ADP121 Analog Devices, ADP121 Datasheet - Page 14

• Manufacturer Part Number: ADP121
• Description: 150 mA, Low Quiescent Current, CMOS Linear Regulator in 5-Lead TSOT or 4-Ball WLCSP
• Manufacturer: Analog Devices

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.
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 con-
ducted 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 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.
To guarantee reliable operation, the junction temperature of the
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 temperature
changes. These parameters include ambient temperature, power
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dissipation in the power device, and thermal resistances between
the junction-and-ambient air (θ
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 6 shows typical θ
sizes and Table 7 shows the typical Ψ
Table 6. Typical θ
Copper Size (mm
0
50
100
300
500
1
Table 7. Typical Ψ
TSOT (°C/W)
42.8
The junction temperature of the ADP121 can be calculated
from the following equation:
where:
T
P
where:
I
I
V
Power dissipation due to ground current is quite small and
can be ignored. Therefore, the junction temperature equation
simplifies to
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,
V
In cases where the board temperature is known, the thermal
characterization parameter, Ψ
junction temperature rise. T
the formula
LOAD
GND
Device soldered to minimum size pin traces.
1
D
A
IN
IN
is the power dissipation in the die, given by
is the ambient temperature.
-to-V
and V
is the ground current.
T
P
T
T
is the load current.
D
J
J
J
= T
= T
= T
= [(V
OUT
OUT
A
A
B
+ (P
+ (P
+ {[(V
differentials, and areas of PCB copper.
IN
are input and output voltages, respectively.
− V
D
D
2
× Ψ
× θ
JA
)
IN
OUT
JB
Values
− V
Values
JA
) × I
JB
)
)
TSOT (°C/W)
170
152
146
134
131
OUT
LOAD
) × I
JA
J
is calculated from T
values for various PCB copper
JB
] + (V
JA
, can be used to estimate the
LOAD
). The θ
WLCSP (°C/W)
58.4
] × θ
JB
IN
values for the ADP121.
× I
JA
JA
GND
}
number is dependent
)
WLCSP (°C/W)
260
159
157
153
151
Data Sheet
B
and P
D
using
(2)
(3)
(4)
(5)