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楼主使用元件都不看规格书的吗？
OUT (Pin 10): Output. This pin supplies power to the load.
For stability, use a minimum 4.7μF output capacitor with
an ESR below 50mΩ and an ESL below 2nH. Large load
transients require larger output capacitance to limit peak
voltage transients. Refer to the Applications Information
section for more information on output capacitance. A
parasitic substrate diode exists between OUT and GND pins
of the LT3042; do not drive OUT more than 0.3V below
GND during normal operation or during a fault condition.
Stability and Output Capacitance
The LT3042 requires an output capacitor for stability.
Given its high bandwidth, LTC recommends low ESR and
ESL ceramic capacitors. A minimum 4.7μF output capacitor with an ESR below 50mΩ and an ESL below 2nH is
required for stability.
Given the high PSRR and low noise performance attained
using a single 4.7μF ceramic output capacitor, larger values
of output capacitor only marginally improves the performance because the regulator bandwidth decreases with
increasing output capacitance — hence, there is little to
be gained by using larger than the minimum 4.7μF output
capacitor. Nonetheless, larger values of output capacitance
do decrease peak output deviations during a load transient.
Note that bypass capacitors used to decouple individual
components powered by the LT3042 increase the effective
output capacitance.
Give extra consideration to the type of ceramic capacitors
used. They are manufactured with a variety of dielectrics,
each with different behavior across temperature and applied
voltage. The most common dielectrics used are specified
with EIA temperature characteristic codes of Z5U, Y5V,
X5R and X7R. The Z5U and Y5V dielectrics are good for
providing high capacitance in the small packages, but they
tend to have stronger voltage and temperature coefficients
as shown in Figure 4 and Figure 5. When used with a 5V
regulator, a 16V10μF Y5V capacitor can exhibit an effective
value as low as 1μF to 2μF for the DC bias voltage applied
over the operating temperature range.
X5R and X7R dielectrics result in more stable characteristics and are thus more suitable for LT3042. The X7R
dielectric has better stability across temperature, while
the X5R is less expensive and is available in higher values.
Nonetheless, care must still be exercised when using
X5R and X7R capacitors. The X5R and X7R codes only
specify operating temperature range and the maximum
capacitance change over temperature. While capacitance
change due to DC bias for X5R and X7R is better than
Y5V and Z5U dielectrics, it can still be significant enough
to drop capacitance below sufficient levels. As shown in
Figure 6, capacitor DC bias characteristics tend to improve
as component case size increases, but verification of
expected capacitance at the operating voltage is highly
recommended. |
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发表于 2023-4-23 16:07
楼主
