ADP172

Rev. B | Page 13 of 20

In this example, the worst-case temperature coefficient

(TEMPCO) over −40°C to +85°C is assumed to be 15% for an

X5R dielectric. The tolerance of the capacitor (TOL) is assumed

Substituting these values in Equation 1 yields

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 ADP172, 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 ADP172 has an internal undervoltage lockout circuit that

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

than approximately 1.2 V. This ensures that the ADP172 inputs

and the output behave in a predictable manner during power-up.

ENABLE FEATURE

The ADP172 uses the EN pin to enable and disable the VOUT

pin under normal operating conditions. As shown in Figure 27,

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.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Figure 27. ADP172 Typical EN Pin Operation

As shown in Figure 27, 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 28 shows typical EN active/inactive thresholds

when the input voltage varies from 1.6 V to 3.6 V.

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

EN ACTIVE

EN INACTIVE

Figure 28. Typical EN Pin Thresholds vs. Input Voltage

The ADP172 uses 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. As shown in Figure 29, the start-up time is

dependent on the output voltage setting.

CH2 1.00V

B

W

M20.0µs

A CH1

2.72V

1

2

T

79.8000µs

EN

Figure 29. Typical Start-Up Time