LP2989

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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

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

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 m

Ω. 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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