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LT3645
15
3645f
large enough to create an additional zero, and the phase
lead might not be necessary. If the output capacitor is
different than the recommended capacitor, stability should
be checked across all operating conditions, including input
voltage and temperature.
Figure 7 shows the transient response of the LT3645 with a
few output capacitor choices. The output is 3.3V. The load
current is stepped from 0.25A to 0.5A and back to 0.25A,
and the oscilloscope traces show the output voltage. The
upper photo shows the recommended value. The second
photo shows the improved response (faster recovery)
resulting from a phase lead capacitor.
APPLICATIONS INFORMATION
Frequency Compensation (LDO)
The LT3645 LDO requires an output capacitor for stability.
It is designed to be stable with most low ESR capacitors
(typically ceramic, tantalum or low ESR electrolytic). A
minimum output capacitor of 2.2糉 with an ESR of 0.5?
or less is recommended to prevent oscillations. Larger
values of output capacitance decrease peak deviations
and provide improved transient response for larger load
current changes. Bypass capacitors, used to decouple
individual components powered by the LT3645, increase
the effective output capacitor value. For improvement in
transient performance, place a capacitor across the OUT2
Figure 7.
With Phase Lead Capacitor
No Phase Lead Capacitor
pin and the FB2 pin. Capacitors up to 1nF can be used. This
bypass capacitor reduces system noise as well.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used are speci ed with EIA temperature char-
acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but they tend to have strong voltage
and temperature coef cients as shown in Figures 8 and 9.
When used with a 5V regulator, a 16V 10糉 Y5V capaci-
tor can exhibit an effective value as low as 1糉 to 2糉
for the DC bias voltage applied and over the operating
Figure 8. Ceramic Capacitor DC Bias Characteristics
Figure 9. Ceramic Capacitor Temperature Characteristics
DC BIAS VOLTAGE (V)
3645 F08
20
0
20
40
60
80
100
0
4
8
10
2
6
12  14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10糉
TEMPERATURE (癈)
50
40
20
0
20
40
60
80
100
25
75
3645 F09
25   0
50
100  125
Y5V
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10糉
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