AMS1085
APPLICATION HINTS
The AMS1085 series of adjustable and fixed regulators are easy
to use and have all the protection features expected in high
performance voltage regulators: short circuit protection and
thermal shutdown.
D1
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.
AMS1085
VOUT
VIN
IN
OUT
ADJ
+
COUT
R1
150mF
CADJ
Stability
R2
10mF
The circuit design used in the AMS1085 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.
Overload Recovery
When the adjustment terminal is bypassed 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.
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.
When the power is first turned on, as the input voltage rises, the
output follows the input, permitting the regulator to start up into
heavy loads. During the start-up, as the input voltage is rising,
the input-to-output voltage differential remains small, allowing
the regulator to supply large output currents. A problem can occur
with a heavy output load when the input voltage is high and the
output voltage is low, when the removal of an output short will
not permit the output voltage to recover. The load line for such a
load may intersect two points on the output current curve. In this
case, there are two stable output operating points for the
regulator. With this double intersection, the power supply may
need to be cycled down to zero and brought up again to make the
output recover.
Protection Diodes
Unlike older regulators, the AMS1085 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
AMS1085 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 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 AMS1085 is used. Normal
power supply cycling or even plugging and unplugging in the
system will not generate current large enough to do any damage.
The adjustment pin can be driven on a transient basis ±25V, with
respect to the output without any device degradation. As with any
IC regulator, none the protection circuitry will be functional and
the internal transistors will break down if the maximum input to
output voltage differential is exceeded.
Ripple Rejection
The ripple rejection values are measured with the adjustment pin
bypassed. The impedance of the adjust pin capacitor at the ripple
frequency should be less than the value of R1 (normally 100W
to120W) for a proper bypassing and ripple rejection approaching
the values shown. The size of the required adjust pin capacitor is
a function of the input ripple frequency. If R1=100W at 120Hz
the adjust pin capacitor should be 25mF. At 10kHz only 0.22mF is
needed.
The ripple rejection will be a function of output voltage, in
circuits without an adjust pin bypass capacitor. The output ripple
will increase directly as a ratio of the output voltage to the
reference voltage (VOUT / VREF ).
Output Voltage
The AMS1085 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.
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
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