ADP121 Analog Devices, ADP121 Datasheet - Page 13

• Manufacturer Part Number: ADP121
• Description: 150 mA, Low Quiescent Current, CMOS Linear Regulator in 5-Lead TSOT or 4-Ball WLCSP
• Manufacturer: Analog Devices

Data Sheet
Equation 1 can be used to determine the worst-case capacitance
accounting for capacitor variation over temperature, compo-
nent tolerance, and voltage.
where:
C
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
Substituting these values in Equation 1 yields
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.
BIAS
BIAS
C
C
is 0.94 μF at 1.8 V from the graph in Figure 30.
is the effective capacitance at the operating voltage.
EFF
EFF
= C
= 0.94 μF × (1 − 0.15) × (1 − 0.1) = 0.719 μF
V
V
C
I
LOAD
IN
OUT
IN
BIAS
= 5V
= C
Figure 31. ADP121 Typical EN Pin Operation
= 1.8V
= 100mA
OUT
× (1 − TEMPCO) × (1 − TOL)
= 1µF
40ms/DIV
EN
VOUT
Rev. E | Page 13 of 20
(1)
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.
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.
1.10
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
6
5
4
3
2
1
0
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
0
Figure 32. Typical EN Pin Thresholds vs. Input Voltage
20
40
Figure 33. Typical Start-Up Time
60
80
EN ACTIVE
TIME (µs)
EN INACTIVE
V
100
EN
IN
(V)
120
3.3V
1.8V
1.2V
140
160
180
ADP121
200