LP3853ET-33 Datasheet, PDF (11/25 Pages) Texas Instruments – LP3853/LP3856 3A Fast Response Ultra Low Dropout Linear Regulators

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LP3853, LP3856
SNVS173G – FEBRUARY 2003 – REVISED APRIL 2013
100
3A
2A
1A
10
10
100
1000
MAX. ALLOWABLE CERAMIC
OUTPUT CAPACITANCE (PF)
Figure 29. 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.
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.
Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or
Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V
also exhibit a severe temperature effect, losing more than 50% of nominal capacitance at high and low limits of
the temperature range.
X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically
maintain a capacitance range within ±20% of nominal over full operating ratings of temperature and voltage. Of
course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance.
TANTALUM: Solid Tantalum capacitors are typically recommended for use on the output because their ESR is
very close to the ideal value required for loop compensation.
Tantalums also have good temperature stability: a good quality Tantalum will typically show a capacitance value
that varies less than 10-15% across the full temperature range of 125°C to −40°C. ESR will vary only about 2X
going from the high to low temperature limits.
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