LP2985-13DBVT datasheet(10/15 Pages) TI

10 / 15 page

LP2985LV

150mA LOW NOISE, LOW DROPOUT REGULATOR

WITH SHUTDOWN FOR OUTPUT VOLTAGES <2.3 V

SLVS552 − OCTOBER 2004

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

output capacitor (COUT) (continued)

It is critical that both the minimum capacitance and ESR requirement be met over the entire operating

temperature range. Depending on the type of capacitors used, both these parameters can vary significantly with

temperature (see Capacitor Characteristics section).

noise bypass capacitor (CBYPASS)

The LP2985LV allows for low noise performance with the use of a bypass capacitor that is connected to the

internal band-gap reference via the BYPASS pin. This high-impedance band-gap circuitry is biased in the

microampere range and, thus, cannot be loaded significantly, else its output − and correspondingly the output

of the regulator − will change. Thus, for best output accuracy, dc leakage current through CBYPASS should be

minimized as much as possible and should never exceed 100 nA.

A 10-nF capacitor is recommended for CBYPASS; ceramic and film capacitors are well suited for this purpose.

capacitor characteristics

ceramic

Ceramic capacitors are ideal choices for use on the output of the LP2985LV for several reasons. For

capacitances in the range of 2.2

µF to 4.7 µF, ceramic capacitors have the lowest cost and the lowest ESR,

making them choice candidates for filtering high-frequency noise. For instance, a typical 2.2-

µF ceramic

capacitor has an ESR in the range of 10 m

Ω to 20mΩ and satisfies minimum ESR requirements of the regulator.

Ceramic capacitors have one glaring disadvantage that must be taken into account − a poor temperature

coefficient, where the capacitance can vary significantly with temperature. For instance, a large-value ceramic

capacitor (

≥2.2 µF) can lose more than half of its capacitance as temperature rises from 25°C to 85°C. Thus,

a 2.2

µF at 25°C will drop well below the minimum COUT required for stability as ambient temperature rises. For

this reason, select an output capacitor that maintains the minimum 2.2

µF required for stability over the entire

operating temperature range. Note that there are some ceramic capacitors that can maintain a

±15%

capacitance tolerance over temperature.

tantalum

Tantalum capacitors can be used at the output of the LP2985LV, but there are significant disadvantages that

could prohibit their use.

• In the 1-µF to 4.7-µF range, tantalum capacitors are more expensive than ceramics of the equivalent

capacitance and voltage ratings.

• Tantalum capacitors have higher ESRs than their equivalent-sized ceramic counterparts. Thus, to meet

the ESR requirements, a higher-capacitance tantalum may be required, at the expense of larger size

and higher cost.

• The ESR of a tantalum capacitor increases as temperature drops, as much as double from 25°C to

−40

°C. Thus, ESR margins must be maintained over the temperature range in order to prevent

regulator instability.

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