Advanced Monolithic Systems, Inc.

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

AMS2501/AMS2502

APPLICATION HINTS

The

AMS2501/AMS2502

series

of

adjustable

and

fixed

regulators 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 1.2V 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

voltage the current requirement is small, equal to about 1% of the

output current or approximately 10mA for a 1A 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 40

µA at 25°C and varies proportional to

absolute temperature.

The improved frequency compensation of AMS2501/AMS2502

permits the use of capacitors with very low ESR. Output voltage

tolerances are tighter and include transient response as part of the

specification. Designed to meet the fast current load step, the

AMS2501/AMS2502 also saves total cost by needing less output

capacitance to maintain regulation.

Careful design of the AMS2501/AMS2502 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 AMS2501/AMS2502 can also be operated as a

single supply device. In single supply operation the dropout will

be determined by the minimum control voltage.

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 1A with a 2.5V output, the typical load

regulation is less than 1mV. For AMS2502 the reference voltage

is brought out to pin 7, 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 Reference pin. The value chosen

will depend on the amount of output capacitance in the system.

This devices 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 AMS2501/AMS2502 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 1A of output current with a maximum

dropout of 0.8V the AMS2501/AMS2502 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 AMS2501/AMS2502 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 AMS2501/AMS2502. 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

and /or connector resistances (in the case of a modular regulator)

between the regulator and the load. Trace A of figure 3 illustrates

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 AMS2501/AMS2502

will be negligible. The effect on output regulation can be seen in

trace B of figure 3.

CONTROL

POWER

SENSE

AMS2501

OUTPUT

ADJ

LOAD

+

-

V

OUT

R

P

R

P

R1

R2

5V

3.3V

Figure 1. Conventional Load Sensing