ADP151ACBZ-2.75-R7(Rev0) View Datasheet(PDF) - Analog Devices
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ADP151ACBZ-2.75-R7 Datasheet PDF : 24 Pages
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ADP151
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP151 is protected against damage due to excessive
power dissipation by current and thermal overload protection
circuits. The ADP151 is designed to current limit when the
output load reaches 300 mA (typical). When the output load
exceeds 300 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 dissipation) 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 ground
occurs. At first, the ADP151 current limits, so that only 300 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 300 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
300 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 that junction temperatures do not exceed 125°C.
THERMAL CONSIDERATIONS
In most applications, the ADP151 does not dissipate much heat
due to its high efficiency. However, in 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.
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 5 shows typical θJA values of the 5-lead TSOT package for
various PCB copper sizes. Table 6 shows the typical ΨJB values of
the 5-lead TSOT and 4-ball WLCSP.
Table 5. Typical θJA Values
Copper Size (mm2)
TSOT
01
170
50
152
100
146
300
134
500
131
1 Device soldered to minimum size pin traces.
θJA (°C/W)
WLCSP
260
159
157
153
151
Table 6. Typical ΨJB Values
Model
TSOT
WLCSP
ΨJB (°C/W)
43
58
The junction temperature of the ADP151 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
PD = [(VIN − VOUT) × ILOAD] + (VIN × IGND)
(3)
where:
ILOAD is the load current.
IGND is the ground current.
VIN and VOUT 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:
TJ = TA + {[(VIN − VOUT) × ILOAD] × θJA}
(4)
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.
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 37 to Figure 50 show junction temperature calculations
for different ambient temperatures, load currents, VIN-to-VOUT
differentials, and areas of PCB copper.
To guarantee reliable operation, the junction temperature of
the ADP151 must not exceed 125°C. To ensure that the junction
temperature stays below this maximum value, the user must be
aware of the parameters that contribute to junction temperature
Rev. 0 | Page 15 of 24
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