ADP120
To guarantee reliable operation, the junction temperature of
the ADP120 and ADP120-1 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 6 shows typical θJA values of the
5-lead TSOT and 4-ball WLCSP packages for various PCB copper
sizes. Table 7 shows the typical ΨJB value of the 5-lead TSOT and
4-ball WLCSP.
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP120 and ADP120-1 are protected against damage due
to excessive power dissipation by current and thermal overload
protection circuits. The ADP120 and ADP120-1 are designed to
current limit when the output load reaches 150 mA (typical).
When the output load exceeds 150 mA, the output voltage
reduces to maintain a constant current limit.
Thermal overload protection is built-in limiting the junction
temperature to a maximum of 150°C (typical). Under extreme
conditions (that is, high ambient temperature and power dissi-
pation) when the junction temperature starts to rise above 150°C,
the output turns off, reducing the output current to zero. When
the junction temperature drops below 135°C, the output turns
on again restoring output current to its nominal value.
Table 6. Typical θJA Values
θJA (°C/W)
Copper Size (mm2)
01
ADP120
170
ADP120-1
260
Consider the case where a hard short from VOUT to GND
occurs. At first, the ADP120 and ADP120-1 current limit
conducting only 150 mA 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 150 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
150 mA and 0 mA that continues as long as the short remains at
the output.
50
152
159
100
300
500
146
134
131
157
153
151
1 Device soldered to minimum size pin traces.
Table 7. Typical ΨJB Values
ΨJB (°C/W)
ADP120, TSOT
ADP120-1, WLCSP
58.4
42.8
The junction temperature of the ADP120 and ADP120-1 can be
calculated from the following equation:
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
to prevent junction temperatures from exceeding 125°C.
TJ = TA + (PD × θJA)
where:
(2)
TA is the ambient temperature.
PD is the power dissipation in the die, given by
THERMAL CONSIDERATIONS
In most applications, the ADP120 and ADP120-1 do not dissi-
pate much heat due to their high efficiency. However, in
PD = [(VIN − VOUT) × ILOAD] + (VIN × IGND
)
(3)
applications with high ambient temperature, 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.
where:
I
I
LOAD is the load current.
GND is the ground current.
V
IN and VOUT are input and output voltages, respectively.
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.
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. The following
figures show junction temperature calculations for different
ambient temperatures, load currents, VIN-to-VOUT differentials,
and areas of PCB copper.
Rev. 0 | Page 14 of 20
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