Application Hints

All pins of any unused comparators should be tied to the

negative supply.

The bias network of the LP339 establishes a drain current

which is independent of the magnitude of the power supply

voltage over the range of from 2 V

It is usually unnecessary to use a bypass capacitor across

the power supply line.

The differential input voltage may be larger than V+ without

damaging the device. Protection should be provided to pre-

vent the input voltages from going negative more than −0.3

V

in the application section.

The output section of the LP339 has two distinct modes of

operation-a Darlington mode and a grounded emitter mode.

This unique drive circuit permits the LP339 to sink 30 mA at

V

(grounded emitter mode).

Figure 1 is a simplified schematic

diagram of the LP339 output section.

Notice that the output section is configured in a Darlington

connection (ignoring Q3). Therefore, if the output voltage is

held high enough (V

output current is limited only by the product of the betas of

Q1, Q2 and I1 (and the 60

Ω R

capable of driving LED’s, relays, etc. in this mode while

maintaining an ultra-low power supply current of typically

60 µA.

If transistor Q3 were omitted, and the output voltage allowed

to drop below about 0.8 V

and the output current would drop to zero. The circuit would,

therefore, be unable to “pull” low current loads down to

ground (or the negative supply, if used). Transistor Q3 has

been included to bypass transistor Q1 under these condi-

tions and apply the current I1 directly to the base of Q2. The

output sink current is now approximately I1 times the beta of

Q2 (700 µA at V

its a bi-modal characteristic with a smooth transition be-

tween modes. (See Output Sink Current graphs in Typical

Performance Characteristics section.)

It is also important to note that in both cases the output is an

uncommitted collector. Therefore, many collectors can be

tied together to provide an output OR’ing function. An output

pull-up resistor can be connected to any available power

supply voltage within the permitted power supply voltage

range and there is no restriction on this voltage due to the

magnitude of the voltage which is applied to the V+ terminal

of the LP339 package.

DS005226-11

FIGURE 1.

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