LP2985 Datasheet, PDF (14/24 Pages) National Semiconductor (TI) – Micropower 150 mA Low-Noise Ultra Low-Dropout Regulator Designed for Use with Very Low ESR Output Capacitors

LP2985
150-mA LOW-NOISE LOW-DROPOUT REGULATOR
WITH SHUTDOWN
SLVS522F – JULY 2004 – REVISED AUGUST 2005
APPLICATION INFORMATION
www.ti.com
Capacitors
Input Capacitor (C_IN)
A minimum value of 1 µF (over the entire operating temperature range) is required at the input of the LP2985. In
addition, this input capacitor should be located within 1 cm of the input pin and connected to a clean analog
ground. There are no equivalent series resistance (ESR) requirements for this capacitor, and the capacitance
can be increased without limit.
Output Capacitor (C_OUT)
As an advantage over other regulators, the LP2985 permits the use of low-ESR capacitors at the output,
including ceramic capacitors that can have an ESR as low as 5 mΩ. Tantalum and film capacitors also can be
used if size and cost are not issues. The output capacitor also should be located within 1 cm of the output pin
and be returned to a clean analog ground.
As with other PNP LDOs, stability conditions require the output capacitor to have a minimum capacitance and an
ESR that falls within a certain range.
• Minimum C_OUT: 2.2 µF (can be increased without limit to improve transient response stability margin)
• ESR range: see Figure 28
Load Current − mA
Figure 28. 2.2-V/3.3-µF ESR Curves
It is critical that both the minimum capacitance and ESR requirement be met over the entire operating
temperature range. Depending on the type of capacitors used, both these parameters can vary significantly with
temperature (see capacitor characteristics).
Noise Bypass Capacitor (C_BYPASS)
The LP2985 allows for low-noise performance with the use of a bypass capacitor that is connected to the internal
bandgap reference via the BYPASS pin. This high-impedance bandgap circuitry is biased in the microampere
range and, thus, cannot be loaded significantly, otherwise, its output – and, correspondingly, the output of the
regulator – changes. Thus, for best output accuracy, dc leakage current through C_BYPASS should be minimized as
much as possible and never should exceed 100 nA.
A 10-nF capacitor is recommended for C_BYPASS. Ceramic and film capacitors are well suited for this purpose.
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