AMS1503

APPLICATION HINTS

CONTROL

POWER

SENSE

AMS1503

OUTPUT

ADJ

LOAD

+

-

V

OUT

R

P

R

P

R1

R2

5V

3.3V

Figure 2. Remote Load Sensing

(

∆I

TIME

V

OUT

FIGURE 1

V

OUT

FIGURE 2

I

OUT

Figure 3. Remote Sensing Improves Load Regulation

dropout voltage of the regulator regardless of whether they are

inside or outside the regulation loop. The AMS1503 can control

the voltage at the load as long as the input-output voltage is

greater than the total of the dropout voltage of the device plus the

Stability

The circuit design used in the AMS1503 series requires the use of

an output capacitor as part of the device frequency compensation.

µF aluminum electrolytic or a 22µF solid

tantalum on the output will ensure stability for all operating

conditions. For best frequency response use capacitors with an

ESR of less than 1

Ω.

In order to meet the transient requirements of the processor larger

value capacitors are needed. Tight voltage tolerances are required

in the power supply. To limit the high frequency noise generated

by the processor high quality bypass capacitors must be used. In

order to limit parasitic inductance (ESL) and resistance (ESR) in

the capacitors to acceptable limits, multiple small ceramic

capacitors in addition to high quality solid tantalum capacitors are

required.

When the adjustment terminal is bypassed to improve the ripple

rejection, the requirement for an output capacitor increases. The

Adjust pin is brought out on the fixed voltage device specifically

to allow this capability. To ensure good transient response with

heavy load current changes capacitor values on the order of 100

µF

are used in the output of many regulators. To further improve

stability and transient response of these devices larger values of

output capacitor can be used.

The modern processors generate large high frequency current

transients. The load current step contains higher order frequency

components than the output coupling network must handle until

the regulator throttles to the load current level. Because they

contain parasitic resistance and inductance, capacitors are not

ideal elements. These parasitic elements dominate the change in

output voltage at the beginning of a transient load step change.

The ESR of the output capacitors produces an instantaneous step

in output voltage (

∆V=∆I)(ESR). The ESL of the output

capacitors produces a droop proportional to the rate of change of

the output current (V= L)(

∆I/∆t). The output capacitance produces

a change in output voltage proportional to the time until the

regulator can respond (

∆V=∆t) (∆I/C). Figure 4 illustrates these

transient effects.

CAPACITANCE

EFFECTS

ESR

EFFECTS

ESL

EFFECTS

POINT AT WHICH REGULATOR

TAKES CONTROL

SLOPE, V/t =

∆I/C

Figure 4.

Output Voltage

The AMS1503 series develops a 1.25V reference voltage between

the Sense pin and the Adjust pin (Figure5). Placing a resistor

between these two terminals causes a constant current to flow

through R1 and down through R2 to set the overall output voltage.

In general R1 is chosen so that this current is the specified

minimum load current of 10mA.The current out of the Adjust pin

is small, typically 50

µA and it adds to the current from R1.

very precise output voltage setting is required. For best regulation

the top of the resistor divider should be connected directly to the

Sense pin.

CONTROL

POWER

OUTPUT

AMS1503

SENSE

ADJ

R1

R2

V

OUT

V

REF

V

POWER

I

ADJ

50

µA

+

+

+

V

CONTROL

Figure 5. Setting Output Voltage

Advanced Monolithic Systems, Inc.

www.advanced-monolithic.com

Phone (925) 443-0722

Fax (925) 443-0723