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 μV(RMS) to 160 μV(RMS) for a 100 kHz bandwidth at 5V output.
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, θJC, is 14.2°C/W,
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
θJC
θJA
JEDEC 2-Layer JESD 51-3
None
14.2°C/W
14.2°C/W
14.2°C/W
14.2°C/W
14.2°C/W
185°C/W
68°C/W
60°C/W
51°C/W
48°C/W
1
2
4
6
JEDEC 4-Layer JESD 51-7
16
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