LP3988

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SNVS161E – OCTOBER 2001 – REVISED OCTOBER 2015

NOTE

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 ensured by the manufacturer to have a surge

current rating sufficient for the application.

There are no requirements for the equivalent series resistance (ESR) on the input capacitor, but tolerance and

temperature coefficient must be considered when selecting the capacitor to ensure the capacitance is at least 1

μF over the entire operating temperature range.

9.2.2.3 Output Capacitors

The LP3988 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor

(dielectric types Z5U, Y5V or X7R) in 1-

μF to 22-μF range with 5-mΩ to 500-mΩ ESR range is suitable in the

LP3988 application circuit.

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 a stable range (5 m

Ω to 500 mΩ).

9.2.2.4 No-Load Stability

The LP3988 remains stable and in regulation with no external load. This is specially important in CMOS RAM

keep-alive applications.

9.2.2.5 Capacitor Characteristics

The LP3988 is designed to work with ceramic capacitors on the output to take advantage of the benefits they

offer: for capacitance values in the range of 1

μF to 4.7 μF range, ceramic capacitors are the smallest, least

expensive and have the lowest ESR values (which makes them best for eliminating high frequency noise). The

ESR of a typical 1-

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

requirement for stability by the LP3988.

Capacitance of the ceramic capacitor can vary with temperature. Most large-value ceramic capacitors (around

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, which holds the capacitance within ±15%.

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 1-

μF to 4.7-μF range.

Another important consideration is that tantalum capacitors have higher ESR values than equivalent size

ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the

stable range, it would have to be larger in capacitance (which means bigger and more costly ) than a ceramic

capacitor with the same ESR value. The ESR of a typical tantalum increases about 2:1 as the temperature goes

from 25°C down to

−40°C, so some guard band must be allowed.

9.2.2.6 Power Dissipation

Knowing the device power dissipation and proper sizing of the thermal plane connected to the tab or pad is

critical to ensuring reliable operation. Device power dissipation depends on input voltage, output voltage, and

load conditions and can be calculated with Equation 1.

(1)

Power dissipation can be minimized, and greater efficiency can be achieved, by using the lowest available

voltage drops result in better dynamic (that is, PSRR and transient) performance.

On the DSBGA (YKA) package, the primary conduction path for heat is through the four bumps to the PCB.

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