Data Sheet ADP130
Rev. C | Page 15 of 20
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP130 is protected against damage due to excessive power
dissipation by current limit and thermal overload protection
circuits. The ADP130 is designed to current limit when the
output load reaches 550 mA (typical). When the output load
exceeds 550 mA, the output voltage is reduced to maintain
a constant current limit.
Thermal overload protection 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 starts to rise above 150°C, the output is
turned off, reducing 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 nominal value.
Consider the case where a hard short from VOUT to GND occurs.
At first, the ADP130 current limits so that only 550 mA is con-
ducted 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 550 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 550 mA and 0 mA that continues as long
as the short remains at the output.
Current limit and thermal overload protections protect the device
against accidental overload conditions. For reliable operation,
device power dissipation must be externally limited so that
junction temperatures do not exceed 125°C.
THERMAL CONSIDERATIONS
To guarantee reliable operation, the junction temperature of the
ADP130 must not exceed 125°C. To ensure that the junction tem-
perature 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 resistances between
the junction and ambient air (θ
JA
). The value of θ
JA
is dependent on
the package assembly compounds used and the amount of copper
to which the GND pins of the package are soldered on the PCB.
Table 6 shows typical θ
JA
values of the 5-lead TSOT package for
various PCB copper sizes.
Table 6. Typical θ
JA
Values for Specified PCB Copper Sizes
Copper Size (mm
2
) θ
JA
(°C/W)
0
1
170
50 152
100 146
300 134
500 131
1
Device soldered to minimum size pin traces.
The junction temperature of the ADP130 can be 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:
V
IN
and V
OUT
are the input and output voltages, respectively.
I
LOAD
is the load current.
I
GND
is the ground current.
Power dissipation due to ground current is quite small and can
be ignored. Therefore, the junction temperature equation can
be simplified as follows:
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,
a minimum copper size requirement exists for the PCB to ensure
that the junction temperature does not rise above 125°C. Figure 40
through Figure 46 show junction temperature calculations for
different ambient temperatures, load currents, V
IN
to V
OUT
differentials, and areas of PCB copper.
