MAS 100-110 (serv.man5) - Harman Kardon Audio Service Manual (repair manual). Page 15

Advanced Monolithic Systems, Inc.    6680B Sierra Lane, Dublin, CA  94568   Phone (925) 556-9090   Fax (925) 556-9140

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.

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.

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

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 I

ADJ

 is very small and constant it

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

V

OUT

= V

REF

(1+ R2/R1)+I

ADJ

R2

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:

                R

P

 x ( R2+R1 ) ,     R

P

 = Parasitic Line Resistance

                         R1

harman/kardon

MAS 100 / MAS 110 / MAS 1 Service Manual

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