LP38690-ADJ_16 Datasheet, PDF (15/27 Pages) Texas Instruments – 1-A Low Dropout CMOS Linear Regulator

www.ti.com
LP38690-ADJ, LP38692-ADJ
SNVS323I – DECEMBER 2004 – REVISED FEBRUARY 2016
8.2.2.2.2 Output Capacitor
An output capacitor is required for loop stability. It must be located less than 1 centimeter from the device and
connected directly to the OUT and GND pins using traces which have no other currents flowing through them.
The minimum amount of output capacitance that can be used for stable operation is 1 µF. Ceramic capacitors
are recommended (the LP3869x-ADJ was designed for use with ultralow-ESR capacitors). The LP3869x-ADJ is
stable with any output capacitor ESR between 0 Ω and 100 Ω.
8.2.2.2.3 Selecting A Capacitor
It is important to note that capacitance tolerance and variation with temperature must be taken into consideration
when selecting a capacitor so that the minimum required amount of capacitance is provided over the full
operating temperature range.
8.2.2.2.3.1 Ceramic
For values of capacitance in the 10-µF to 100-µF range, ceramics are usually larger and more costly than
tantalum capacitors but give superior AC performance for bypassing high frequency noise because of very low
ESR (typically less than 10 mΩ). However, some dielectric types do not have good capacitance characteristics
as a function of voltage and temperature.
Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or
Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V
also exhibit a severe temperature effect, losing more than 50% of nominal capacitance at high and low limits of
the temperature range.
X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically
maintain a capacitance range within ±20% of nominal over full operating ratings of temperature and voltage.
They are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance.
8.2.2.2.3.2 Tantalum
Solid tantalum capacitors have good temperature stability: a high-quality tantalum typically show a capacitance
value that varies less than 10-15% across the full temperature range of –40°C to +125°C. ESR varies 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).
8.2.2.3 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 IN 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. 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 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, 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.
Copyright © 2004–2016, Texas Instruments Incorporated
Submit Documentation Feedback
15
Product Folder Links: LP38690-ADJ LP38692-ADJ