Applications
As a result, over an output current range of 100mA
to 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 0.1µF to
1µF for bypassing the Adjust pin. The value chosen
will depend on the amount of output capacitance in
the system.
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,
guaranteed, when combined with ratiometrically
accurate internal divider resistors and operating
with an input/output differential of well under 1V.
Typical applications for the AMS1505 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 5A of output current with a maximum
dropout of 0.8V the AMS1505 also has a fast
transient response that allows it to handle large
current changes. The device is fully protected
The AMS1505 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 15mA for a
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 60µA at 25°C and
varies proportional to absolute temperature.
The improved frequency compensation of AMS1505
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 AMS1505 also saves total cost
by needing less output capacitance to maintain
regulation.
against
conditions.
overcurrent
and
overtemperature
Grounding and Output Sensing
The AMS1505 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
AMS1505. 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.
Careful design of the AMS1505 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 AMS1505 can also be
operated as a single supply device. In single supply
operation the dropout will be determined by the
minimum control voltage.
The new features of the AMS1505 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.
Advanced Monolithic Systems http://www.ams-semitech.com
AMS [ Advanced Monolithic Systems Ltd ]
