AMS1084 Advanced Monolithic Systems, AMS1084 Datasheet - Page 4

• Manufacturer Part Number: AMS1084
• Description: 5A LOW DROPOUT VOLTAGE REGULATOR
• Manufacturer: Advanced Monolithic Systems

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
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 F tantalum or
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 100 F 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 1000 F to 5000 F 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.
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone [phone] Fax [phone]
150
F aluminum electrolytic or a 22 F solid
150
F
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 100
to120 ) 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=100 at 120Hz the
adjust pin capacitor should be 25 F. At 10kHz only 0.22 F 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 (V
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.
V
IN
OUT
IN
AMS1084
/ V
ADJ
D1
REF
OUT
).
C
10 F
ADJ
AMS1084
R
R
1
2
+
C
150 F
OUT
V
OUT