ams1501 Advanced Monolithic Systems, ams1501 Datasheet - Page 4

• Manufacturer Part Number: ams1501
• Description: 1.5a Low Dropout Voltage Regulators
• Manufacturer: Advanced Monolithic Systems

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.
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 60µA 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.
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 0.1µF 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,
Advanced Monolithic Systems, Inc. www.advanced-monolithic.com
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.
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. R
between the device and the load. Typically R
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. R
connected inside the regulation loop of the AMS1501 and for
reasonable values of R
negligible. The effect on output regulation can be seen in trace B
of figure 3.
3.3V
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Figure 1. Conventional Load Sensing
POWER
P
is the parasitic resistance of the connections
AMS1501
CONTROL
5V
ADJ
OUTPUT
R2
P
SENSE
the load regulation at the load will be
R1
R
R
P
P
AMS1501
LOAD
P
is made up of the
V
+
-
OUT
P
is now