AMS2501/AMS2502
APPLICATION HINTS
The AMS2501/AMS2502 series of adjustable and fixed
regulators 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.
Typical applications for the AMS2501/AMS2502 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 1A of output current with a maximum
dropout of 0.8V the AMS2501/AMS2502 also has a fast transient
response that allows it to handle large current changes. The
device is fully protected against overcurrent and overtemperature
conditions.
The second supply is at least 1.2V 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 10mA for a 1A 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 40mA at 25°C and varies proportional to
absolute temperature.
The improved frequency compensation of AMS2501/AMS2502
permits the use of capacitors with very low ESR. Output voltage
tolerances are tighter and include transient response as part of the
specification. Designed to meet the fast current load step, the
AMS2501/AMS2502 also saves total cost by needing less output
capacitance to maintain regulation.
Careful design of the AMS2501/AMS2502 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 AMS2501/AMS2502 can also be operated as a
single supply device. In single supply operation the dropout will
be determined by the minimum control voltage.
Grounding and Output Sensing
The AMS2501/AMS2502 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 AMS2501/AMS2502. The
advantages of remote sensing are illustrated in figures 1 through
3.
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. RP is made up of the PCB traces
and /or connector resistances (in the case of a modular regulator)
between the regulator and the load. 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 AMS2501/AMS2502
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.
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 1A with a 2.5V output, the typical load
regulation is less than 1mV. For AMS2502 the reference voltage
is brought out to pin 7, 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
0.1mF to 1mF for bypassing the Reference pin. The value chosen
will depend on the amount of output capacitance in the system.
This devices can hold 1% accuracy over the full temperature
range and load current range, guaranteed, when combined with
ratiometrically accurate internal divider resistors and operating
with an input/output differential of well under 1V.
5V
CONTROL
POWER
SENSE
3.3V
AMS2501
OUTPUT
+
RP
ADJ
LOAD
VOUT
R1
R2
RP
-
Figure 1. Conventional Load Sensing
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
