Datasheet

ADP160/ADP161/ADP162/ADP163 Data Sheet
Rev. H | Page 16 of 24
Consider the case where a hard short from OUT to ground
occurs. At first, the ADP16x current limit so that only 320 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 tempera-
ture cools and drops below 135°C, the output turns on and
conducts 320 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
320 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 junction temperatures do not exceed 125°C.
THERMAL CONSIDERATIONS
In most applications, the ADP16x do not dissipate much heat due
to their high efficiency. However, in applications with high ambient
temperature and high supply voltage to output voltage differential,
the heat dissipated in the package is large enough that it can 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 ADP16x must not exceed 125°C. To ensure 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 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 8 shows the typical θ
JA
values of the 5-lead TSOT and the
4-ball WLCSP for various PCB copper sizes. Table 9 shows the
typical Ψ
JB
value of the 5-lead TSOT and 4-ball WLCSP.
Table 8. Typical θ
JA
Values
θ
JA
(°C/W)
Copper Size (mm
2
) TSOT WLCSP
0
1
170 260
50 152 159
100 146 157
300 134 153
500
131
151
1
Device soldered to minimum size pin traces.
Table 9. Typical Ψ
JB
Values
Ψ
JB
(°C/W)
TSOT WLCSP
42.8 58.4
The junction temperature of the ADP16x 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
the junction temperature does not rise above 125°C. Figure 41 to
Figure 48 show the junction temperature calculations for the
different ambient temperatures, load currents, V
IN
-to-V
OUT
differentials, and areas of PCB copper.
In the case where the board temperature is known, use the
thermal characterization parameter, Ψ
JB
, to estimate the junction
temperature rise (see Figure 49 and Figure 50). Maximum
junction temperature (T
J
) is calculated from the board
temperature (T
B
) and power dissipation (P
D
) using the
following formula:
T
J
= T
B
+ (P
D
× Ψ
JB
) (5)
The typical value of Ψ
JB
is 58°C/W for the 4-ball WLCSP package
and 43°C/W for the 5-lead TSOT package.
140
120
100
80
60
40
20
0
0.3 4.84.33.83.32.82.31.81.30.8
V
IN
– V
OUT
(V)
JUNCTION TEMPERATURE, T
J
(°C)
MAXIMUM JUNCTION TEMPERATURE
I
LOAD
= 1mA
I
LOAD
= 10mA
I
LOAD
= 50mA
I
LOAD
= 100mA
I
LOAD
= 150mA
I
LOAD
= 200mA
08628-039
Figure 41. 500 mm
2
of PCB Copper, WLCSP, T
A
= 25°C