ADP121-ACBZ155R7 데이터 시트보기 (PDF) - Analog Devices
부품명
상세내역
제조사
ADP121-ACBZ155R7 Datasheet PDF : 20 Pages
| |||
ADP121
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP121 is protected against damage due to excessive
power dissipation by current and thermal overload protection
circuits. The ADP121 is designed to current limit when the
output load reaches 225 mA (typical). When the output load
exceeds 225 mA, the output voltage is reduced to maintain a
constant current limit.
temperature changes. These parameters include ambient temper-
ature, 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 used
and the amount of copper to which the GND pins of the package
are soldered on the PCB. Table 5 shows typical θJA values for
various PCB copper sizes and Table 6 shows the typical ΨJB values
for the ADP121.
Thermal overload protection is built-in, 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 zero. 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 GND
occurs. At first, the ADP121 current limits, so that only
225 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 225 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 225 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 ADP121 does not dissipate a lot of
heat due to high efficiency. However, in applications with a high
ambient temperature and high supply voltage to an 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 tempera-
ture rise of the package due to the power dissipation, as shown
in Equation 2.
Table 5. Typical θJA Values
Copper Size (mm2) TSOT (°C/W)
01
170
50
152
100
146
300
134
500
131
WLCSP (°C/W)
260
159
157
153
151
1 Device soldered to minimum size pin traces.
Table 6. Typical ΨJB Values
TSOT (°C/W)
42.8
WLCSP (°C/W)
58.4
The junction temperature of the ADP121 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
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 125°C. Figure 34 to Figure 47 show junction temperature
calculations for different ambient temperatures, load currents,
VIN-to-VOUT differentials, and areas of PCB copper.
In cases where the board temperature is known, the thermal
characterization parameter, ΨJB, can be used to estimate the
junction temperature rise. TJ is calculated from TB and PD using
the formula
To guarantee reliable operation, the junction temperature of the
TJ = TB + (PD × ΨJB)
(5)
ADP121 must not exceed 125°C. To ensure that the junction
temperature stays below this maximum value, the user needs
to be aware of the parameters that contribute to junction
Rev. 0 | Page 14 of 20
Share Link: 