Datasheet

Data Sheet ADP7102
Rev. C | Page 21 of 28
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP7102 is protected against damage due to excessive
power dissipation by current and thermal overload protection
circuits. The ADP7102 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 its operating value.
Consider the case where a hard short from VOUT to ground
occurs. At first, the ADP7102 current limits, so that only 400 mA
is conducted into the short. If self heating of the junction is
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 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 are intended to protect
the device against accidental overload conditions. For reliable
operation, device power dissipation must be externally limited
so the junction temperature does not exceed 125°C.
THERMAL CONSIDERATIONS
In applications with low input-to-output voltage differential,
the ADP7102 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 that it causes 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 tempera-
ture rise of the package due to the power dissipation, as shown
in Equation 2.
To guarantee reliable operation, the junction temperature
of the ADP7102 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
(θ
JA
). The θ
JA
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 θ
JA
values of the 8-lead SOIC and 8-lead
LFCSP packages for various PCB copper sizes. Table 7 shows
the typical Ψ
JB
values of the 8-lead SOIC and 8-lead LFCSP.
Table 6. Typical θ
JA
Values
Copper Size (mm
2
)
θ
JA
(°C/W)
LFCSP SOIC
25
1
165.1 167.8
100 125.8 111
500 68.1 65.9
1000 56.4 56.1
6400 42.1 45.8
1
Device soldered to minimum size pin traces.
Table 7. Typical Ψ
JB
Values
Model Ψ
JB
(°C/W)
LFCSP 15.1
SOIC 31.3
The junction temperature of the ADP7102 is calculated from
the following equation:
T
J
= T
A
+ (P
D
× θ
JA
) (2)
where:
T
A
is the ambient temperature.
P
D
is the power dissipation in the die, given by
P
D
= [(V
IN
− V
OUT
) × I
LOAD
] + (V
IN
× I
GND
) (3)
where:
I
LOAD
is the load current.
I
GND
is the ground current.
V
IN
and V
OUT
are input and output voltages, respectively.
Power dissipation due to ground current is quite small and
can be ignored. Therefore, the junction temperature equation
simplifies to the following:
T
J
= T
A
+ {[(V
IN
− V
OUT
) × I
LOAD
] × θ
JA
} (4)
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 71 to Figure 78 show junction temperature
calculations for different ambient temperatures, power dissipa-
tion, and areas of PCB copper.