Datasheet

LT3071
20
3071fc
For more information www.linear.com/LT3071
APPLICATIONS INFORMATION
The X7R type has better stability across temperature,
while the X5R is less expensive and is available in higher
values. Care still must be exercised when using X5R and
X7R capacitors; the X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to
DC bias with X5R and X7R capacitors is better than Y5V
and Z5U capacitors, but can still be significant enough to
drop capacitor values below appropriate levels. Capacitor
DC bias characteristics tend to improve as component
case size increases, but expected capacitance at operat
-
ing voltage
should be verified. Voltage and temperature
coefficients
are not the only sources of problems. Some
ceramic capacitors have a piezoelectric response. A piezo
-
electric device generates voltage across its terminals due
to mechanical stress, similar to the way a piezoelectric
microphone works. For a ceramic capacitor the stress
can be induced by vibrations in the system or thermal
transients.
Stability and Input Capacitance
The LT3071 is stable with a minimum capacitance of
47µF connected to its IN pins. Use low ESR capacitors to
minimize instantaneous voltage drops under large load
transient conditions.
Large V
IN
droops during large load
transients may cause the regulator to enter dropout with
corresponding degradation in load transient response.
Increased values of input and output capacitance may be
necessary depending on an application’s requirements.
Sufficient input capacitance is critical as the circuit is in
-
tentionally operated
close to dropout to minimize power.
Ideally, the output impedance of the supply that powers
IN should be less than 10mΩ to support a 5A load with
large transients.
In cases where wire is used to connect a power supply
to the input of the LT3071 (and also from the ground of
the LT3071 back to the power supply ground), large input
capacitors are required to avoid an unstable application.
This is due to the inductance of the wire forming an LC
tank circuit with the input capacitor and not a result of the
LT3071 being unstable. The self inductance, or isolated
inductance, of a wire is directly proportional to its length.
However, the diameter of a wire does not have a major
influence on its self inductance. For example, one inch of
18-AWG, 0.04 inch diameter wire has 28nH of self induc
-
tance. The self inductance of a 2-
AWG isolated wire with
a diameter of 0.26 inch is about half the inductance of a
18-AWG wire. The overall self inductance of a wire can be
reduced in two ways. One is to divide the current flowing
towards the LT3071 between two parallel conductors
which flows in the same direction in each. In this case,
the farther the wires are placed apart from each other, the
more inductance will be reduced, up to a 50% reduction
when placed a few inches apart. Splitting the wires basi
-
cally connects
two equal inductors in parallel. However,
when placed in close proximity from each other, mutual
inductance is added to the overall self inductance of the
DC BIAS VOLTAGE (V)
0
–100
CHANGE IN VALUE (%)
–80
–60
–40
–20
4 8
12
16
3071 F04
0
20
2 6
10
X5R
Y5V
14
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
TEMPERATURE (°C)
–50
–20
0
40
25 75
X5R
Y5V
3071 F05
–40
–60
–25 0
50 100 125
–80
–100
20
CHANGE IN VALUE (%)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 4. Ceramic Capacitor DC Bias Characteristics
Figure 5. Ceramic Capacitor Temperature Characteristics
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