ADP1761

Data Sheet

Rev. 0 | Page 14 of 18

Therefore, the capacitor chosen in this example meets the

minimum capacitance requirement of the LDO over temperature

and tolerance at the chosen output voltage.

To guarantee the performance of the ADP1761, it is imperative

that the effects of dc bias, temperature, and tolerances on the

behavior of the capacitors be evaluated for each application.

UNDERVOLTAGE LOCKOUT

The ADP1761 has an internal undervoltage lockout circuit that

disables all inputs and the output when the input voltage is less

than approximately 1.06 V. The UVLO ensures that the ADP1761

inputs and the output behave in a predictable manner during

power-up.

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP1761 is protected against damage due to excessive power

dissipation by current-limit and thermal overload protection

circuits. The ADP1761 is designed to reach the current limit

when the output load reaches 2 A (typical). When the output

load exceeds 2 A, 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 conditions (that is, high ambient temperature and power

dissipation) when the junction temperature begins to rise above

150°C, the output is turned off, reducing the output current to

zero. When the junction temperature drops below 135°C (typical),

the output is turned on again, and the output current is restored

to its nominal value.

Consider the case where a hard short from VOUT to ground

occurs. At first, the ADP1761 reaches the current limit so that

only 2 A is conducted into the short circuit. If self-heating of the

junction becomes great enough to cause its temperature to rise

above 150°C, thermal shutdown activates, turning off the

output and reducing the output current to zero. As the junction

temperature cools and drops below 135°C, the output turns on

and conducts 2 A into the short circuit, again causing the junction

temperature to rise above 150°C. This thermal oscillation between

135°C and 150°C causes a current oscillation between 2 A and

0 A that continues as long as the short circuit remains at the

output.

Current-limit and thermal overload protections are intended to

protect the device against accidental overload conditions. For

reliable operation, limit the device power dissipation externally so

that junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP1761 must not exceed 125°C. To ensure that the junction

temperature stays below this maximum value, the user needs to be

aware of the parameters that contribute to junction temperature

changes. These parameters include ambient temperature, power

dissipation in the power device, and thermal resistance between

on the package assembly compounds used and the amount of

copper to which the GND pin and the exposed pad (EPAD) of the

for the 16-lead LFCSP for various PCB copper sizes. Table 8

25

138.1

100

102.9

500

76.9

1000

67.3

6400

56

100

33.3

500

28.9

1000

28.5

To calculate the junction temperature of the ADP1761, use the

following equation:

(5)

where:

(6)

where:

As shown in Equation 6, for a given ambient temperature and

computed power dissipation, a minimum copper size requirement

exists for the PCB to ensure that the junction temperature does

not rise above 125°C.