LT3045 datasheet(32/62 Pages) AD

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Data Sheet

LT3074

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Rev 0.

32 of 62

pin are worse than that on the SETRES pin as the leakage current flows through the filter resistor creating additional

voltage drop. If necessary, use high-quality insulation (for example, Teflon or Kel-F). Moreover, cleaning of all

insulating surfaces to remove fluxes and other residues can be required. High humidity environments can require a

surface coating at the SETRES and SETCAP pins to provide moisture barrier.

Minimize board leakage by encircling the SETRES and SETCAP pins with a guard ring that operates at a potential

close to itself, ideally connected to the OUT pins. Guarding both sides of the circuit board is recommended. Bulk

leakage reduction depends on the guard ring width. Leakages of 100nA into or out of the SETRES creates a 0.1% or

more error in the reference voltage. Leakages of this magnitude, coupled with other sources of leakage, can cause

significant errors in the output voltage, especially over a wide operating temperature range. Figure 82 illustrates a

typical guard ring layout.

Table 8. 1% Resistors for Common Output Voltages

VOUT (V)

RSETRES (kΩ)

0.5

4.99

0.8

8.06

1.0

10.0

1.2

12.1

2.5

24.9

3.3

33.2

Figure 82. Guard Ring Layout

Output Sensing

The SENSE pin of the LT3074 provides a Kelvin-sense connection to the output capacitor. The GND side of the SETRES

pin resistor and the SETCAP pin capacitor provide a Kelvin-sense connection to the GND side of the output capacitor

(see Figure 83). The LT3074 corrects for the parasitic package and the PCB trace resistance drops when the SENSE

pin is Kelvin connected to the output capacitor. The LT3074 handles moderate levels of output line impedance, but

excessive impedance between OUT pin and the output capacitor causes an excessive phase shift in the feedback

loop and adversely affects stability. This Kelvin connection also regulates the output voltage at the output capacitor,

which optimizes noise, PSRR, load transient, and regulation performance. All measured at the output capacitor.

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