Data Sheet

ADP1761

APPLICATIONS INFORMATION

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Rev. D | 16 of 21

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 mini-

mum 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 disa-

bles 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 the 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 be-

comes 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 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 pro-

tect 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 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 6 shows typical θJA values for the

16-lead LFCSP for various PCB copper sizes. Table 7 shows typical

ΨJB values for the 16-lead LFCSP.

Table 6. 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 7. 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 ADP1761, 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.

Figure 34 through Figure 39 show the junction temperature calcula-

tions for the different ambient temperatures, load currents, VIN to

VOUT differentials, and areas of PCB copper.