LP38691, LP38693

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SNVS321K – JANUARY 2005 – REVISED APRIL 2013

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 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 2 (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 LP38691SD and LP38693SD in the NGG0006A 6-Lead WSON package, the junction-to-case thermal

rating,

θJC, is 10.4°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 LP38691SD and LP38693SD in the NGG0006A 6-Lead WSON package,

using the JEDEC JESD51 standards is summarized in the following table:

Board

Thermal

Type

Vias

JEDEC

2-Layer

None

10.4°C/W

237° C/W

JESD 51-3

1

10.4°C/W

74° C/W

JEDEC

2

10.4°C/W

60° C/W

4-Layer

4

10.4°C/W

49° C/W

JESD 51-7

6

10.4°C/W

45° C/W

RFI/EMI SUSCEPTIBILITY

RFI (radio frequency interference) and EMI (electromagnetic interference) can degrade any integrated circuit’s

performance because of the small dimensions of the geometries inside the device. In applications where circuit

sources are present which generate signals with significant high frequency energy content (> 1 MHz), care must

be taken to ensure that this does not affect the IC regulator.

If RFI/EMI noise is present on the input side of the regulator (such as applications where the input source comes

from the output of a switching regulator), good ceramic bypass capacitors must be used at the input pin of the IC.

If a load is connected to the IC output which switches at high speed (such as a clock), the high-frequency current

pulses required by the load must be supplied by the capacitors on the IC output. Since the bandwidth of the

regulator loop is less than 100 kHz, the control circuitry cannot respond to load changes above that frequency.

This means the effective output impedance of the IC at frequencies above 100 kHz is determined only by the

output capacitor(s).

In applications where the load is switching at high speed, the output of the IC may need RF isolation from the

load. It is recommended that some inductance be placed between the output capacitor and the load, and good

RF bypass capacitors be placed directly across the load.

PCB layout is also critical in high noise environments, since RFI/EMI is easily radiated directly into PC traces.

Noisy circuitry should be isolated from "clean" circuits where possible, and grounded through a separate path. At

MHz frequencies, ground planes begin to look inductive and RFI/ EMI can cause ground bounce across the

ground plane. In multi-layer PCB applications, care should be taken in layout so that noisy power and ground

planes do not radiate directly into adjacent layers which carry analog power and ground.

OUTPUT NOISE

Noise is specified in two ways: Spot Noise or Output Noise Density is the RMS sum of all noise sources,

measured at the regulator output, at a specific frequency (measured with a 1Hz bandwidth). This type of noise is

usually plotted on a curve as a function of frequency. Total Output Noise or Broad-Band Noise is the RMS

sum of spot noise over a specified bandwidth, usually several decades of frequencies.

Attention should be paid to the units of measurement. Spot noise is measured in units µV/root-Hz or nV/root-Hz

and total output noise is measured in µV(rms)

The primary source of noise in low-dropout regulators is the internal reference. Noise can be reduced in two

ways: by increasing the transistor area or by increasing the current drawn by the internal reference. Increasing

the area will decrease the chance of fitting the die into a smaller package. Increasing the current drawn by the

internal reference increases the total supply current (ground pin current).

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Product Folder Links: LP38691 LP38693