lt3032 Linear Technology Corporation, lt3032 Datasheet - Page 15

• Manufacturer Part Number: lt3032
• Description: Dual 150ma Positive/negative Low Noise Low Dropout Linear Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Figure 5. Noise Resulting From Tapping on a Ceramic Capacitor
Figure 4. Ceramic Capacitor Temperature Characteristics
Figure 3. Ceramic Capacitor DC Bias Characteristics
–100
–20
–40
–60
–80
–100
20
–20
–40
–60
–80
0
40
20
0
0
–50
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
2
–25
4
OUTPUT SET TO 5V
DC BIAS VOLTAGE (V)
0
TEMPERATURE (°C)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
6
25
8
Y5V
X5R
50
10
Y5V
75
12
X5R
14
100
3032 F05
3032 F03
3032 F04
16
125
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-
specifi c 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 modifi cations/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 fl owing 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-
fi fth that of a single isolated wire.
LT3032
15
3032f