Advanced Monolithic Systems, Inc.

6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

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 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 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 22

µF 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 100

µF 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 1000

µF 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).

AMS1117

IN

OUT

ADJ

+

C

OUT

2

2µF

V

OUT

R

1

R

2

C

ADJ

10

µF

D1

V

IN

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.

AMS1117

IN

OUT

ADJ

R1

R2

V

OUT

V

REF

V

IN

I

ADJ

50

µA

(1+ R2/R1)+I

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:

R1