LMP8640, LMP8640-Q1, LMP8640HV

SNOSB28G – AUGUST 2010 – REVISED NOVEMBER 2014

www.ti.com

9.3 Do's and Don'ts

Do properly bypass the power supplies.

Do add series resistance to the output when driving capacitive loads, cables, long traces, muxes and ADC

inputs.

Do not exceed the input common mode range.

10 Power Supply Recommendations

The input circuitry of the device can accurately measure signals on common-mode voltages beyond its power

supply voltage, V+. For example, the voltage applied to the VS power supply terminal can be 5 V, whereas the

load power-supply voltage being monitored (the common-mode voltage) can be as high as +76 V. Power-supply

bypass capacitors are required for stability and should be placed as closely as possible to the supply and ground

terminals of the device. A typical value for this supply bypass capacitor is 0.1

μF close to the V+ pin. The

capacitors should be rated for at least twice the maximum expected applied voltage. Applications with noisy or

high-impedance power supplies may require additional decoupling capacitors to reject power-supply noise.

11 Layout

11.1 Layout Guidelines

•

Use 4-wire (Kelvin) connections to the sense resistor. Connect to the resistor at a point that is not within a

direct high-current path (See Figure 29).

•

Do not "share" part of the sense trace path with the load current.

•

Maintain proper clearance and spacings around the input traces, as they may be at a higher voltage (up to 76

V) than the surrounding traces and planes.

•

Input traces from the sense resistor pads should follow the same path, be spaced tightly together, and ideally

on the same copper layer.

•

Vias used in the input traces should be of continuous plated copper to avoid creating thermocouples.

•

Avoid routing inputs through jumpers and connectors. Even the best connectors introduce thermal errors

("thermocouples"). Each input trace should have the same number of "thermocouples" if they cannot be

avoided.

•

Keep the amplifier away from heat generating devices. The copper-solder-lead junction on the input pins will

form a thermocouple. The copper mass should be equal on both input pins.

•

Avoid temperature gradients across the input pins. Input pins should be equal distance to the heat source.

•

Place the LMP8640 in the most temperature-stable area possible and away from high-velocity air flows. It is

better to carefully extend the input traces than to place the amplifier in a less-than-ideal environment.

•

Give the sense resistor as much copper trace area as possible to dissipate heat as the resistor value will

change slightly with temperature. Also see the resistor manufacturers datasheet or application notes for

further layout guidelines.

•

The power-supply bypass capacitor should be placed as closely as possible to the supply and ground

terminals. The recommended value of this bypass capacitor is 0.1

μF. Additional decoupling capacitance can

be added to compensate for noisy or high-impedance power supplies.

20

Submit Documentation Feedback

Copyright © 2010–2014, Texas Instruments Incorporated

Product Folder Links: LMP8640 LMP8640-Q1 LMP8640HV