ADP170 datasheet(13/20 Pages) AD

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ADP170/ADP171

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

to be 10%, and CBIAS is 0.94 μF at 1.8 V, as shown in Figure 29.

Substituting these values in 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 temper-

ature and tolerance at the chosen output voltage.

To guarantee the performance of the ADP170/ADP171, it is

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

ances on the behavior of the capacitors are evaluated for each

application.

UNDERVOLTAGE LOCKOUT

The ADP170/ADP171 have 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

ADP170/ADP171 inputs and the output behave in a predictable

manner during power-up.

ENABLE FEATURE

The ADP170/ADP171 use the EN pin to enable and disable the

VOUT pin under normal operating conditions. As shown in

Figure 30, 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.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

VEN

Figure 30. ADP170/ADP171 Typical EN Pin Operation

As shown in Figure 30, 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 31 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

VIN (V)

EN ACTIVE

EN INACTIVE

Figure 31. Typical EN Pin Thresholds vs. Input Voltage

The ADP170/ADP171 utilize 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 32, the start-up time is

dependent on the output voltage setting.

CH1 1.00V

CH2 1.00V

M20.0µs

A CH1

2.72V

79.8000µs

VOUT = 0.8V

VOUT = 1.8V

VOUT = 3.0V

Figure 32. Typical Start-Up Time

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