LP2989_15 Datasheet, PDF (17/34 Pages) Texas Instruments – LP2989 Micropower and Low-Noise, 500-mA Ultra Low-Dropout Regulator for Use With Ceramic Output Capacitors

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LP2989
SNVS083O – FEBRUARY 2005 – REVISED MARCH 2015
8.2.2 Detailed Design Procedure
At 500-mA loading, the dropout of the LP2989 has 650-mV maximum dropout over temperature, thus an 1500-
mV headroom is sufficient for operation over both input and output voltage accuracy. The efficiency of the
LP2989 in this configuration is V_OUT / V_IN = 76.9%. To achieve the smallest form factor, the WSON package is
selected. Input and output capacitors are selected in accordance with the capacitor recommendations. Ceramic
capacitances of 2.2 μF for the input and one 4.7-μF capacitor for the output are selected. With an efficiency of
76.9% and a 500-mA maximum load, the internal power dissipation is 750 mW, which corresponds to a 26.1°C
junction temperature rise for the WSON package. With an 85°C maximum ambient temperature, the junction
temperature is at 111.1°C. To minimize noise, a bypass capacitance (C_BYPASS) of 0.01 µF is placed from the
BYPASS pin (device pin 1) to device ground (device pin 3).
8.2.2.1 WSON Package Devices
The LP2989 is offered in the 8-lead WSON surface mount package to allow for increased power dissipation
compared to the SOIC and VSSOP packages. For details on thermal performance as well as mounting and
soldering specifications, refer to Application Note AN-1187 Leadless Leadframe Package (LLP) (SNOA401).
For output voltages < 2 V, see LP2989LV (SNVS086) data sheet.
8.2.2.2 External Capacitors
Like any low-dropout regulator, the LP2989 requires external capacitors for regulator stability. These capacitors
must be correctly selected for good performance.
8.2.2.2.1 Input Capacitor
An input capacitor whose value is at least 2.2 µF is required between the LP2989 input and ground (the amount
of capacitance may be increased without limit).
Characterization testing performed on the LP2989 has shown that if the value of actual input capacitance drops
below about 1.5 µF, an unstable operating condition may result. Therefore, the next larger standard size (2.2 µF)
is specified as the minimum required input capacitance. Capacitor tolerance and temperature variation must be
considered when selecting a capacitor (see Capacitor Characteristics section) to assure the minimum
requirement of 1.5 µF is met over all operating conditions.
The input capacitor must be located at a distance of not more than 0.5 inches from the input pin and returned to
a clean analog ground. Any good quality ceramic or tantalum may be used for this capacitor, assuming the
minimum capacitance requirement is met.
8.2.2.2.2 Output Capacitor
The LP2989 requires a ceramic output capacitor whose value is at least 4.7 µF. The actual amount of
capacitance on the output must never drop below about 3.5 µF or unstable operation may result. For this reason,
capacitance tolerance and temperature characteristics must be considered when selecting an output capacitor.
The LP2989 is designed specifically to work with ceramic output capacitors, using 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
4 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 the Capacitor Characteristics section).
The output capacitor must meet the requirement for minimum amount of capacitance and also have an
equivalent series resistance (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 41).
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