ADP7157ARDZ-02-R7 datasheet(17/24 Pages) AD

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ADP7157

Data Sheet

Rev. A | Page 16 of 23

PROGRAMMABLE PRECISION ENABLE

The ADP7157 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.

The ADP7157 includes the discharge resistor on each VOUT,

VREG, VREF, and BYP pin. These resistors are turned on when

the device is disabled, helping to quickly discharge the associated

capacitor.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1.00

1.05

1.10

1.15

1.20

1.25

1.30

EN PIN VOLTAGE (V)

–40°C

–5°C

+25°C

+85°C

+125°C

Figure 49. Typical VOUT Response to EN Pin Operation

0.5

1.0

1.5

2.0

2.5

3.0

3.5

–2

–1

TIME (ms)

VOUT

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

VOUT = 3.3 V, VIN = 5 V, CBYP = 1 µF

1.100

1.125

1.150

1.175

1.200

1.225

1.250

2.5

3.0

3.5

4.0

4.5

5.0

5.5

INPUT VOLTAGE (V)

RISING

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 × (VEN − 1.22 V)/1.22 V

where:

REN2 typically ranges from 10 kΩ to 100 kΩ.

VEN 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.

BYP

VREG

ADP7157

REF_SENSE

VIN

VOUT

VOUT_SENSE

COUT

10µF

VOUT = 3.3V

REF

GND

CIN

10µF

CREG

1µF

CBYP

1µF

CREF

1µF

VOUT = 1.2V × (R1 + R2)/R2

1kΩ < R2 < 200kΩ

VIN = 3.8V

REN2

100kΩ

REN1

100kΩ

OFF

Figure 52. Typical EN Pin Voltage Divider

Figure 52 shows the typical hysteresis of the EN pin. This

hysteresis prevents on/off oscillations that can occur due to

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

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