Advanced Monolithic Systems, Inc.

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

AMS1501

APPLICATION HINTS

CONTROL

POWER

SENSE

AMS1501

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 AMS1501 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 AMS1501 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 load larger

value capacitors are needed. Tight voltage tolerances are required

in the power supply. 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

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 systems 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 AMS1501 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

only 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

AMS1501

SENSE

ADJ

R1

R2

V

OUT

V

REF

V

POWER

I

ADJ

50

µA

+

+

+

V

CONTROL

Figure 5. Setting Output Voltage