LT3032IDE datasheet(15/20 Pages) LINER

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LT3032

3032f

Stability and Input Capacitance

Low ESR, ceramic input bypass capacitors are acceptable

for applications without long input leads. However,

applications connecting a power supply to an LT3032’s

circuit’s INP/INN and GND pins with long input wires

combined with 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/solutions are then required.

This behavior is not indicative of LT3032 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 about 465nH of

self-inductance.

One of two ways reduces a wire’s self-inductance. One

method divides the current ﬂowing towards the LT3032

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 basically connects

two equal inductors in parallel, but placing them in close

proximity gives the wires mutual 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.

APPLICATIONS INFORMATION

DC BIAS VOLTAGE (V)

3032 F03

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

Figure 3. Ceramic Capacitor DC Bias Characteristics

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

3032 F04

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

Figure 4. Ceramic Capacitor Temperature Characteristics

OUTPUT SET TO 5V

3032 F05

Figure 5. Noise Resulting From Tapping on a Ceramic Capacitor

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