Data Sheet

LT3073

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Rev 0.

31 of 37

of each LT3073 is connected to the common load using a small piece of PC trace as a ballast resistor (≅2mΩ) or an

actual sense resistor beyond the feedback SENSE tap of each regulator. The ballast resistor ensures output current

sharing. Keep this ballast trace area free of solder to maintain a controlled resistance.

Table 8 shows a simple guideline for PCB trace resistance as a function of weight and trace width.

Table 8. PC Board Trace Resistance

WEIGHT (oz)

10mil WIDTH

20mil WIDTH

1

54.3

27.1

2

27.1

13.6

Trace resistance is measured in mΩ/in.

Output Noise

The LT3073 offers many advantages for noise performance. Traditional linear regulators have several sources of

noise. The most critical noise sources for a conventional regulator are its voltage reference, error amplifier, noise

from the resistor divider network used for setting output voltage, and the noise gain created by this resistor divider.

LT3073’s unity-gain follower architecture presents no gain from the REF pin to the output. Therefore, if a capacitor

bypasses the REF pin internal resistor DAC, the output noise is independent of the programmed output voltage. The

resultant output noise is set just by the error amplifier’s noise — typically 3nV/√ from 10kHz to 1MHz and 1.2μVRMS

in a 10Hz to 100kHz bandwidth using a 4.7μF REF pin capacitor. Paralleling multiple LT3073s further reduces noise

by √ for N parallel regulators.

Filtering High Frequency Spikes

For applications where the LT3073 is used to post-regulate a switching converter, its high PSRR effectively

suppresses any “noise” present at the switcher’s switching frequency — typically 100kHz to 4MHz. However, the high

frequency (hundreds of MHz) “spikes” — beyond the LT3073’s bandwidth — associated with the switcher’s power

switch transition times will almost directly pass through the LT3073. While the output capacitor is partly intended to

absorb these spikes, its ESL will limit its ability at these frequencies. A ferrite bead or the inductance associated with

a short (example: 0.5 inch) PCB trace between the switcher’s output and the LT3073’s input can serve as an LC filter

to suppress these very high-frequency spikes.

Fast Start-Up

For ultralow noise applications that require low 1/f noise (i.e., at frequencies below 100Hz), a larger value REF pin

capacitor is required, up to 22μF. While this would normally significantly increase the regulator’s start-up time, the

LT3073 incorporates fast start-up circuitry that increases the REF pin current to about 2mA during start-up. For a

22μF capacitor, this will reduce the start-up time from 100ms to 5ms.

The 2mA current source remains engaged until REF is 98.8% of its final value on the rising edge. It will restart when

REF is below 91% of the output setting on the falling edge unless the regulator is in the current limit, thermal

shutdown or any UVLO situation.

Temperature Monitoring

The TEMP pin outputs a voltage proportional to the average junction temperature. The pin voltage is 250mV for 25°C

and has a slope of 10mV/°C. The TEMP pin is stable with no bypass capacitor or a bypass capacitor with a value

between 100pF and 1nF. A 100pF capacitor is recommended to improve TEMP pin power supply rejection. If not used,

leave TEMP unconnected.