AMS317
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. For guaranteed specifications and test conditions, see the
Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Parameters identified with boldface type apply over the full
operating temperature range.
Note 2: Line and Load regulation are guaranteed up to the maximum power dissipation of 1.2 W. Power dissipation is determined by the input/output differential
and the output current. Guaranteed maximum power dissipation will not be available over the full input/output range.
Note 3: See thermal regulation specifications for changes in output voltage due to heating effects. Line and load regulation are measured at a constant junction
temperature by low duty cycle pulse testing. Load regulation is measured at the output lead ~1/8” from the package.
Note 4: Minimum load current is defined as the minimum output current required to maintain regulation. When 3V £ (VIN - VOUT) £ 25V the device is guaranteed
to regulate if the output current is greater than 10mA.
APPLICATION HINTS
The AMS317 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.
output capacitors are used, such as 1000mF to 5000mF 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 AMS317 is used (Figure 1).
D1
Stability
AMS317
IN OUT
ADJ
VOUT
COUT
VIN
The circuit design used in the AMS317 series requires the use of
an output capacitor as part of the device frequency compensation.
The addition of 150mF aluminum electrolytic or a 22mF solid
tantalum on the output will ensure stability for all operating
conditions.
+
R1
R2
m
F
150
CADJ
m
F
10
When the adjustment terminal is bypassed with a capacitor to
improve the ripple rejection, the requirement for an output
capacitor increases. The value of 22mF tantalum or 150mF
aluminum covers all cases of bypassing the adjustment terminal.
Without bypassing the adjustment terminal smaller capacitors can
be used with equally good results.
Figure 1.
Output Voltage
To ensure good transient response with heavy load current
changes capacitor values on the order of 100mF 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.
The AMS317 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.
Protection Diodes
Unlike older regulators, the AMS317 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
AMS317 adjustment pin, therefore even with capacitors on the
adjustment pin no protection diode is needed to ensure device
safety under short-circuit conditions.
AMS317
IN OUT
ADJ
VOUT
VIN
VREF
R1
R2
IADJ
50
A
m
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
VOUT = VREF (1+ R2/R1)+IADJR2
Figure 2. Basic Adjustable Regulator
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
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