LMP8480, LMP8481

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SNVS829C – MARCH 1999 – REVISED SEPTEMBER 2015

10 Power Supply Recommendations

10.1 Power Supply Decoupling

In order to decouple the LMP848x from AC noise on the power supply, TI recommends using a 0.1-

μF bypass

pins. In some cases, an additional 10-

μF bypass capacitor may further reduce the supply noise.

Do not forget that these bypass capacitors must be rated for the full supply and / or load source voltage. TI

recommends that the working voltage of the capacitor (WVDC) should be at least two times the maximum

expected circuit voltage.

11 Layout

11.1 Layout Guidelines

The traces leading to and from the sense resistor can be significant error sources. With small value sense

resistors (< 100 m

Ω), any trace resistance shared with the load current can cause significant errors.

The amplifier inputs should be directly connected to the sense resistor pads using “Kelvin” or “4-wire” connection

techniques. The traces should be one continuous piece of copper from the sense resistor pad to the amplifier

input pin pad, and ideally on the same copper layer with minimal vias or connectors. This can be important

around the sense resistor if it is generating any significant heat gradients.

To minimize noise pickup and thermal errors, the input traces should be treated as a differential signal pair and

routed tightly together with a direct path to the input pins. The input traces should be run away from noise

sources, such as digital lines, switching supplies or motor drive lines. Remember that these traces can contain

high voltage, and should have the appropriate trace routing clearances.

Since the sense traces only carry the amplifier bias current (about 7 µA at room temperature), the connecting

input traces can be thinner, signal level traces. Excessive Resistance in the trace should also be avoided.

The paths of the traces should be identical, including connectors and vias, so that these errors will be equal and

cancel.

The sense resistor will heat up as the load increases. As the resistor heats up, the resistance generally goes up,

which will cause a change in the readings The sense resistor should have as much heatsinking as possible to

remove this heat through the use of heatsinks or large copper areas coupled to the resistor pads. A reading

drifting over time after turn-on can usually be traced back to sense resistor heating.

11.2 Layout Example

Figure 32. “Kelvin” or “4–wire” Connection to the Sense Resistor

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