 9 8020fb APPLICATIONS INFORMATION PCB Layout Most of the headaches associated with PCB layout have been alleviated or even eliminated by the high level of integration of the LTM8020. The LTM8020 is never-the- less a switching power supply, and care must be taken to minimize EMI and ensure proper operation. Even with the high level of integration, you may fail to achieve specified operation with a haphazard or poor layout. See Figure 3 for a suggested layout. Ensure that the grounding and heatsinking are acceptable. A few rules to keep in mind are: 1. Place the CIN capacitor as close as possible to the VIN and GND connection of the LTM8020. 2. Place the COUT capacitor as close as possible to the VOUT and GND connection of the LTM8020. 3. Place the CIN and COUT capacitors such that their ground current flows directly adjacent or underneath the LTM8020. 4. Connect all of the GND connections to as large a copper pour or plane area as possible on the top layer. Avoid breaking the ground connection between the external components and the LTM8020. 5. The copper pours also serve as the heatsink for the LTM8020. Place several vias in the GND plane to act as heat pipes to other layers of the printed circuit board. Positive to Negative Voltage Regulation The LTM8020 can generate a negative output by tying the VOUTpadstosystemgroundandconnectingGNDasshown in the Typical Applications section. In this configuration, SHDN must be level shifted or referenced to GND, and the available output current may be reduced. Hot-Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LTM8020. However, these capacitors can cause problems if the LTM8020 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor combined with stray inductance in series with the power source forms an under damped tank circuit, and the volt- age at the VIN pin of the LTM8020 can ring to twice the nominal input voltage, possibly exceeding the LTM8020’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LTM8020 into an energized supply, the input network should be designed to prevent this overshoot. Figure 4 shows the waveforms that result when an LTM8020 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2μF ceramic capacitor at the input. The input voltage rings as high as 35V and the input current peaks at 20A. One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 4b an aluminum electrolytic capacitor has been added. This capacitor’s high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 4c. A 1 Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1μF capacitor improves high frequency filtering. This solution is smaller and less expensive than the electrolytic capacitor. For high input voltages its impact on efficiency is minor, reducing efficiency less than one half percent for a 5V output at full load operating from 24V. Figure 3. Layout showing suggested external components, GND plane and thermal vias GND VIAs TO GND PLANE VOUT ADJ COPPER BIAS SHDN VIN COUT RADJ CIN 8020 F03 |
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