AMS1117CD datasheet(6/12 Pages) ADMOS

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AMS1117

1A Low Dropout Voltage Regulator

09/14/2012

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Applications

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 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 22µF

tantalum covers all cases of bypassing the adjustment

terminal. Without bypassing the adjustment terminal

smaller capacitors can be used with equally good results.

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 current of 10mA. Because IADJ is very small

and constant it represents a small error and it can usually be

ignored.

VOUT = VREF (1+ R2/R1)+IADJR2

Figure 2. Basic Adjustable Regulator

Load Regulation

For optimum load regulation R1 and R2 should be tied as

close to the point of load as possible. This compensates for

any output voltage drop related to load current conduction

though PCB traces.

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