Datasheet

14
LT1764A Series
1764afb
APPLICATIONS INFORMATION
WUU
U
Ceramic Capacitors
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior over
temperature and applied voltage. The most common
dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but exhibit strong voltage and tem-
perature coefficients as shown in Figures 3 and 4. When
used with a 5V regulator, a 10µF Y5V capacitor can exhibit
an effective value as low as 1µF to 2µF over the operating
temperature range. The X5R and X7R dielectrics result in
more stable characteristics and are more suitable for use
as the output capacitor. The X7R type has better stability
across temperature, while the X5R is less expensive and
is available in higher values.
Voltage and temperature coefficients are not the only
sources of problems. 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 micro-
phone works. For a ceramic capacitor the stress can be
induced by vibrations in the system or thermal transients.
“FREE” Resistance with PC Traces
The resistance values shown in Table 1 can easily be made
using a small section of PC trace in series with the output
capacitor. The wide range of noncritical ESR makes it easy
to use PC trace. The trace width should be sized to handle
the RMS ripple current associated with the load. The
output capacitor only sources or sinks current for a few
microseconds during fast output current transitions. There
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
1764 F11
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 3. Ceramic Capacitor DC Bias Characteristics
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
1764 F10
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 4. Ceramic Capacitor Temperature Characteristics
Table 2. PC Trace Resistors
10mΩ 20mΩ 30mΩ
0.5oz C
U
Width 0.011 " (0.28mm) 0.011 " (0.28mm) 0.011 " (0.28mm)
Length 0.102 " (2.6mm) 0.204 " (5.2mm) 0.307 " (7.8mm)
1.0oz C
U
Width 0.006 " (0.15mm) 0.006 " (0.15mm) 0.006 " (0.15mm)
Length 0.110
" (2.8mm) 0.220 " (5.6mm) 0.330 " (8.4mm)
2.0oz C
U
Width 0.006 " (0.15mm) 0.006 " (0.15mm) 0.006 " (0.15mm)
Length 0.224
" (5.7mm) 0.450 " (11.4mm) 0.670 " (17mm)
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