AMS1505V
APPLICATION HINTS
The AMS1505V series of adjustable and fixed regulators are
designed to power the new generation of microprocessors. The
AMS1505V 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 0.35V 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 50mA 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 AMS1505V 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 the video-processors, the AMS1505V also saves
total cost by needing less output capacitance to maintain
regulation.
Careful design of the AMS1505V 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.
The new features of the AMS1505V 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
5A with a 2.5V output, the typical load regulation is less than
1mV. 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
±1%
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 AMS1505V include 3.3V to 2.9V
conversion with a 5V control supply, 5V to 4.7V conversion with
a 12V control supply or 5V to 3.6V conversion with a 12V
control supply. Capable of 5A of output current with a maximum
dropout of 350mV the AMS1505V also has a fast transient
response that allows it to handle large current changes associated
with the new generation of video-processors. The device is fully
protected against overcurrent and overtemperature conditions.
Grounding and Output Sensing
The AMS1505V 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 AMS1505V. 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. Typically the load is a
microprocessor and R
P
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. R
P
is now
connected inside the regulation loop of the AMS1505V 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
AMS1505V
OUTPUT
ADJ
LOAD
R1
R2
R
P
R
P
+
V
OUT
-
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 ]
