Data Sheet

LT3077

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Rev. 0

32 of 38

Table 7 shows a simple guideline for PCB trace resistance as a function of weight and trace width.

Table 7. PC Board Trace Resistance 1

WEIGHT (oz)

10mil WIDTH

20mil WIDTH

1

54.3

27.1

2

27.1

13.6

1

Trace resistance is measured in mΩ/in.

Output Noise

The LT3077 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.

LT3077’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 3.5nV/√ from 10kHz to 1MHz and 1.2μVRMS

in a 10Hz to 100kHz bandwidth using a 4.7μF REF pin capacitor. Paralleling multiple LT3077s further reduces noise

by √ for N parallel regulators.

Filtering High Frequency Spikes

For applications where the LT3077 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 LT3077’s bandwidth — associated with the switcher’s power

switch transition times almost directly pass through the LT3077. While the output capacitor is partly intended to

absorb these spikes, its ESL limits 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 LT3077’s input can serve as an LC filter to

suppress these very high-frequency spikes.

Fast Start-Up

For ultra-low 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 normally significantly increases the regulator’s start-up time, the LT3077

incorporates fast start-up circuitry that increases the REF pin current to about 2mA during start-up. For a 22μF

capacitor, this reduces 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 restarts 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.

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