LP38691_15 Datasheet, PDF (17/34 Pages) Texas Instruments – LP3869x/-Q1 500-mA Low-Dropout CMOS Linear Regulators Stable With Ceramic Output Capacitors

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LP38691, LP38693, LP38691-Q1, LP38693-Q1
SNVS321N – JANUARY 2005 – REVISED MARCH 2015
100%
0603, 10V, X5R
80%
60%
40%
0402, 6.3V, X5R
20%
0
1.0 2.0 3.0 4.0 5.0
DC BIAS (V)
Figure 27. Typical Variation In Capacitance vs DC Bias
The value of the ceramic capacitor can vary with temperature. The capacitor type X7R, which operates over a
temperature range of –55°C to 125°C, will only vary the capacitance to within ±15%. The capacitor type X5R has
a similar tolerance over a reduced temperature range of –55°C to 85°C. Many large value ceramic capacitors,
larger than 1 µF, are manufactured with Z5U or Y5V temperature characteristics. Their capacitance can drop by
more than 50% as the temperature varies from 25°C to 85°C. Therefore, X7R and X5R types are recommended
over Z5U and Y5V in applications where the ambient temperature will change significantly above or below 25°C.
Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more costly
when comparing equivalent capacitance and voltage ratings in the 0.47-µF to 4.7-µF range.
Another important consideration is that tantalum capacitors have higher ESR values than equivalent size
ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the
stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic
capacitor with the same ESR value. It should also be noted that the ESR of a typical tantalum will increase about
2:1 as the temperature goes from 25°C down to –40°C, so some guard band must be allowed.
9.2.2.7 RFI/EMI Susceptibility
Radio frequency interference (RFI) and electromagnetic interference (EMI) 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 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 IC 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 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, 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.
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