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LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

Input Capacitance and Stability

EachLDOisstablewithaninputcapacitortypicallybetween

1μFand10μF.Thisinputcapacitormustbeplacedasclose

as possible to the corresponding input pin. Applications

operating with smaller input to output differential voltages

and that experience large load transients may require a

higher input capacitor value to prevent input voltage droop

and letting the regulator enter dropout.

Very low ESR ceramic capacitors may be used. However,

in cases where long wires connect the power supply to

the LDOs input and ground, use of low value input capaci-

tors may result in instability. The resonant LC tank circuit

formed by the wire inductance and the input capacitor is

the cause and not a result of LDO instability.

The minimum input capacitance needed to stabilize the

application also varies with power supply output imped-

ancevariations.PlacingadditionalcapacitanceonanLDO’s

output also helps. However, this requires an order of

magnitudemorecapacitanceincomparisonwithadditional

inputbypassing.Seriesresistancebetweenthesupplyand

an LDO’s input also helps stabilize the application; as little

as 0.1Ω to 0.5Ω suffices. This impedance dampens the

LC tank circuit at the expense of dropout voltage. A better

alternative is to use higher ESR tantalum or electrolytic

capacitors at the input in place of ceramic capacitors.

Output Capacitance, Transient Response, Stability

Each LT3668’s LDO is stable with a wide range of output

capacitors. The ESR of the output capacitor affects stabil-

ity, most notably with small capacitors. Use a minimum

output capacitor of 10μF to prevent oscillations. The ESR

of the output capacitor must not exceed 3Ω.

The LT3668 is a micropower device and output load

transient response is a function of output capacitance.

Larger values of output capacitance decrease the peak

deviations and provide improved transient response for

larger load current changes, especially for low output

voltages. Bypass capacitors, used to decouple individual

components powered by the LT3668, increase the effec-

tive output capacitor value. For applications with large

load current transients, a low ESR ceramic capacitor in

parallel with a bulk tantalum capacitor often provides an

optimally damped response.

Note that some ceramic capacitors have a piezoelectric

response. A piezoelectric device generates voltage across

its terminals due to mechanical stress, similar to the way

a piezoelectric accelerometer or microphone works. For

a ceramic capacitor, the stress is induced by vibrations in

the system or thermal transients. The resulting voltages

produced cause appreciable amounts of noise. A ceramic

capacitorproducedthetraceinFigure5inresponsetolight

tapping from a pencil. Similar vibration induced behavior

can masquerade as increased output voltage noise.

Figure 5. Noise Resulting from Tapping On a Ceramic Capacitor

VOUT2

1mV/DIV

3668 F05

2ms/DIV

VOUT2 = 5V

COUT2 = 10µF

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