AMS1503
APPLICATION HINTS
The AMS1503 series of adjustable and fixed regulators are
designed to power the new generation of microprocessors. The
AMS1503 is designed to make use of multiple power supplies,
existing in most systems, to reduce the dropout voltage. One of the
advantages of the two supply approach is maximizing the
efficiency.
The second supply is at least 1V greater than output voltage and is
providing the power for the control circuitry and supplies the drive
current to the NPN output transistor. This allows the NPN to be
driven into saturation; thereby reducing the dropout voltage by a
VBE compared to conventional designs. For the control voltage
the current requirement is small equal to about 1% of the output
current or approximately 30mA for a 3A load. Most of this current
is drive current for the NPN output transistor. This drive current
becomes part of the output current. The maximum voltage on the
Control pin is 13V. The maximum voltage at the Power pin is 7V.
Ground pin current for fixed voltage devices is typical 6mA and is
constant as a function of load. Adjust pin current for adjustable
devices is 60µA at 25°C and varies proportional to absolute
temperature.
guaranteed, when combined with ratiometrically accurate internal
divider resistors and operating with an input/output differential of
well under 1V.
Typical applications for the AMS1503 include 3.3V to 2.5V
conversion with a 5V control supply, 5V to 4.2V conversion with
a 12V control supply or 5V to 3.6V conversion with a 12V control
supply. Capable of 3A of output current with a maximum dropout
of 0.8V the AMS1503 also has a fast transient response that
allows it to handle large current changes associated with the new
generation of microprocessors. The device is fully protected
against overcurrent and overtemperature conditions.
Grounding and Output Sensing
The AMS1503 allows true Kelvin sensing for both the high and
low side of the load. As a result the voltage regulation at he load
can be easily optimized. Voltage drops due to parasitic resistances
between the regulator and the load can be placed inside the
regulation loop of the AMS1503. The advantages of remote
sensing are illustrated in figures 1 through 3.
The improved frequency compensation of AMS1503 permits the
use of capacitors with very low ESR. This is critical in addressing
the needs of modern, low voltage high sped microprocessors. The
new generation of microprocessors cycle load current from several
hundred mA to several A in tens of nanoseconds. Output voltage
tolerances are tighter and include transient response as part of the
specification. Designed to meet the fast current load step
requirements of these microprocessors, the AMS1503 also saves
total cost by needing less output capacitance to maintain
regulation.
Careful design of the AMS1503 has eliminated any supply
sequencing issues associated with a dual supply system. The
output voltage will not turn on until both supplies are operating. If
the control voltage comes up first, the output current will be
limited to a few milliamperes until the power input voltage comes
up. If power input comes up first the output will not turn on at all
until the control voltage comes up. The output can never come up
unregulated. By tying the control and power inputs together the
AMS1503 can also be operated as a single supply device. In single
supply operation the dropout will be determined by the minimum
control voltage.
Figure 1 shows the device connected as a conventional 3 terminal
regulator with the Sense lead connected directly to the output of
the device. RP is the parasitic resistance of the connections
between the device and the load. Typically the load is a
microprocessor and RP is made up of the PC traces and /or
connector resistances (in the case of a modular regulator) between
the regulator and the processor. Trace A of figure 3 illustrates the
effect of RP. Very small resistances cause significant load
regulation steps.
Figure 2 shows the device connected to take advantage of the
remote sense feature. The Sense pin and the top of the resistor
divider are connected to the top of the load; the bottom of the
resistor divider is connected to the bottom of the load. RP is now
connected inside the regulation loop of the AMS1503 and for
reasonable values of RP the load regulation at the load will be
negligible. The effect on output regulation can be seen in trace B
of figure 3.
5V
The new features of the AMS1503 require additional pins over the
traditional 3-terminal regulator. Both the fixed and adjustable
versions have remote sense pins, permitting very accurate
regulation of output voltage at the load, rather than at the
regulator. As a result, over an output current range of 100mA to
3A with a 2.5V output, the typical load regulation is less than
1mV. For the fixed voltages the adjust pin is brought out allowing
the user to improve transient response by bypassing the internal
resistor divider. Optimum transient response is provided using a
capacitor in the range of 0.1µF to 1µF for bypassing the Adjust
pin. The value chosen will depend on the amount of output
capacitance in the system.
CONTROL
POWER
SENSE
3.3V
AMS1503
OUTPUT
+
RP
ADJ
LOAD
VOUT
R1
R2
RP
-
In addition to the enhancements mentioned, the reference accuracy
has been improved by a factor of two with a guaranteed initial
tolerance of ±0.6% at 25°C. This device can hold 1% accuracy
over the full temperature range and load current range,
Figure 1. Conventional Load Sensing
Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723
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