ADP121 datasheet(13/20 Pages) AD

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ADP121

Rev. 0 | Page 13 of 20

Equation 1 can be used to determine the worst-case capacitance

accounting for capacitor variation over temperature, compo-

nent tolerance, and voltage.

CEFF = CBIAS × (1 − TEMPCO) × (1 − TOL)

(1)

where:

CBIAS is the effective capacitance at the operating voltage.

TEMPCO is the worst-case capacitor temperature coefficient.

TOL is the worst-case component tolerance.

In this example, TEMPCO over −40°C to +85°C is assumed to

be 15% for an X5R dielectric. TOL is assumed to be 10%, and

CBIAS is 0.94 μF at 1.8 V from the graph in Figure 30.

Substituting these values in Equation 1 yields

CEFF = 0.94 μF × (1 − 0.15) × (1 − 0.1) = 0.719 μF

Therefore, the capacitor chosen in this example meets the

minimum capacitance requirement of the LDO over

temperature and tolerance at the chosen output voltage.

To guarantee the performance of the ADP121, 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 ADP121 has an internal undervoltage lockout circuit that

disables all inputs and the output when the input voltage is less

than approximately 2.2 V. This ensures that the inputs of the

ADP121 and the output behave in a predictable manner during

power-up.

ENABLE FEATURE

The ADP121 uses the EN pin to enable and disable the VOUT

pin under normal operating conditions. Figure 31 shows a

rising voltage on EN crossing the active threshold, and then

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

inactive threshold, VOUT turns off.

40ms/DIV

VIN = 5V

VOUT = 1.8V

CIN = COUT = 1µF

ILOAD = 100mA

VOUT

Figure 31. ADP121 Typical EN Pin Operation

As shown in Figure 31, 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/inactive thresholds of the EN pin are derived from

the VIN voltage. Therefore, these thresholds vary with changing

input voltage. Figure 32 shows typical EN active/inactive thresholds

when the input voltage varies from 2.3 V to 5.5 V.

1.10

1.05

1.00

0.95

0.90

0.85

0.80

0.75

0.70

2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 5.25 5.50

VIN (V)

EN ACTIVE

EN INACTIVE

Figure 32. Typical EN Pin Thresholds vs. Input Voltage

The ADP121 utilizes an internal soft start to limit the inrush

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

1.8 V option is approximately 120 μs from the time the EN

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

final value. The start-up time is somewhat dependant on the

output voltage setting and increases slightly as the output

voltage increases.

100

120

140

160

180

200

(µs)

1.2V

1.8V

3.3V

Figure 33. Typical Start-Up Time

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