Advanced Monolithic Systems, Inc.

www.advanced-monolithic.com

Phone (925) 443-0722

Fax (925) 443-0723

AMS2501/AMS2502

APPLICATION HINTS

CONTROL

POWER

SENSE

AMS2501

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 AMS2501/AMS2502

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

Stability

The circuit design used in the AMS2501/AMS2502 series requires

the use of an output capacitor as part of the device frequency

µF aluminum electrolytic or a

22

µF 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 increase the transient response larger value capacitors

are needed. To limit the high frequency noise generated by the

load 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

reference voltage is brought out specifically to allow this

capability.

Some of the loads 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 AMS2501/AMS2502 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 40

µA and it adds to the current from

when 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

AMS2501

SENSE

ADJ

R1

R2

V

OUT

V

REF

V

POWER

I

ADJ

50

µA

+

+

+

V

CONTROL

(1+ R2/R1)+I

Figure 5. Setting Output Voltage