LP5990

SNVS438B – APRIL 2007 – REVISED DECEMBER 2007

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Important: To ensure stable operation it is essential that good PCB practices are employed to minimize ground

impedance and keep input inductance low. If these conditions cannot be met, or if long leads are to be used to

connect the battery or other power source to the LP5990, then it is recommended to increase the input capacitor

to at least 2.2µF. Also, tantalum capacitors can suffer catastrophic failures due to surge current when connected

to a low-impedance source of power (like a battery or a very large capacitor). If a tantalum capacitor is used at

the input, it must be guaranteed by the manufacturer to have a surge current rating sufficient for the application.

There are no requirements for the ESR (Equivalent Series Resistance) on the input capacitor, but tolerance and

temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain

0.3

μF over the entire operating temperature range.

OUTPUT CAPACITOR

The LP5990 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor

(dielectric types X5R or X7R) 1.0

μF, and with ESR between 5 mΩ to 500 mΩ, is suitable in the LP5990

application circuit.

Other ceramic capacitors such as Y5V and Z5U are less suitable owing to their inferior temperature

characteristics. (See section in Capacitor Characteristics).

and should be mounted within 1 cm of the device.

attractive for reasons of size and cost (see the section Capacitor Characteristics).

The output capacitor must meet the requirement for the minimum value of capacitance (0.3

μF) and have an ESR

value that is within the range 5 m

Ω to 500 mΩ for stability.

CAPACITOR CHARACTERISTICS

The LP5990 is designed to work with ceramic capacitors on the input and output to take advantage of the

benefits they offer. For capacitance values in the range of 1.0

μF to 4.7 μF, ceramic capacitors are the smallest,

least expensive and have the lowest ESR values, thus making them best for eliminating high frequency noise.

The ESR of a typical 1.0

μF ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which easily meets the ESR

requirement for stability for the LP5990

For both input and output capacitors careful interpretation of the capacitor specification is required to ensure

correct device operation. The capacitor value can change greatly depending on the conditions of operation and

capacitor type.

In particular the output capacitor selection should take account of all the capacitor parameters to ensure that the

specification is met within the application.Capacitance value can vary with DC bias conditions as well as

temperature and frequency of operation. Capacitor values will also show some decrease over time due to aging.

The capacitor parameters are also dependant on particular case size with smaller sizes giving poorer

performance figures in general. As an example Figure 2 shows a typical graph showing a comparison of

capacitor case sizes in a Capacitance versus DC Bias plot. As shown in the graph, as a result of the DC Bias

condition, the capacitance value may drop below the minimum capacitance value given in the recommended

capacitor table (0.3µF in this case). Note that the graph shows the capacitance out of spec for the 0402 case

size capacitor at higher bias voltages. It is therefore recommend that the capacitor manufacturer's specifications

for the nominal value capacitor are consulted for all conditions as some capacitors may not be suited in the

application.

The temperature performance of ceramic capacitors varies by type and manufacturer. Most large value ceramic

capacitors (

≥2.2 µF) are manufactured with Z5U or Y5V temperature characteristics, which results in the

capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C.

A better choice for temperature coefficient in a ceramic capacitor is X7R. This type of capacitor is the most stable

and holds the capacitance within ±15% over the temperature range. Tantalum capacitors are less desirable than

ceramic for use as output capacitors because they are more expensive when comparing equivalent capacitance

and voltage ratings in the 0.47

μF to 4.7 μF range.

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