Advanced Monolithic Systems, Inc.
www.advanced-monolithic.com
Phone (925) 443-0722
Fax (925) 443-0723
AMS1084
APPLICATION HINTS
The AMS1084 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 shut-down.
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 AMS1084 series requires the use of
an output capacitor as part of the device frequency compensation.
The addition of 150µF aluminum electrolytic or a 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 or 150µF
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 100µF 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 AMS1084 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
AMS1084 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 AMS1084 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.
AMS1084
IN
OUT
ADJ
+
C
OUT
150µF
V
OUT
R
1
R
2
C
ADJ
10
µF
D1
V
IN
Overload Recovery
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.
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 100Ω
to120Ω) 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=100Ω at 120Hz the
adjust pin capacitor should be 25µF. At 10kHz only 0.22µF 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 AMS1084 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.
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