AMS1501
APPLICATION HINTS
The AMS1501 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.
guaranteed, when combined with ratiometrically accurate internal
divider resistors and operating with an input/output differential of
well under 1V.
Typical applications for the AMS1501 include 3.3V to 2.5V
conversion with a 5V control supply, 5V to 4.2V conversion with
a 12V control supply or 5V to3.6V conversion with a 12V control
supply. Capable of 1.5A of output current with a maximum
dropout of 0.8V the AMS1501 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 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 15mA for a 1.5A 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 60mA at 25°C and varies
proportional to absolute temperature.
The improved frequency compensation of AMS1501 permits the
use of capacitors with very low ESR. This is critical in addressing
the needs of modern, low voltage high sped microprocessors.
Output voltage tolerances are tighter and include transient
response as part of the specification. Designed to meet the fast
current load step requirements, the AMS1501 also saves total
cost by needing less output capacitance to maintain regulation.
Careful design of the AMS1501 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
AMS1501 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 AMS1501 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 AMS1501. 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. Typically RP is made up of the
PC 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 AMS1501 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.
The new features of the AMS1501 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
1.5A 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 0.1mF to 1mF for bypassing the Adjust
pin. The value chosen will depend on the amount of output
capacitance in the system.
5V
CONTROL
POWER
SENSE
3.3V
AMS1501
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. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
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