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.
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 V
BE
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 40µA 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.
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
0.1µF to 1µF 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.
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.
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. R
P
is the parasitic resistance of the connections
between the device and the load. R
P
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 R
P
. 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. R
P
is now
connected inside the regulation loop of the AMS2501/AMS2502
and for reasonable values of R
P
the load regulation at the load will
be negligible. The effect on output regulation can be seen in trace
B of figure 3.
5V
CONTROL
POWER
SENSE
3.3V
AMS2501
OUTPUT
ADJ
R
P
LOAD
R1
R2
R
P
+
V
OUT
-
Figure 1. Conventional Load Sensing
Advanced Monolithic Systems, Inc.
www.advanced-monolithic.com
Phone (925) 443-0722
Fax (925) 443-0723
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
