ADP160/ADP161
Consider the case where a hard short from OUT to ground occurs.
At first, the ADP160/ADP161 current limits so that only ꢀ20 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 1ꢀ5°C, the output turns on and conducts ꢀ20 mA
into the short, again causing the junction temperature to rise
above 150°C. This thermal oscillation between 1ꢀ5°C and
150°C causes a current oscillation between ꢀ20 mA and 0 mA
that continues as long as the short remains at the output.
Table 9. Typical ΨJB Values
ΨJB (°C/W)
TSOT
WLCSP
42.8
58.4
The junction temperature of the ADP160/ADP161 can be
calculated from the following equation:
TJ = TA + (PD × θJA)
(2)
(ꢀ)
where:
TA is the ambient temperature.
PD is the power dissipation in the die, given by
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.
PD = [(VIN − VOUT) × ILOAD] + (VIN × IGND
)
where:
LOAD is the load current.
GND is the ground current.
I
I
V
THERMAL CONSIDERATIONS
IN and VOUT are input and output voltages, respectively.
In most applications, the ADP160/ADP161 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.
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}
(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 ꢀ7 to
Figure 44 show the junction temperature calculations for the
different ambient temperatures, load currents, VIN-to-VOUT
differentials, and areas of PCB copper.
When the junction temperature exceeds 150°C, the converter enters
thermal shutdown. It recovers only after the junction temperature
has decreased below 1ꢀ5°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.
In the case where the board temperature is known, use the
thermal characterization parameter, ΨJB, to estimate the junction
temperature rise (see Figure 45 and Figure 46). Maximum junction
temperature (TJ) is calculated from the board temperature (TB)
and power dissipation (PD) using the following formula:
To guarantee reliable operation, the junction temperature of the
ADP160/ADP161 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 (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 8 shows the typical θJA values of the 5-lead TSOT and the
4-ball WLCSP for various PCB copper sizes. Table 9 shows the
typical ΨJB value of the 5-lead TSOT and 4-ball WLCSP.
TJ = TB + (PD × ΨJB)
(5)
The typical value of ΨJB is 58°C/W for the 4-ball WLCSP package
and 4ꢀ°C/W for the 5-lead TSOT package.
140
MAXIMUM JUNCTION TEMPERATURE
120
100
80
Table 8. Typical θJA Values
60
θJA (°C/W)
Copper Size (mm2)
TSOT
170
152
146
134
131
WLCSP
260
159
157
153
40
01
20
I
I
I
= 1mA
= 10mA
= 50mA
I
I
I
= 100mA
= 150mA
= 200mA
LOAD
LOAD
LOAD
LOAD
LOAD
LOAD
50
100
300
500
0
0.3
0.8
1.3
1.8
2.3
– V
2.8
(V)
3.3
3.8
4.3
4.8
V
IN
OUT
151
Figure 37. 500 mm2 of PCB Copper, WLCSP, TA = 25°C
1 Device soldered to minimum size pin traces.
Rev. 0 | Page 15 of 20