Data Sheet

ADP1761

THEORY OF OPERATION

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Rev. D | 13 of 21

ADJUSTABLE OUTPUT VOLTAGE

The output voltage of the ADP1761 can be set over a 0.5 V to 1.5 V

range. Connect a resistor (RADJ) from the VADJ pin to ground to set

the output voltage. To calculate the output voltage, use the following

equation:

VOUT=AD× RADJ×IADJ

(3)

where:

AD is the gain factor with a typical value of 3.0 between the VADJ

pin and the VOUT pin.

IADJ is the 50.0 µA constant current out of the VADJ pin.

ENABLE FEATURE

The ADP1761 uses the EN pin to enable and disable the VOUT

pins under normal operating conditions. As shown in Figure 27,

when a rising voltage on EN crosses the active threshold, VOUT

turns on. When a falling voltage on EN crosses the inactive thresh-

old, VOUT turns off.

Figure 27. Typical EN Pin Operation

As shown in Figure 28, the EN pin has hysteresis built in. This

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

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

Figure 28. Output Voltage vs. Typical EN Pin Voltage, VOUT = 1.3 V

POWER-GOOD (PG) FEATURE

The ADP1761 provides a power-good pin (PG) to indicate the

status of the output. This open-drain output requires an external

pull-up resistor that can be connected to VIN or VOUT. If the device

is in shutdown mode, current-limit mode, or thermal shutdown, or

if it falls below 90% of the nominal output voltage, PG immediately

transitions low. During soft start, the rising threshold of the power-

good signal is 95% of the nominal output voltage.

The open-drain output is held low when the ADP1761 has a suffi-

cient input voltage to turn on the internal PG transistor. An optional

soft start delay can be detected. The PG transistor is terminated via

a pull-up resistor to VOUT or VIN.

Power-good accuracy is 92.5% of the nominal regulator output

voltage when this voltage is rising, with a 95% trip point when this

voltage is falling.

Regulator input voltage brownouts or glitches trigger a power no

good if VOUT falls below 92.5%.

A normal power-down triggers a power good when VOUT is at 95%.

Figure 29. Typical PG Behavior vs. VOUT, VIN Rising (VOUT = 1.3 V)