Data Sheet

ADP122/ADP123

Rev. E | Page 13 of 24

Therefore, the capacitor chosen in this example meets the

minimum capacitance requirement of the LDO over tem-

perature and tolerance at the chosen output voltage.

To guarantee the performance of the ADP122/ADP123, it is

imperative that the effects of dc bias, temperature, and tolerances

on the behavior of the capacitors are evaluated for each application.

UNDERVOLTAGE LOCKOUT

The ADP122/ADP123 have an internal undervoltage lockout

circuit that disables all inputs and the output when the input

voltage is less than approximately 2 V. This ensures that the

ADP122/ADP123 inputs and the output behave in a predictable

manner during power-up.

ENABLE FEATURE

The ADP122/ADP123 uses the EN pin to enable and disable the

VOUT pin under normal operating conditions. As shown in

Figure 35, when a rising voltage on EN crosses the active threshold,

VOUT turns on. Conversely, when a falling voltage on EN crosses

the inactive threshold, VOUT turns off.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Figure 35. Typical EN Pin Operation

As shown in Figure 35, the EN pin has built-in hysteresis. This

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

EN pin as it passes through the threshold points.

The active and inactive thresholds of the EN pin are derived from

the VIN voltage. Therefore, these thresholds vary as the input

voltage changes. Figure 36 shows typical EN active and inactive

thresholds when the VIN voltage varies from 2.3 V to 5.5 V.

0.5

0.6

0.7

0.8

0.9

1.0

1.1

2.2

2.7

3.2

3.7

4.2

4.7

5.2

RISING

FALLING

Figure 36. Typical EN Pin Thresholds vs. Input Voltage

The ADP122/ADP123 utilize an internal soft start to limit the

in-rush current when the output is enabled. The start-up time

for the 2.8 V option is approximately 350 µs from the time the

EN active threshold is crossed to when the output reaches 90%

of its final value. As shown in Figure 37, the start-up time is

dependent on the output voltage setting and increases slightly

as the output voltage increases.

CH2 1.00V

M200µs

A CH1

3.08V

2

1

T

600.000µs

Figure 37. Typical Start-Up Time