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LT3089

3089f

For more information www.linear.com/LT3089

APPLICATIONS INFORMATION

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress. In a

ceramic capacitor, the stress can be induced by vibrations

in the system or thermal transients.

Paralleling Devices

Higher output current is obtained by paralleling multiple

LT3089s together. Tie the individual SET pins together and

tie the individual IN pins together. Connect the outputs in

commonusingsmallpiecesofPCtraceasballastresistors

to promote equal current sharing. PC trace resistance in

milliohms/inch is shown in Table 2. Ballasting requires

only a tiny area on the PCB.

Table 2. 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 ±1.5mV

between the SET pin and the OUT pin, allows the use of

very small ballast resistors.

As shown in Figure 9, each LT3089 has a small 20mΩ

ballast resistor, which at full output current gives better

than 80% equalized sharing of the current. The external

resistance of 20mΩ (10mΩ for the two devices in paral-

lel) only adds about 16mV of output regulation drop at an

output of 1.6A. Even with an output voltage as low as 1V,

thisonlyadds1.6%totheregulation.Ofcourse,paralleling

more than two LT3089s yields even higher output current.

Spreading the devices on the PC board also spreads the

heat. Series input resistors can further spread the heat if

the input-to-output difference is high.

If the increase in load regulation from the ballast resis-

tors is unacceptable, the IMON output can be used to

compensate for these drops (see Using IMON Cancels

Ballast Resistor Drop in the Typical Applications section).

Regulatorparallelingwithouttheuseofballastresistorsis

accomplished by comparing the IMON outputs of regula-

tors (see Load Current Sharing Without Ballasting in the

Typical Applications section).

Quieting the Noise

The LT3089 offers numerous noise performance advan-

tages. Every linear regulator has its sources of noise. In

general, a linear regulator’s critical noise source is the

reference. In addition, consider the error amplifier’s noise

contribution along with the resistor divider’s noise gain.

Many traditional low noise regulators bond out the voltage

reference to an external pin (usually through a large value

resistor) to allow for bypassing and noise reduction. The

LT3089 does not use a traditional voltage reference like

other linear regulators. Instead, it uses a 50µA reference

current. The 50µA current source generates noise current

levels of 18pA/√Hz (5.7nARMS over a 10Hz to 100kHz

bandwidth). The equivalent voltage noise equals the RMS

noise current multiplied by the resistor value.

The SET pin resistor generates spot noise equal to √4kTR

(k = Boltzmann’s constant, 1.38 • 10–23J/°K, and T is abso-

lute temperature) which is RMS summed with the voltage

noise. If the application requires lower noise performance,

bypassthevoltagesettingresistorwithacapacitortoGND.

Notethatthisnoise-reductioncapacitorincreasesstart-up

time as a factor of the RC time constant.

SET

–

LT3089

50µA

20m

VIN

4.8V TO 40V

VOUT

3.3V

1.6A

OUT

10µF

1µF

33k

3089 F09

SET

–

LT3089

50µA

OUT

Figure 9. Parallel Devices

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