LP2950-N, LP2951-N

SNVS764N – JANUARY 2000 – REVISED MAY 2013

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Stray capacitance to the LP2951-N Feedback terminal can cause instability. This may especially be a problem

when using high value external resistors to set the output voltage. Adding a 100 pF capacitor between the Output

pin and the Feedback pin,and increasing the output capacitor to at least 3.3

μF, will fix this problem.

Reducing Output Noise

In reference applications it may be advantageous to reduce the AC noise present at the output. One method is to

reduce the regulator bandwidth by increasing the size of the output capacitor. This is the only way noise can be

reduced on the 3 lead LP2950-N but is relatively inefficient, as increasing the capacitor from 1

μF to 220 μF only

decreases the noise from 430

Noise can be reduced fourfold by a bypass capacitor across R1, since it reduces the high frequency gain from 4

to unity. Pick

(8)

or about 0.01

μF. When doing this, the output capacitor must be increased to 3.3 μF to maintain stability. These

changes reduce the output noise from 430

μV to 100 μV rms for a 100 kHz bandwidth at 5V output. With the

bypass capacitor added, noise no longer scales with output voltage so that improvements are more dramatic at

higher output voltages.

WSON Mounting

The NGT (No Pullback) 8-Lead WSON package requires specific mounting techniques which are detailed in

Application Note 1187 (literature number SNOA401). Referring to the PCB Design Recommendations section

(literature number SNOA401), it should be noted that the pad style which should be used with the WSON

package is the NSMD (non-solder mask defined) type. Additionally, it is recommended the PCB terminal pads to

be 0.2 mm longer than the package pads to create a solder fillet to improve reliability and inspection.

The thermal dissipation of the WSON package is directly related to the printed circuit board construction and the

amount of additional copper area connected to the DAP.

The DAP (exposed pad) on the bottom of the WSON package is connected to the die substrate with a conductive

die attach adhesive. The DAP has no direct electrical (wire) connection to any of the eight pins. There is a

parasitic PN junction between the die substrate and the device ground. As such, it is strongly recommend that

the DAP be connected directly to the ground at device lead 4 (i.e. GND). Alternately, but not recommended, the

DAP may be left floating (i.e. no electrical connection). The DAP must not be connected to any potential other

than ground.

For the LP2951-N in the NGT 8-Lead WSON package, the junction-to-case thermal rating,

where the case is the bottom of the package at the center of the DAP. The junction-to-ambient thermal

performance for the LP2951-N in the NGT 8-Lead WSON package, using the JEDEC JESD51 standards is

summarized in the following table:

Board Type

Thermal Vias

JEDEC 2-Layer JESD 51-3

None

14.2°C/W

185°C/W

1

14.2°C/W

68°C/W

2

14.2°C/W

60°C/W

JEDEC 4-Layer JESD 51-7

4

14.2°C/W

51°C/W

6

14.2°C/W

48°C/W

16

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