LP3961_15 Datasheet, PDF (17/31 Pages) Texas Instruments – LP396x 800-mA Fast Ultra-Low-Dropout Linear Regulators

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LP3961, LP3964
SNVS056J – MAY 2000 – REVISED JUNE 2015
8.2.2.4 RFI and EMI Susceptibility
Radio frequency interference (RFI) and electromagnetic interference (EMI) can degrade the performance of any
integrated circuit 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 and EMI noise is present on the input side of the LP396x 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 LP396x.
If a load is connected to the LP396x 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 LP396x output. Because the
bandwidth of the regulator loop is less than 100 kHz, the control circuitry cannot respond to load changes above
that frequency. The means the effective output impedance of the LP396x at frequencies above 100 kHz is
determined only by the output capacitors.
In applications where the load is switching at high speed, the output of the LP396x may need RF isolation from
the load. It is recommended that some inductance be placed between the LP396x 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, because RFI and 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 multilayer 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.
8.2.2.5 Output Adjustment
An adjustable output device has output voltage range of 1.216 V to 5.1 V. To obtain a desired output voltage, the
following equation can be used with R1 always a 10-kΩ resistor.
(3)
For output stability, C_F must be between 68 pF and 100 pF.
8.2.2.6 Turnon Characteristics for Output Voltages Programmed to 2.0 V or Below
As V_IN increases during start-up, the regulator output will track the input until V_IN reaches the minimum operating
voltage (typically about 2.2 V). For output voltages programmed to 2 V or below, the regulator output may
momentarily exceed its programmed output voltage during start up. Outputs programmed to voltages above 2 V
are not affected by this behavior.
8.2.2.7 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 1-Hz bandwidth). This type of noise is usually plotted on a curve as a
function of frequency.
• Total output noise or broadband 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/√Hz or nV/√Hz and
total output noise is measured in µV_(rms).
The primary source of noise in low-dropout regulators is the internal reference. In CMOS regulators, noise has a
low-frequency component and a high-frequency component, which depend strongly on the silicon area and
quiescent current. 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). Using an optimized trade-off of ground pin current and die size, LP396x achieves low noise
performance and low quiescent current operation.
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