LP38691-ADJ_16 Datasheet, PDF (15/27 Pages) Texas Instruments – 500-mA Low-Dropout CMOS Linear Regulators Stable

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LP38691-ADJ, LP38693-ADJ, LP38691-ADJ-Q1, LP38693-ADJ-Q1
SNVS324K – JANUARY 2005 – REVISED JANUARY 2016
8.2.2.5.2 Tantalum Capacitors
Solid tantalum capacitors have good temperature stability: a high-quality tantalum capacitor typically shows a
capacitance value that varies less than 10-15% across the full temperature range of -40°C to 125°C. ESR will
vary only about 2× going from the high to low temperature limits.
The increasing ESR at lower temperatures can cause oscillations when marginal quality capacitors are used (if
the ESR of the capacitor is near the upper limit of the stability range at room temperature).
8.2.2.6 RFI/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 device 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
device.
If a load is connected to the device 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 device output. Because 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 device 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 device 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, because 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.
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 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√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. 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 (GND pin current).
8.2.2.8 Power Dissipation
Knowing the device power dissipation and proper sizing of the thermal plane connected to the tab or pad is
critical to ensuring reliable operation. Device power dissipation depends on input voltage, output voltage, and
load conditions and can be calculated with Equation 2.
P_D(MAX) = (V_IN(MAX) – V_OUT) × I_OUT(MAX)
(2)
Power dissipation can be minimized, and greater efficiency can be achieved, by using the lowest available
voltage drop option that would still be greater than the dropout voltage (V_DO). However, keep in mind that higher
voltage drops result in better dynamic (that is, PSRR and transient) performance.
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