Datasheet

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.
C
EFF
= C
BIAS
× (1 − TEMPCO) × (1 − TOL) (1)
where:
C
BIAS
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
C
BIAS
is 0.94 μF at 1.8 V from the graph in Figure 31.
Substituting these values into Equation 1 yields
C
EFF
= 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 V
OUTx
turns on. When a falling voltage on ENx crosses the
inactive threshold, V
OUTx
turns off.
1
CH1 500mV
B
W
CH2 500mV
B
W
M10.0ms A CH2 1.76V
T 27.40%
T
07572-032
ENx
V
OUTx
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
ENx PINS THRESHOLD (V)
0.60
EN INACTIVE
EN ACTIVE
07572-033
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.
1
CH1 5.00V
B
W
CH2 2.00V
B
W
M40.0µs A CH1 2.10V
T 9.80%
T
CH3 2.00V
B
W
2
3
07572-034
Figure 34. Typical Start-Up Time