Advanced Monolithic Systems

6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

AMS1503

APPLICATION HINTS

The AMS1503 series of adjustable and fixed regulators are

designed to power the new generation of microprocessors. The

AMS1503 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 30mA for a 3A 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 60

µA at 25°C and varies proportional to

absolute temperature.

The improved frequency compensation of AMS1503 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 these microprocessors, the AMS1503 also saves

total cost by needing less output capacitance to maintain

regulation.

Careful design of the AMS1503 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

AMS1503 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 AMS1503 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

3A 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 0.1

µF 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 AMS1503 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 3A of output current with a maximum

dropout of 0.8V the AMS1503 also has a fast transient response

that allows it to handle large current changes associated with the

new generation of microprocessors. The device is fully protected

against overcurrent and overtemperature conditions.

Grounding and Output Sensing

The AMS1503 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 AMS1503. 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

between the device and the load. Typically the load is a

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

connected inside the regulation loop of the AMS1503 and for

negligible. The effect on output regulation can be seen in trace B

of figure 3.

CONTROL

POWER

SENSE

AMS1503

OUTPUT

ADJ

LOAD

+

-

V

OUT

R

P

R

P

R1

R2

5V

3.3V

Figure 1. Conventional Load Sensing