Datasheet

ADP172 Data Sheet
Rev. D | Page 14 of 20
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP172 is protected against damage due to excessive
power dissipation by current and thermal overload protection
circuits. The ADP172 is designed to limit the current when the
output load reaches 450 mA (typical). When the output load
exceeds 450 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 power dissip-
ation), when the junction temperature starts to rise above 150°C,
the output is turned off, reducing the output current to 0. 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 ADP172 limits the current so that only 450 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 0. As the junction temperature cools and
drops below 135°C, the output turns on and conducts 450 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 450 mA and 0 mA, which
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.
THERMAL CONSIDERATIONS
To guarantee reliable operation, the junction temperature of the
ADP172 must not exceed 125°C. To ensure that the junction temp-
erature 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 used and the amount of
copper to which the GND pin of the package is soldered on the
PCB. Table 6 shows typical θ
JA
values of the 4-ball WLCSP package
for various PCB copper sizes.
Table 6. Typical θ
JA
Values
Copper Size (mm
2
) θ
JA
(°C/W)
0
1
260
50 159
100 157
300 153
500
151
1
Device soldered to minimum size pin traces.
The junction temperature of the ADP172 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:
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 30 to Figure 35 show junction temperature calculations
for different ambient temperatures, load currents, V
IN
to V
OUT
differentials, and areas of PCB copper.
0
20
40
60
80
100
120
140
0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5
V
IN
– V
OUT
(V)
JUNCTION TEMPERATURE, T
J
(°C)
I
LOAD
= 300mA
T
J
MAX
I
LOAD
= 1mA
I
LOAD
= 50mA
I
LOAD
= 100mA
I
LOAD
= 200mA
I
LOAD
= 10mA
06111-030
Figure 30. 500 mm
2
of PCB Copper, T
A
= 25°C