ADP7104ARDZ-3.3-R7 datasheet(18/28 Pages) AD

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ADP7104

Data Sheet

Rev. B | Page 18 of 28

PROGRAMABLE UNDERVOLTAGE LOCKOUT (UVLO)

The ADP7104 uses the EN/UVLO pin to enable and disable

the VOUT pin under normal operating conditions. As shown

in Figure 64, when a rising voltage on EN crosses the upper

threshold, VOUT turns on. When a falling voltage on EN/

UVLO crosses the lower threshold, VOUT turns off. The

hysteresis of the EN/UVLO threshold is determined by

the Thevenin equivalent resistance in series with the EN/

UVLO pin.

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

1.00

VOUT, EN RISE

VOUT, EN FALL

Figure 64. Typical VOUT Response to EN Pin Operation

The upper and lower thresholds are user programmable and can

be set using two resistors. When the EN/UVLO pin voltage is

below 1.22 V, the LDO is disabled. When the EN/UVLO pin

voltage transitions above 1.22 V, the LDO is enabled and 10 µA

hysteresis current is sourced out the pin raising the voltage, thus

providing threshold hysteresis. Typically, two external resistors

program the minimum operational voltage for the LDO. The

resistance values, R1 and R2 can be determined from:

R1 = VHYS/10 μA

R2 = 1.22 V × R1/(VIN − 1.22 V)

where:

VIN is the desired turn-on voltage.

VHYS is the desired EN/UVLO hysteresis level.

Hysteresis can also be achieved by connecting a resistor in

series with EN/UVLO pin. For the example shown in Figure 65,

the enable threshold is 2.44 V with a hysteresis of 1 V.

VOUT = 5V

VIN = 8V

VOUT

VIN

GND

SENSE

EN/

UVLO

100k

Ω

100k

Ω

100k

Ω

COUT

1µF

CIN

1µF

OFF

Figure 65. Typical EN Pin Voltage Divider

Figure 64 shows the typical hysteresis of the EN/UVLO pin.

This prevents on/off oscillations that can occur due to noise

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

The ADP7104 uses an internal soft-start to limit the inrush

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

3.3 V option is approximately 580 μs from the time the EN

active threshold is crossed to when the output reaches 90%

of its final value. As shown in Figure 66, the start-up time is

dependent on the output voltage setting.

TIME (µs)

500

1000

1500

2000

3.3V

ENABLE

Figure 66. Typical Start-Up Behavior

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