Data Sheet ADP7118
Rev. D | Page 17 of 23
CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP7118 is protected against damage due to excessive
power dissipation by current and thermal overload protection
circuits. The ADP7118 is designed to current limit when the
output load reaches 400 mA (typical). When the output load
exceeds 400 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 conditions (that is, high ambient temperature and/or
high power dissipation) when the junction temperature starts
to rise above 150°C, the output is turned off, reducing the
output current to zero. When the junction temperature drops
below 135°C, the output is turned on again, and output current
is restored to the operating value.
Consider the case where a hard short from VOUT to ground
occurs. At first, the ADP7118 current limits, so that only 400 mA
is conducted into the short. If self heating of the junction is
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
400 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 400 mA and
0 mA that continues as long as the short remains at the output.
Current and thermal limit protections protect the device against
accidental overload conditions. For reliable operation, device
power dissipation must be externally limited so that the junction
temperature does not exceed 125°C.
THERMAL CONSIDERATIONS
In applications with a low input-to-output voltage differential,
the ADP7118 does not dissipate much heat. However, in
applications with high ambient temperature and/or high input
voltage, the heat dissipated in the package may become large
enough to cause the junction temperature of the die to exceed
the maximum junction temperature of 125°C.
When the junction temperature exceeds 150°C, the converter
enters thermal shutdown. It recovers only after the junction
temperature has decreased below 135°C to prevent any permanent
damage. Therefore, thermal analysis for the chosen application
is very important to guarantee reliable performance over all
conditions. The junction temperature of the die is the sum of
the ambient temperature of the environment and the temperature
rise of the package due to the power dissipation, as shown in
Equation 2.
To guarantee reliable operation, the junction temperature of the
ADP7118 must not exceed 125°C. To ensure that the junction
temperature 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 and ambient air (θJA). The θJA number is
dependent on the package assembly compounds that are used
and the amount of copper used to solder the package GND pins
to the PCB.
Table 6 shows typical θJA values of the 8-lead SOIC, 6-lead LFCSP,
and 5-lead TSOT packages for various PCB copper sizes. Table 7
shows the typical ΨJB values of the 8-lead SOIC, 6-l e a d L F C S P,
and 5-lead TSOT.
Table 6. Typical θJA Values
Copper Size (mm2)
θJA (°C/W)
LFCSP SOIC TSOT
251 182.8 N/A2 N/A2
50 N/A2 181.4 152
100 142.6 145.4 146
500 83.9 89.3 131
1000 71.7 77.5 N/A2
6400 57.4 63.2 N/A2
1 Device soldered to minimum size pin traces.
2 N/A means not applicable.
Table 7. Typical ΨJB Values
Model ΨJB (°C/W)
6-Lead LFCSP 24
5-Lead TSOT 43
To calculate the junction temperature of the ADP7118, use
Equation 1.
TJ = TA + (PD × θJA)
where:
TA is the ambient temperature.
PD is the power dissipation in the die, given by
PD = [(VIN − VOUT) × ILOAD] + (VIN × IGND) (10)
where:
VIN and VOUT are input and output voltages, respectively.
ILOAD is the load current.
IGND is the ground current.
Power dissipation due to ground current is quite small and can
be ignored. Therefore, the junction temperature equation
simplifies to the following:
TJ = TA + {[(VIN − VOUT) × ILOAD] × θJA} (11)
As shown in Equation 4, for a given ambient temperature, input-
to-output voltage differential, and continuous load current,
there exists a minimum copper size requirement for the PCB
to ensure that the junction temperature does not rise above 125°C.
Figure 52 to Figure 60 show junction temperature calculations
for different ambient temperatures, power dissipation, and areas
of PCB copper.