ADP221ACBZ-1818-R7 datasheet(13/20 Pages) AD

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Data Sheet

ADP220/ADP221

Rev. H | 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 31.

Substituting these values into 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 ADP220/ADP221, it is

imperative that the effects of dc bias, temperature, and toler-

ances on the behavior of the capacitors be evaluated for each

application.

UNDERVOLTAGE LOCKOUT

The ADP220/ADP221 have 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 ADP220/ADP221 and the output behave in a

predictable manner during power-up.

ENABLE FEATURE

The ADP220/ADP221 use the ENx pins to enable and disable

the VOUTx pins under normal operating conditions. Figure 32

shows a rising voltage on ENx crossing the active threshold,

then VOUTx turns on. When a falling voltage on ENx crosses the

inactive threshold, VOUTx turns off.

CH1 500mV BW CH2 500mV BW M10.0ms

A CH2

1.76V

T 27.40%

ENx

VOUTx

Figure 32. Typical ENx Pin Operation

As shown in Figure 32, the ENx pins have built-in hysteresis.

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

the ENx pins as it passes through the threshold points.

The active/inactive thresholds of the ENx pins are derived from

the VIN voltage. Therefore, these thresholds vary with changing

input voltage. Figure 33 shows typical ENx active/inactive thresh-

olds when the input voltage varies from 2.5 V to 5.5 V.

2.5

3.0

3.5

4.0

4.5

5.0

5.5

INPUT VOLTAGE (V)

1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

EN INACTIVE

EN ACTIVE

Figure 33. Typical ENx Pins Thresholds vs. Input Voltage

The ADP220/ADP221 utilize an internal soft start to limit the

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

for the 2.8 V option is approximately 220 µs from the time the

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

of its final value. The start-up time is somewhat dependent on

the output voltage setting and increases slightly as the output

voltage increases.

CH1 5.00V BW

CH2 2.00V BW

M40.0µs

A CH1

2.10V

T 9.80%

CH3 2.00V BW

Figure 34. Typical Start-Up Time

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