AMS1117

800mA LOW DROPOUT VOLTAGE REGULATOR

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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 the junction temperature exceed 165℃ at the

sense point.

Pin compatible with older three terminal adjustable regulators,

these devices offer the advantage of a lower dropout voltage,

more precise reference tolerance and improved reference

stability with temperature.

Stability

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.

When the adjustment terminal is bypassed with a capacitor to

improve the ripple rejection, the requirement for an output

capacitor increases. The value of 22 µF tantalum covers all

cases of bypassing the adjustment terminal. Without bypassing

the adjustment terminal smaller capacitors can be used with

equally good results.

To ensure good transient response with heavy load current

changes capacitor values on the order of 100µF are used in the

output of many regulators. To further improve stability and

transient response of these devices larger values of output

capacitor can be used.

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 (Figure 1).

Figure 1.

Output Voltage

The AMS1117 series develops a 1.25V reference voltage

between the output and the adjust terminal. Placing a resistor

between these two terminals causes a constant current to flow

through R1 and down through R2 to set the overall output

voltage. This current is normally the specified minimum load

represents a small error and it can usually be ignored.

Figure 2. Basic Adjustable Regulator

Load Regulation

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: