Data Sheet

ADP172

APPLICATIONS INFORMATION

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Rev. F | 13 of 17

UNDERVOLTAGE LOCKOUT

The ADP172 has an internal undervoltage lockout circuit that disa-

bles all inputs and the output when the input voltage is less than

approximately 1.2 V. This ensures that the ADP172 inputs and the

output behave in a predictable manner during power-up.

ENABLE FEATURE

The ADP172 uses the EN pin to enable and disable the VOUT pin

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 threshold, VOUT

turns off.

Figure 27. ADP172 Typical EN Pin Operation

As shown in Figure 27, the EN pin has hysteresis built in. This

prevents on/off oscillations that can occur due to noise on the EN

pin as it passes through the threshold points.

The EN pin active/inactive thresholds are derived from the VIN volt-

age. Therefore, these thresholds vary with changing input voltage.

Figure 28 shows typical EN active/inactive thresholds when the

input voltage varies from 1.6 V to 3.6 V.

Figure 28. Typical EN Pin Thresholds vs. Input Voltage

The ADP172 uses an internal soft start to limit the inrush current

when the output is enabled. The start-up time for the 1.8 V option

is approximately 120 μs from the time the EN active threshold is

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

shown in Figure 29, the start-up time is dependent on the output

voltage setting.

Figure 29. Typical Start-Up Time

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP172 is protected against damage due to excessive power

dissipation by current and thermal overload protection circuits. The

ADP172 is designed to limit the current when the output load

reaches 450 mA (typical). When the output load exceeds 450 mA,

the output voltage is reduced to maintain a constant current limit.

Thermal overload protection is included, which limits the junction

temperature to a maximum of 150°C (typical). Under extreme con-

ditions (that is, high ambient temperature and power dissipation),

when the junction temperature starts to rise above 150°C, the

output is turned off, reducing the output current to 0. When the

junction temperature drops below 135°C, the output is turned on

again, and output current is restored to its nominal value.

Consider the case where a hard short from VOUT to GND occurs.

At first, the ADP172 limits the current so that only 450 mA is

conducted into the short. If self-heating of the junction is great

enough to cause its temperature to rise above 150°C, thermal

shutdown activates, turning off the output and reducing the output

current to 0. As the junction temperature cools and drops below

135°C, the output turns on and conducts 450 mA into the short,

again causing the junction temperature to rise above 150°C. This

thermal oscillation between 135°C and 150°C causes a current

oscillation between 450 mA and 0 mA, which continues as long as

the short remains at the output.

Current and thermal limit protections are intended to protect the

device against accidental overload conditions.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP172 must not exceed 125°C. To ensure that the junction tem-

perature stays below this maximum value, the user must be aware

of the parameters that contribute to junction temperature changes.

These parameters include ambient temperature, power dissipation

in the power device, and thermal resistances between the junction

assembly compounds used and the amount of copper to which the

GND pin of the package is soldered on the PCB. Table 6 shows