The ceramic capacitor’s capacitance can vary with tempera-

ture. The capacitor type X7R, which operates over a tem-

perature range of -55˚C to +125˚C, will only vary the capaci-

tolerance over a reduced temperature range of -55˚C to

+85˚C.

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

1µF to 4.7µF 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. It should also be noted that the ESR of a typical

tantalum will increase about 2:1 as the temperature goes

from 25˚C down to -40˚C, so some guard band must be

allowed.

LDO Input Capacitor

An input capacitor is required for stability. The input capacitor

should be at least equal to or greater than the output capaci-

tor. It is recommended that a 1µF capacitor be connected

between V

may be increased without limit).

This capacitor must be located a distance of not more than

1cm from the input pin and returned to a clean analogue

ground. Any good quality ceramic, tantalum, or film capacitor

may be used at the input.

Important: Tantalum capacitors can suffer catastrophic fail-

ures 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 guaranteed by the manufacturer to have a surge current

rating sufficient for the application. There are no require-

ments for the ESR (Equivalent Series Resistance) on the

input capacitor, but tolerance and temperature coefficient

must be considered when selecting the capacitor to ensure

operating voltage and temperature range.

LDO Output Capacitor

The LP3905 LDOs are designed specifically to work with

very small ceramic output capacitors. A ceramic capacitor

(dielectric types X5R or X7R) in the 0.47µF to 10µF range,

and with ESR between 5m

Ω to 500mΩ, is suitable in the

application circuit. For this device the output capacitor

should be connected between the LDO1 and LDO2 pins and

a good ground connection and should be mounted within

1cm of the device.

The output capacitor must meet the requirement for the

minimum value of capacitance and also have an ESR value

that is within the range 5m

Ω to 500mΩ for stability.

No-Load Stability

The LP3905 LDOs will remain stable and in regulation with

no external load.

Enable Control

A1M

Ω pulldown resistor ties the EN

ensures that the device will remain off when the enable pin is

left open circuit. To ensure proper operation, the signal

source used to drive the EN

above and below the specified turn-on/off voltage thresholds

listed in the Electrical Characteristics section under V

V

case Buck1 will be turned on first. Once Buck1 is powered

up, after a typical 150µs delay LDO1/2 will be turned on

concurrently.

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