MAX. ALLOWABLE CERAMIC

OUTPUT CAPACITANCE (PF)

100

1000

10

100

LP3855-ADJ

www.ti.com

SNVS244F – SEPTEMBER 2003 – REVISED APRIL 2013

OPERATION WITH CERAMIC OUTPUT CAPACITORS

LP385X voltage regulators can operate with ceramic output capacitors if the values of the input and output

capacitors are selected appropriately. The total ceramic output capacitance must be equal to or less than a

specified maximum value in order for the regulator to remain stable over all operating conditions. This maximum

amount of ceramic output capacitance is dependent upon the amount of ceramic input capacitance used as well

as the load current of the application. This relationship is shown in Figure 19, which graphs the maximum stable

value of ceramic output capacitance as a function of ceramic input capacitance for load currents of 1.5A.

Figure 19. Maximum Ceramic Output Capacitance vs Ceramic Input Capacitance

If the maximum load current is 1.5A and a 10µF ceramic input capacitor is used, the regulator will be stable with

ceramic output capacitor values from 10µF up to about 150µF. When calculating the total ceramic output

capacitance present in an application, it is necessary to include any ceramic bypass capacitors connected to the

regulator output.

capacitance depends on the value selected for R1 (see Typical Application Circuit). The capacitor should be

selected such that the zero frequency as given by the equation shown below is approximately 45 kHz:

Fz = 45,000 = 1 / ( 2 x

(2)

A good quality ceramic with X5R or X7R dielectric should be used for this capacitor.

SELECTING A CAPACITOR

It is important to note that capacitance tolerance and variation with temperature must be taken into consideration

when selecting a capacitor so that the minimum required amount of capacitance is provided over the full

operating temperature range. In general, a good Tantalum capacitor will show very little capacitance variation

with temperature, but a ceramic may not be as good (depending on dielectric type). Aluminum electrolytics also

typically have large temperature variation of capacitance value.

Equally important to consider is a capacitor's ESR change with temperature: this is not an issue with ceramics,

as their ESR is extremely low. However, it is very important in Tantalum and aluminum electrolytic capacitors.

Both show increasing ESR at colder temperatures, but the increase in aluminum electrolytic capacitors is so

severe they may not be feasible for some applications (see Capacitor Characteristics Section).

CAPACITOR CHARACTERISTICS

CERAMIC: For values of capacitance in the 10 to 100 µF range, ceramics are usually larger and more costly

than tantalums but give superior AC performance for bypassing high frequency noise because of very low ESR

(typically less than 10 m

Ω). However, some dielectric types do not have good capacitance characteristics as a

function of voltage and temperature.

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