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LT3065 Series

3065fc

For more information www.linear.com/LT3065

applicaTions inForMaTion

Figure 8. Noise Resulting from Tapping On a Ceramic Capacitor

VOUT

1mV/DIV

10ms/DIV

3065 F08

VOUT = 5V

COUT = 10µF

CREF/BYP = 10nF

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltageacrossitsterminalsduetomechanicalstress,simi-

lar to the way a piezoelectric accelerometer or microphone

works. For a ceramic capacitor, the stress is induced by

vibrationsinthesystemorthermaltransients.Theresulting

voltages produced cause appreciable amounts of noise.

A ceramic capacitor produced the trace in Figure 8 in

response to light tapping from a pencil. Similar vibration

induced behavior can masquerade as increased output

voltage noise.

Stability and Input Capacitance

Low ESR, ceramic input bypass capacitors are acceptable

for applications without long input leads. However, appli-

cations connecting a power supply to an LT3065 circuit’s

IN and GND pins with long input wires combined with a

low ESR, ceramic input capacitors are prone to voltage

spikes, reliability concerns and application-speciﬁc board

oscillations.

The input wire inductance found in many battery-powered

applications, combined with the low ESR ceramic input

capacitor, forms a high Q LC resonant tank circuit. In

some instances this resonant frequency beats against the

output current dependent LDO bandwidth and interferes

with proper operation. Simple circuit modifications/solu-

tions are then required. This behavior is not indicative of

LT3065 instability, but is a common ceramic input bypass

capacitor application issue.

The self-inductance, or isolated inductance, of a wire is

directly proportional to its length. Wire diameter is not a

major factor on its self-inductance. For example, the self-

inductance of a 2-AWG isolated wire (diameter = 0.26") is

about half the self-inductance of a 30-AWG wire (diameter

= 0.01"). One foot of 30-AWG wire has approximately

465nH of self-inductance.

Twomethodscanreducewireself-inductance.Onemethod

divides the current ﬂowing towards the LT3065 between

two parallel conductors. In this case, the farther apart the

wires are from each other, the more the self-inductance is

reduced; up to a 50% reduction when placed a few inches

apart. Splitting the wires connects two equal inductors in

parallel,butplacingthemincloseproximitycreatesmutual

inductance adding to the self-inductance. The second and

most effective way to reduce overall inductance is to place

both forward and return current conductors (the input

and GND wires) in very close proximity. Two 30-AWG

wires separated by only 0.02", used as forward and return

current conductors, reduce the overall self-inductance

to approximately one-ﬁfth that of a single isolated wire.

Ifabattery,mountedincloseproximity,powerstheLT3065,

a 10µF input capacitor suffices for stability. However, if a

distant supply powers the LT3065, use a larger value input

capacitor. Use a rough guideline of 1µF (in addition to the

10µF minimum) per 8 inches of wire length. The minimum

input capacitance needed to stabilize the application also

varies with power supply output impedance variations.

Placing additional capacitance on the LT3065’s output

also helps. However, this requires an order of magnitude

more capacitance in comparison with additional LT3065

inputbypassing.Seriesresistancebetweenthesupplyand

the LT3065 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 elec-

trolytic capacitors at the LT3065 input in place of ceramic

capacitors.

IMAX Pin Operation

The IMAX pin is the collector of a PNP that sources a cur-

rent equal to 1/500th of output load current (see Block

Diagram). The IMAX pin is also the input to the precision

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