ADP1762

Data Sheet

Rev. D | Page 14 of 19

Substituting these values in Equation 4 yields

CEFF = 10 μF × (1 − 0.15) × (1 − 0.1) = 7.65 μF

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 ADP1762, 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 ADP1762 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 ADP1762

inputs and the output behave in a predictable manner during

power-up.

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP1762 is protected against damage due to excessive power

dissipation by current-limit and thermal overload protection

circuits. The ADP1762 is designed to reach the current limit

when the output load reaches 3 A (typical). When the output

load exceeds 3 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

the nominal value.

Consider the case where a hard short from VOUT to ground

occurs. At first, the ADP1762 reaches the current limit so that

only 3 A is conducted into the short circuit. If self heating of the

junction becomes great enough to cause the 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 3 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

3 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 externally so that

junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of

the ADP1762 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 the junction and ambient air (θJA).

The θJA value is dependent on the package assembly compounds

used and the amount of copper to which the GND pin and the

exposed pad (EPAD) of the package are soldered on the PCB.

Table 7 shows typical θJA values for the 16-lead LFCSP for

various PCB copper sizes. Table 8 shows typical ΨJB values for

the 16-lead LFCSP.

Table 7. Typical θJA Values

Copper Size (mm2)

θJA (°C/W), LFCSP

25

138.1

100

102.9

500

76.9

1000

67.3

6400

56

Table 8. Typical ΨJB Values

Copper Size (mm2)

ΨJB (°C/W) at 1 W

100

33.3

500

28.9

1000

28.5

To calculate the junction temperature of the ADP1762, use the

following equation:

TJ = TA + (PD × θJA)

(5)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

PD = ((VIN − VOUT) × ILOAD) + (VIN × IGND)

(6)

where:

VIN and VOUT are the input and output voltages, respectively.

ILOAD is the load current.

IGND is the ground current.

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.