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AMS1501CT

1.5A LOW DROPOUT VOLTAGE REGULATORS

Advanced Monolithic Systems Inc 

APPLICATION HINTS

5V

3.3V

CONTROL

POWER SENSE

AMS1501

OUTPUT

ADJ

R1

R2

RP

LOAD

RP

+

VOUT

-

Figure 2. Remote Load Sensing

VOUT

FIGURE 1

VOUT

FIGURE 2

(∆IOUT)(RP)

IOUT

TIME

Figure 3. Remote Sensing Improves Load Regulation

Voltage drops due to RP are not eliminated; they will add to the

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

voltage drop across RP.

Stability

The circuit design used in the AMS1501 series requires the use of

an output capacitor as part of the device frequency compensation.

The addition of 150µF 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 1Ω.

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

AMS1501

to allow this capability. To ensure good transient response with

heavy load current changes capacitor values on the order of 100µF

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.

ESR

EFFECTS

ESL

EFFECTS

CAPACITANCE

EFFECTS

SLOPE, V/t = ∆I/C

POINT AT WHICH REGULATOR

TAKES CONTROL

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 50µA and it adds to the current from R1.

Because IADJ is very small it needs to be considered 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.

VCONTROL

+

VPOWER

+

CONTROL

POWER OUTPUT

AMS1501

SENSE

ADJ

VREF

+

R1

IADJ

50µA

R2

VOUT = VREF (1+ R2/R1)+IADJR2

Figure 5. Setting Output Voltage

VOUT

Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723

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