AMS1508

APPLICATION HINTS

The AMS1508 series of adjustable and fixed regulators are

designed to power the new generation of microprocessors. The

AMS1508 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 70mA for a8A 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 AMS1508 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 AMS1508 also saves

total cost by needing less output capacitance to maintain

regulation.

Careful design of the AMS1508 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

AMS1508 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 AMS1508 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

8A 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 AMS1508 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. Due to the innovative design of the AMS1508 it is easy to

obtain dropout voltages of less than 0.5V at 4A along with

excellent static and dynamic specifications. Capable of 8A of

output current with a maximum dropout of 0.8V the AMS1508

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 AMS1508 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 AMS1508. 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 AMS1508 and for

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

of figure 3.

CONTROL

POWER

SENSE

AMS1508

OUTPUT

ADJ

LOAD

+

-

V

OUT

R

P

R

P

R1

R2

5V

3.3V

Figure 1. Conventional Load Sensing

Advanced Monolithic Systems, Inc.

www.advanced-monolithic.com

Phone (925) 443-0722

Fax (925) 443-0723