ADM7154ACPZ-3.3-R7 datasheet(17/23 Pages) AD

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Data Sheet

ADM7154

Rev. B | Page 17 of 23

PROGRAMMABLE PRECISION ENABLE

The ADM7154 uses the EN pin to enable and disable the

VOUT pin under normal operating conditions. As shown in

Figure 49, when a rising voltage on EN crosses the upper threshold,

nominally 1.22 V, VOUT turns on. When a falling voltage on EN

crosses the lower threshold, nominally 1.13 V, VOUT turns off.

The hysteresis of the EN threshold is approximately 90 mV.

EN PIN VOLTAGE (V)

1.3

1.2

1.1

1.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

–40°C

–5°C

+25°C

+85°C

+125°C

Figure 49. Typical VOUT Response to EN Pin Operation

TIME (ms)

4.0

3.6

3.2

2.8

2.4

2.0

1.6

1.2

0.8

0.4

3.5

3.0

2.5

2.0

1.5

1.0

0.5

ENABLE (VEN)

VOUT

Figure 50. Typical VOUT Response to EN Pin Operation (VEN),

VOUT = 3.3 V, VIN = 5 V, CBYP = 1 µF

INPUT VOLTAGE (V)

5.5

5.0

4.5

4.0

3.5

3.0

2.5

1.100

1.250

1.225

1.200

1.175

1.150

1.125

–40°C RISING

+25°C RISING

+125°C RISING

–40°C FALLING

+25°C FALLING

+125°C FALLING

Figure 51. Typical EN Threshold vs. Input Voltages (VIN) for Different

Temperatures

The upper and lower thresholds are user programmable and can

be set higher than the nominal 1.22 V threshold by using two

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

determined from

REN1 = REN2 × (VIN − 1.22 V)/1.22 V

where:

REN2 typically ranges from 10 kΩ to 100 kΩ.

VIN is the desired turn-on voltage.

The hysteresis voltage increases by the factor

(REN1 + REN2)/REN2

For the example shown in Figure 52, the EN threshold is 2.44 V

with a hysteresis of 200 mV.

VOUT

REF_SENSE

REF

VIN

ADM7154

GND

CREF

1µF

CIN

10µF

REN1

100kΩ

REN2

100kΩ

COUT

10µF

OFF

VIN = 2.3V

VOUT = 1.8V

BYP

CBYP

1µF

VREG

CREG

10µF

Figure 52. Typical EN Pin Voltage Divider

Figure 52 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.

START-UP TIME

The ADM7154 uses an internal soft start to limit the inrush current

when the output is enabled. The start-up time for a 3.3 V output is

approximately 1.2 ms from the time the EN active threshold is

crossed to when the output reaches 90% of the final value.

The rise time in seconds of the output voltage (10% to 90%) is

approximately

0.0012 × CBYP

where CBYP is in microfarads.

TIME (ms)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

ENABLE (VEN)

CBYP = 1µF

CBYP = 3.3µF

CBYP = 10µF

Figure 53. Typical Start-Up Behavior with CBYP = 1 µF to 10 µF

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