Table 6‘ Typical e A Values
Data Sheet ADM7150
CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADM7150 is protected against damage due to excessive
power dissipation by current and thermal overload protection
circuits. The ADM7150 is designed to current-limit when the
output load reaches 1.2 A (typical). When the output load
exceeds 1.2 A, 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 155°C (typical). Under
extreme conditions (that is, high ambient temperature and/or
high power dissipation) when the junction temperature starts to
rise above 155°C, the output is turned off, reducing the output
current to zero. When the junction temperature drops below
140°C, the output is turned on again, and output current is
restored to its operating value.
Consider the case where a hard short from VOUT to GND occurs.
At first, the ADM7150 current limits, so that only 1.2 A is
conducted into the short. If self heating of the junction is great
enough to cause its temperature to rise above 155°C, thermal
shutdown activates, turning off the output and reducing the
output current to zero. As the junction temperature cools and
drops below 140°C, the output turns on and conducts 1.2 A into
the short, again causing the junction temperature to rise above
155°C. This thermal oscillation between 140°C and 155°C
causes a current oscillation between 1.2 A and 0 mA that
continues as long as the short remains at the output.
Current-limit 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 the junction temperature does not exceed 150°C.
THERMAL CONSIDERATIONS
In applications with low input to output voltage differential, the
ADM7150 does not dissipate much heat. However, in applications
with high ambient temperature and/or high input voltage, the
heat dissipated in the package may become large enough that it
causes the junction temperature of the die to exceed the maximum
junction temperature of 150°C.
When the junction temperature exceeds 155°C, the converter
enters thermal shutdown. It recovers only after the junction
temperature decreases below 140°C to prevent any permanent
damage. Therefore, thermal analysis for the chosen application
is 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
ADM7150 must not exceed 150°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
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 pin
and exposed pad to the PCB.
Table 6 shows typical θJA values of the 8-lead SOIC and 8-lead
LFCSP packages for various PCB copper sizes.
Table 7 shows the typical ΨJB values of the 8-lead SOIC and
8-lead LFCSP.
Table 6. Typical θJA Values
θJA (°C/W)
Copper Size (mm2) 8-Lead LFCSP 8-Lead SOIC
251 165.1 165
100 125.8 126.4
500 68.1 69.8
1000 56.4 57.8
1 Device soldered to minimum size pin traces.
Table 7. Typical ΨJB Values
Package ΨJB (°C/W)
8-Lead LFCSP 15.1
8-Lead SOIC 17.9
The junction temperature of the ADM7150 is 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:
VIN and VOUT are the input and output voltages, respectively.
ILOAD is the load current.
IGND is the ground current.
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
that the junction temperature does not rise above 150°C.
The heat dissipation from the package can be improved by
increasing the amount of copper attached to the pins and
exposed pad of the ADM7150. Adding thermal planes under
the package also improves thermal performance. However, as
listed in Table 6, a point of diminishing returns is eventually
reached, beyond which an increase in the copper area does not
yield significant reduction in the junction to ambient thermal
resistance.
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