ADP120 datasheet(13/20 Pages) AD

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ADP120

Rev. A | Page 13 of 20

Therefore, the capacitor chosen in this example meets the

minimum capacitance requirement of the LDO over temper-

ature and tolerance at the chosen output voltage.

To guarantee the performance of the ADP120, it is imperative

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

behavior of the capacitors be evaluated for each application.

UNDERVOLTAGE LOCKOUT

The ADP120 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 and the

output of the ADP120 behave in a predictable manner during

power-up.

ENABLE FEATURE

The ADP120 uses the EN pin to enable and disable the VOUT

pin under normal operating conditions. As shown in Figure 31,

when a rising voltage on EN crosses the active threshold, 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. Typical EN Pin Operation

As shown in Figure 31, the EN pin has hysteresis built-in. This

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

EN pin as it passes through the threshold points.

The EN pin active/inactive thresholds 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 ADP120 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 dependent on the

output voltage setting and increases slightly as the output

voltage increases.

180

160

140

120

100

TIME (µs)

3.3V

1.8V

1.2V

Figure 33. Typical Start-Up Time

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