ADP320 Analog Devices, ADP320 Datasheet - Page 17

• Manufacturer Part Number: ADP320
• Description: Triple, 200 mA, Low Noise, High PSRR Voltage Regulator
• Manufacturer: Analog Devices

CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP320 triple LDO is protected against damage due to
excessive power dissipation by current and thermal overload
protection circuits. The ADP320 triple LDO 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 built-in, which limits the
junction temperature to a maximum of 155°C (typical). Under
extreme conditions (that is, high ambient temperature and
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 the output current
is restored to its nominal value.
Consider the case where a hard short from VOUTx to GND
occurs. At first, the ADP320 triple LDO 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
155°C, thermal shutdown activates turning off the output and
reducing the output current to zero. As the junction tempera-
ture cools and drops below 140°C, the output turns on and
conducts 300 mA into the short, again causing the junction
temperature to rise above 155°C. This thermal oscillation
between 140°C and 154°C causes a current oscillation between
0 mA and 300 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 ADP320 triple LDO does not dissipate
a lot of heat due to high efficiency. However, in applications
with a high ambient temperature and high supply voltage to out-
put 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 155°C, the converter
enters thermal shutdown. It recovers only after the junction
temperature has decreased below 140°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.
Rev. A | Page 17 of 20
To guarantee reliable operation, the junction temperature of
the ADP320 triple LDO 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 tem-
perature, power dissipation in the power device, and thermal
resistances between the junction and ambient air (θ
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 6 shows typical θ
ADP320 triple LDO for various PCB copper sizes.
Table 6. Typical θ
Copper Size (mm
JEDEC
100
500
1000
1
The junction temperature of the ADP320 triple LDO 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 the junction temperature does not rise
above 125°C. Figure 47 to Figure 50 show junction temperature
calculations for different ambient temperatures, total power
dissipation, and areas of PCB copper.
In cases where the board temperature is known, the thermal
characterization parameter, Ψ
junction temperature rise. T
the formula
The typical Ψ
25.2°C/W.
LOAD
GND
Device soldered to JEDEC standard board.
D
A
IN
is the power dissipation in the die, given by
is the ambient temperature.
and V
is the ground current.
T
P
T
T
is the load current.
1
J
D
J
J
= T
= T
= T
= Σ[(V
OUT
A
A
B
+ (P
+ {Σ[(V
+ (P
are input and output voltages, respectively.
JB
IN
value for the 16-lead 3 mm × 3 mm LFCSP is
D
D
− V
2
× Ψ
× θ
JA
)
IN
OUT
Values
JA
JB
− V
)
)
) × I
OUT
LOAD
J
) × I
is calculated from T
ADP320 Triple LDO (°C/W)
49.5
83.7
68.5
64.7
JB
, may be used to estimate the
] + Σ(V
LOAD
] × θ
IN
× I
JA
}
GND
JA
)
values for the
B
JA
and P
). The θ
ADP320
D
using
JA
(2)
(3)
(4)
(5)