LP2985-N_15 Datasheet, PDF (16/41 Pages) Texas Instruments – LP2985-N Micropower 150-mA Low-Noise Ultra-Low-Dropout Regulator in SOT-23 and DSBGA Packages Designed for Use with Very Low ESR Output Capacitors

LP2985-N
SNVS018X – MARCH 2000 – REVISED MAY 2015
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9.2.2 Detailed Design Procedure
At 150-mA loading, the dropout of the LP2985-N has 575-mV maximum dropout over temperature, thus an 1000-
mV headroom is sufficient for operation over both input and output voltage accuracy. The efficiency of the
LP2985-N in this configuration is V_OUT / V_IN = 76.7%. To achieve the smallest form factor, the SOT-23 package is
selected.
Input and output capacitors are selected in accordance with the Capacitor Characteristics section. Ceramic
capacitances of 1 μF for the input and one 2.2-μF capacitor for the output are selected. With an efficiency of
76.7% and a 150 mA maximum load, the internal power dissipation is 150-mW, which corresponds to a 26°C
junction temperature rise for the SOT-23 package. With an 85°C maximum ambient temperature, the junction
temperature is at 111°C. To minimize noise, a bypass capacitance (C_BYPASS) of 0.01 μF is selected.
9.2.2.1 External Capacitors
Like any low-dropout regulator, the LP2985-N requires external capacitors for regulator stability. These
capacitors must be correctly selected for good performance.
9.2.2.1.1 Input Capacitor
An input capacitor whose capacitance is ≥ 1 µF is required between the LP2985-N input and ground (the amount
of capacitance may be increased without limit).
This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean
analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.
NOTE
Tantalum capacitors can suffer catastrophic failure due to surge current when connected
to a low-impedance source of power (like a battery or very large capacitor). If a Tantalum
capacitor is used at the input, it must be ensured by the manufacturer to have a surge
current rating sufficient for the application.
There are no requirements for ESR on the input capacitor, but tolerance and temperature coefficient must be
considered when selecting the capacitor to ensure the capacitance will be ≥ 1 µF over the entire operating
temperature range.
9.2.2.1.2 Output Capacitor
The LP2985-N is designed specifically to work with ceramic output capacitors, utilizing circuitry which allows the
regulator to be stable across the entire range of output current with an output capacitor whose ESR is as low as
5 mΩ. It may also be possible to use Tantalum or film capacitors at the output, but these are not as attractive for
reasons of size and cost (see Capacitor Characteristics).
The output capacitor must meet the requirement for minimum amount of capacitance and also have an ESR
value which is within the stable range. Curves are provided which show the stable ESR range as a function of
load current (see Figure 22).
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