Datasheet AMS1117 (Advanced Monolithic Systems) - 4

AMS1117 APPLICATION HINTS
The AMS1117 series of adjustable and fixed regulators are easy to use and are protected against short circuit and thermal overloads. Thermal protection circuitry will shut-down the regulator should D1 the junction temperature exceed 165°C at the sense point. Pin compatible with older three terminal adjustable regulators, these devices offer the advantage of a lower dropout voltage, more AMS1117 precise reference tolerance and improved reference stability with V VIN IN OUT OUT temperature. ADJ + C R OUT 1 22µF
Stability
CADJ R 2 10µF The circuit design used in the AMS1117 series requires the use of an output capacitor as part of the device frequency compensation. The addition of 22µF solid tantalum on the output will ensure stability for all operating conditions. Ĥ
igure 1.
When the adjustment terminal is bypassed with a capacitor to improve the ripple rejection, the requirement for an output capacitor increases. The value of 22µF tantalum covers all cases of
Output Voltage
bypassing the adjustment terminal. Without bypassing the adjustment terminal smaller capacitors can be used with equally The AMS1117 series develops a 1.25V reference voltage between good results. the output and the adjust terminal. Placing a resistor between these To further improve stability and transient response of these two terminals causes a constant current to flow through R1 and devices larger values of output capacitor can be used. down through R2 to set the overall output voltage. This current is normally the specified minimum load current of 10mA. Because IADJ is very small and constant it represents a small error and it
Protection Diodes
can usually be ignored. Unlike older regulators, the AMS1117 family does not need any AMS1117 protection diodes between the adjustment pin and the output and VIN IN OUT V from the output to the input to prevent over-stressing the die. OUT ADJ V Internal resistors are limiting the internal current paths on the REF R1 AMS1117 adjustment pin, therefore even with capacitors on the IADJ adjustment pin no protection diode is needed to ensure device 50µA R2 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 VOUT = VREF (1+ R2/R1)+IADJR2 internal diode between the input and output pins of the device. In normal operations it is difficult to get those values of surge
Figure 2. Basic Adjustable Regulator
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
Load Regulation
occur. A diode from output to input is recommended, when a crowbar circuit at the input of the AMS1117 is used (Figure 1). True remote load sensing it is not possible to provide, because the AMS1117 is a three terminal device. The resistance of the wire connecting the regulator to the load will limit the load regulation. The data sheet specification for load regulation is measured at the bottom of the package. Negative side sensing is a true Kelvin connection, with the bottom of the output divider returned to the negative side of the load. The best load regulation is obtained when the top of the resistor divider R1 is connected directly to the case not to the load. If R1 were connected to the load, the effective resistance between the regulator and the load would be: RP x ( R2+R1 ) , RP = Parasitic Line Resistance R1
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