ADP1762 Data Sheet
Rev. C | 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 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 temp-
erature 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.