LMP8640, LMP8640-Q1, LMP8640HV

SNOSB28G – AUGUST 2010 – REVISED NOVEMBER 2014

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Feature Description (continued)

8.3.1.1 Resistor Power Rating and Thermal Issues

The power dissipated by the sense resistor can be calculated from:

PD = IMAX

2 * R

S

where

•

PD is the power dissipated by the resistor in Watts

•

IMAX is the maximum load current in Amps

•

RS is the sense resistor value in ohms.

(4)

The resistor must be rated for more than the expected maximum power (PD), with margin for temperature

derating. Be sure to observe any power derating curves provided by the resistor manufacturer.

Running the resistor at higher temperatures will also affect the accuracy. As the resistor heats up, the resistance

generally goes up, which will cause a change in the measurement. The sense resistor should have as much

heat-sinking as possible to remove this heat through the use of heatsinks or large copper areas coupled to the

resistor pads. A reading drifting slightly after turn-on can usually be traced back to sense resistor heating.

8.3.1.2 Using PCB Trace as a Sense Resistor

While it may be tempting to use the resistance of a known area of PCB trace or copper area as a sense resistor,

it is not recommended for precision measurements.

The tempco of copper is typically 3300-4000ppm/°K (0.33% to 0.4% per °C), which can vary with PCB

processes.

A typical surface mount sense resistor temperature coefficient (tempco) is in the 50ppm to 500ppm per °C range

offering more measurement consistency and accuracy over the copper trace. Special low tempco resistors are

available in the 0.1 to 50ppm range, but at a much higher cost.

8.3.2 Sense Line Inputs

The sense lines should be connected to a point on the resistor that is not shared with the main current path, as

shown in Figure 26 above. For lowest drift, the amplifier should be mounted away from any heat generating

devices, which may include the sense resistor. The traces should be one continuous trace 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. Vias

in the sense lines should be formed from continuous plated copper and routing through connectors should be

avoided. It is better to extend the sense lines than to place the amplifier in a hostile environment.

To minimize noise pickup and thermal errors, the input traces should be treated like a high-speed differential

signal pair and routed tightly together with a direct path to the input pins on the same copper layer. They do not

need to be "impedance matched", but should follow the same matching rules about vias, spacing and equal

lengths. The input traces should be run away from noise sources, such as digital lines, switching supplies or

motor drive lines. Remember that these input traces can contain high voltage, and should have the appropriate

trace routing clearances to other traces and layers. Since the sense traces only carry the amplifier bias current

(typically about 12µA per input at room temp), the connecting input traces can be thin traces running close

together. This can help with routing or creating the required spacings.

It should also be noted that, due to the nature of the device topology, the positive input bias current will vary with

VSENSE with an extra current approximately equivalent to VSENSE / 5 kΩ on top of the typical 12 uA bias current.

The negative input bias current is not in the feedback path and will not change over VSENSE. High or miss-

matched source impedances should be avoided as this imbalance will create an additional error over input

voltage.

8.3.3 Effects of Series Resistance on Sense Lines

Because the input stage uses precision 5 K

Ω resistors internally to convert the voltage on the input pin to a

current, any resistance added in series with the input pins will change this resistance, and thus alter the gain.

If a resistance is added in series with an input, the gain of that input will not track that of the other input, causing

a constant gain error.

16

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