EXTERNAL CAPACITORS/STABILITY
- Input Bypass Capacitor
An input capacitor is recommended. A 10μF tantalum on the input is a suitable input bypassing for almost all applications.
- Output Capacitor
The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both minimum
amount of capacitance and ESR (Equivalent Series Resistance). The minimum output capacitance required by the LM1117 is 10μF, if a tantalum capacitor is used. Any increase of the output capacitance will merely improve the loop stability and transient response.
The ESR of the output capacitor should range between 0.3Ω - 22Ω. In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22μf tantalum) is required.
- Protection Diodes
Unlike older regulators, the AMS1117 family does not need any protection diodes between the adjustment pin and the output and from the output to the input to prevent over-stressing the die. Internal resistors are limiting the internal current paths on the AMS1117 adjustment pin, therefore even with capacitors on the adjustment pin no protection diode is needed to ensure device safety under short-circuit conditions.
Diodes between the input and output are not usually needed. Microsecond surge currents of 50A to 100A can be handled by the internal diode between the input and output pins of the device. In normal operations it is difficult to get those values of surge currents even with the use of large output capacitances. If high value output capacitors are used, such as 1000μF to 5000μF and the input pin is instantaneously shorted to ground, damage can occur. A diode from output to input is recommended, when a crowbar circuit at the input of the AMS1117 is used
The 7800 Series of fixed voltage regulators are designed with Thermal Overload Protection that shuts down the circuit when subjected to an excessive power overload condition, Internal Short Circuit Protection that limits the maximum current the circuit will pass, and Output Transistor Safe-Area Compensation that reduces the output short circuit current as the voltage across the pass transistor is increased.
In many low current applications, compensation capacitors are not required. However, it is recommended that the regulator input be bypassed with a capacitor if the regulator is connected to the power supply filter with long wire lengths, or if the output load capacitance is large. An input bypass capacitor should be selected to provide good high-frequency characteristics to insure stable operation under all load conditions. A 0.33 uF or larger tantalum, mylar, or other capacitor having low internal impedance at high frequencies should be chosen.
The bypass capacitor should be mounted with the shortest possible leads directly across the regulators input terminals. Normally good construction techniques should be used to minimize ground loops and lead resistance drops since the regulator has no external sense lead.
ตัวอย่างThe bypass capacitor should be mounted with the shortest possible leads directly across the regulators input terminals
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