LP3856-ADJ_15 Datasheet, PDF (10/22 Pages) Texas Instruments – 3A Fast Response Ultra Low Dropout Linear Regulators

LP3856-ADJ
SNVS243E – SEPTEMBER 2003 – REVISED APRIL 2013
www.ti.com
OPERATION WITH CERAMIC OUTPUT CAPACITORS
LP385X voltage regulators can operate with ceramic output capacitors if the values of 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 26, which graphs the maximum stable
value of ceramic output capacitance as a function of ceramic input capacitance for load currents of 1A, 2A, and
3A. For example, if the maximum load current is 1A, a 10µF ceramic input capacitor will allow stable operation
for values of ceramic output capacitance from 10µF up to about 500µF.
100
3A
2A
1A
10
10
100
MAX. ALLOWABLE CERAMIC
OUTPUT CAPACITANCE (PF)
1000
Figure 26. Maximum Ceramic Output Capacitance vs Ceramic Input Capacitance
If the maximum load current is 2A and a 10µF ceramic input capacitor is used, the regulator will be stable with
ceramic output capacitor values from 10µF up to about 50µF. At 3A of load current, the ratio of input to output
capacitance required approaches 1:1, meaning that whatever amount of ceramic output capacitance is used
must also be provided at the input for stable operation. For load currents between 1A, 2A, and 3A, interpolation
may be used to approximate values on the graph. 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.
C_FF (Feed Forward Capacitor)
The capacitor C_FF is required to add phase lead and help improve loop compensation. The correct amount of
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 π x R1 x C_FF )
(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 mΩ). However, some dielectric types do not have good capacitance characteristics as a
function of voltage and temperature.
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