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LM339

36V Low Power Dual/Quad Differential Comparators

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LM393/LM339

36V Low Power Dual/Quad Differential Comparators

PCB Surface Leakage

In applications where low input bias current is critical, Printed Circuit Board (PCB) surface leakage effects need to be

considered. Surface leakage is caused by humidity, dust or other contamination on the board. Under low humidity

conditions, a typical resistance between nearby traces is 1012Ω. A 5V difference would cause 5pA of current to flow,

which is greater than the LM393/LM339’s input bias current at +27°C (±6pA, typical). It is recommended to use multi-

layer PCB layout and route the comparator’s -IN and +IN signal under the PCB surface.

The effective way to reduce surface leakage is to use a guard ring around sensitive pins (or traces). The guard ring is

biased at the same voltage as the sensitive pin. An example of this type of layout is shown in Figure 5 for Inverting

configuration application.

1. For Non-Inverting Configuration:

a) Connect the non-inverting pin (VIN+) to the input with a wire that does not touch the PCB surface.

b) Connect the guard ring to the inverting input pin (VIN–). This biases the guard ring to the same reference as the comparator.

2. For Inverting Configuration:

a) Connect the guard ring to the non-inverting input pin (VIN+). This biases the guard ring to the same reference voltage as the

comparator (e.g., VDD/2 or ground).

b) Connect the inverting pin (VIN–) to the input with a wire that does not touch the PCB surface.

Figure 5. Example Guard Ring Layout for Inverting Comparator

Ground Sensing and Rail to Rail Output

The LM393/339 family implements a rail-to-rail topology that is capable of swinging to within 10mV of either rail. Since

the inputs can go 300mV beyond either rail, the comparator can easily perform ‘true ground’ sensing.

The maximum output current is a function of total supply voltage. As the supply voltage of the comparator increases,

the output current capability also increases. Attention must be paid to keep the junction temperature of the IC below

150°C when the output is in continuous short-circuit condition. The output of the amplifier has reverse-biased ESD

diodes connected to each supply. The output should not be forced more than 0.5V beyond either supply, otherwise

current will flow through these diodes.

ESD

The LM393/339 family has reverse-biased ESD protection diodes on all inputs and output. Input and output pins can

not be biased more than 300mV beyond either supply rail.

Power Supply Layout and Bypass

The LM393/339 family’s power supply pin should have a local bypass capacitor (i.e., 0.01μF to 0.1μF) within 2mm for

good high frequency performance. It can also use a bulk capacitor (i.e., 1μF or larger) within 100mm to provide large,

slow currents. This bulk capacitor can be shared with other analog parts.

Good ground layout improves performance by decreasing the amount of stray capacitance and noise at the

comparator’s inputs and outputs. To decrease stray capacitance, minimize PCB lengths and resistor leads, and place

external components as close to the comparator’ pins as possible.

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