ADM7171ACPZ-4.2-R7 datasheet(19/24 Pages) AD

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ADM7171

Data Sheet

Rev. C | Page 18 of 23

The tolerance of the capacitor (TOL) is assumed to be 10%, and

CBIAS is 4.35 μF at 3.0 V, as shown in Figure 57.

Substituting these values in Equation 1 yields

CEFF = 4.35 μF × (1 − 0.15) × (1 − 0.1) = 3.33 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 of 3.0 V.

To guarantee the performance of the ADM7171, it is imperative

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

behavior of the capacitors be evaluated for each application.

PROGRAMMABLE PRECISION ENABLE

The ADM7171 uses the EN pin to enable and disable the VOUT

pins under normal operating conditions. As shown in Figure 58,

when a rising voltage on EN crosses the upper threshold,

typically 1.2 V, VOUT turns on. When a falling voltage on EN

crosses the lower threshold, typically 1.1 V, VOUT turns off. The

hysteresis of the EN threshold is approximately 100 mV.

Figure 58. Typical VOUT Response to EN Pin Operation

The upper and lower thresholds are user programmable and can

be set higher than the nominal 1.2 V threshold by using two

resistors. The resistance values, REN1 and REN2, can be

determined from

REN1 = REN2 × (VIN − 1.2 V)/1.2 V

where:

REN2 is nominally 10 kΩ to 100 kΩ.

VIN is the desired turn-on voltage.

The hysteresis voltage increases by the factor

(REN1 + REN2)/REN1

For the example shown in Figure 59, the enable threshold is

3.6 V with a hysteresis of 300 mV.

Figure 59. Typical EN Pin Voltage Divider

Figure 58 shows the typical hysteresis of the EN pin. This

prevents on/off oscillations that can occur due to noise

on the EN pin as it passes through the threshold points.

UNDERVOLTAGE LOCKOUT

The ADM7171 also incorporates an internal undervoltage

lockout circuit to disable the output voltage when the input

voltage is less than the minimum input voltage rating of the

regulator. The upper and lower thresholds are internally fixed

with about 200 mV of hysteresis. This hysteresis prevents on/off

oscillations that can occur when caused by noise on the input

voltage as it passes through the threshold points.

SOFT START

The ADM7171 uses an internal soft start (SS pin open) to limit the

inrush current when the output is enabled. The start-up time for

the 5.0 V option is approximately 380 μs from the time the EN

active threshold is crossed to when the output reaches 90% of its

final value. As shown in Figure 60, the start-up time is nearly

independent of the output voltage setting.

Figure 60. Typical Start-Up Behavior

VEN (V)

1.30

1.25

1.20

1.15

1.10

1.05

1.00

0.5

1.0

1.5

2.0

2.5

3.0

VOUT

SENSE

VIN

ADM7171

GND

CIN

4.7µF

COUT

4.7µF

OFF

VOUT = 5.0V

VIN = 6.0V

REN1

200kΩ

REN2

100kΩ

VOUT

VIN

TIME (ms)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.5

1.0

2.0

3.0

4.0

5.0

1.5

2.5

3.5

4.5

VEN

2.5V

3.0V

5.0V

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