ADP151

Rev. 0 | Page 13 of 24

ENABLE FEATURE

The ADP151 uses the EN pin to enable and disable the VOUT

pin under normal operating conditions. As shown in Figure 28,

when a rising voltage on EN crosses the active threshold, VOUT

turns on. When a falling voltage on EN crosses the inactive

threshold, VOUT turns off.

3.0

2.5

2.0

1.5

0.5

1.0

0

0

0.5

1.0

1.5

2.0

2.5

ENABLE VOLTAGE

Figure 28. ADP151 Typical EN Pin Operation

As shown in Figure 28, the EN pin has hysteresis built in. This

prevents on/off oscillations that can occur due to noise on the

EN pin as it passes through the threshold points.

The EN pin active/inactive thresholds are derived from the VIN

voltage. Therefore, these thresholds vary with changing input

voltage. Figure 29 shows typical EN active/inactive thresholds

when the input voltage varies from 2.2 V to 5.5 V.

1200

1000

800

600

200

400

0

2.0

2.5

3.0

3.5

4.5

5.0

4.0

5.5

INPUT VOLTAGE

Figure 29. Typical EN Pin Thresholds vs. Input Voltage

The ADP151 uses an internal soft start to limit the inrush current

when the output is enabled. The start-up time for the 3.3 V

option is approximately 160 μs from the time the EN active

threshold is crossed to when the output reaches 90% of its final

value. As shown in Figure 30, the start-up time is dependent on

the output voltage setting.

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

0

450

400

350

300

250

200

150

100

50

TIME (µs)

ENABLE

3.3V

2.8V

1.1V

Figure 30. Typical Start-Up Behavior

ADJUSTABLE OUTPUT VOLTAGE OPERATION

The unique architecture of the ADP151 makes an adjustable

version difficult to implement in silicon. However, it is possible

to create an adjustable regulator at the expense of increasing the

quiescent current of the regulator circuit.

The ADP151, and similar LDOs, are designed to regulate the

to the GND pin. If the GND pin is at a potential other than 0 V

create an adjustable ADP151 circuit that retains most of the

desirable characteristics of the ADP151.

U1

1

2

3

5

4

C2

C1

VOUT

NC

VIN

GND

EN

C3

R2

R1

Figure 31. Adjustable LDO Using the ADP151

The circuit shown in Figure 31 is an example of an adjustable

passing a known current through R2. The current through R2 is

determined by the voltage across R1. Because the voltage across

R1 is set by the voltage between VOUT and GND, the current

To minimize the effect variation of the ADP151 ground current,

best to size the current passing through R2 to at least 20×

greater than the maximum expected ground current.

To create a 4 V LDO circuit, start with the 3.3 V version of the

R1 is, therefore, 3.3 V/7 mA or 471 Ω. A 470 Ω standard value

introduces less than 1% error. Capacitor C3 is necessary to stabilize

the LDO; a value of 1 μF is adequate.