LT3092 datasheet(12/22 Pages) LINER

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LT3092

3092fb

APPLICATIONS INFORMATION

Quieting the Noise

When a reduction in the noise of the current source is

desired, a small capacitor can be placed across RSET (CSET

in Figure 7). Normally, the 10μA reference current source

generates noise current levels of 2.7pA/√Hz (0.7nARMS

over the 10Hz to 100kHz bandwidth). The SET pin resistor

generates a spot noise equal to in =√4kT/R(k = Boltzmann’s

constant, 1.38 • 10–23J/°K, and T is absolute temperature)

which is RMS-summed with the noise generated by the

10μA reference current source. Placing a CSET capacitor

across RSET (as shown in Figure 7) bypasses this noise

current. Note that this noise reduction capacitor increases

start-up time as a factor of the time constant formed by

RSET • CSET. When using a capacitor across the SET pin

resistor, the external pole introduced usually requires

compensation to maintain stability. See the Stability and

Frequency Compensation section for detailed descriptions

on compensating LT3092 circuits.

A curve in the Typical Performance Characteristics section

depicts noise spectral density for the reference current

over a 10Hz to 100kHz bandwidth.

Paralleling Devices

Obtain higher output current by paralleling multiple

LT3092’s together. The simplest application is to run

two current sources side by side and tie their inputs

together and their outputs together, as shown in Figure

8. This allows the sum of the current sources to deliver

more output current than a single device is capable of

delivering.

Another method of paralleling devices requires fewer

components and helps to share power between devices.

Tie the individual SET pins together and tie the individual

IN pins together. Connect the outputs in common using

small pieces of PC trace as ballast resistors to promote

equal current sharing. PC trace resistance in milliohms/

inch is shown in Table 1. Ballasting requires only a tiny

area on the PCB.

Table 1. PC Board Trace Resistance

WEIGHT (oz)

10mil WIDTH

20mil WIDTH

54.3

27.1

13.6

Trace resistance is measured in mΩ/in

The worst-case room temperature offset, only ±2mV be-

tween the SET pin and the OUT pin, allows the use of very

small ballast resistors.

As shown in Figure 9, each LT3092 has a small 40mΩ

ballast resistor, which at full output current gives better

than 80% equalized sharing of the current. The external

resistance of 40mΩ (20mΩ for the two devices in paral-

lel) only adds about 8mV of output voltage compliance at

an output of 0.4A. Of course, paralleling more than two

LT3092’s yields even higher output current. Spreading the

device on the PC board also spreads the heat. Series input

resistors can further spread the heat if the input-to-output

difference is high.

Thermal Considerations

The LT3092’s internal power and thermal limiting circuitry

protects itself under overload conditions. For continuous

normal load conditions, do not exceed the 125°C maximum

junction temperature. Carefully consider all sources of

thermal resistance from junction-to-ambient. This includes

(but is not limited to) junction-to-case, case-to-heat sink

Figure 7. Adding CSET Lowers Current Noise

3092 F07

SET

OUT

–

LT3092

10μA

CCOMP OR

RSET

ROUT

RCOMP

CCOMP

CSET

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