ADP151 Analog Devices, ADP151 Datasheet - Page 15

• Manufacturer Part Number: ADP151
• Description: Ultra Low Noise, 200 mA CMOS Linear Regulator
• Manufacturer: Analog Devices

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
causes 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 a high
ambient temperature and a high supply voltage to output voltage
differential, the heat dissipated in the package 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.
Rev. D | Page 15 of 24
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
changes. These parameters include ambient temperature, power
dissipation in the power device, and thermal resistances between
the junction and ambient air (θ
on the package assembly compounds that are used and the amount
of copper used to solder the package GND pins to the PCB.
Table 6 shows typical θ
LFCSP, and 4-ball WLCSP packages for various PCB copper sizes.
Table 7 shows the typical Ψ
LFCSP, and 4-ball WLCSP.
Table 6. Typical θ
Copper Size (mm
0
50
100
300
500
1
Table 7. Typical Ψ
Model
TSOT
WLCSP
LFCSP
The junction temperature of the ADP151 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 the following:
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 39
through Figure 59 show junction temperature calculations for
various ambient temperatures, load currents, V
differentials, and areas of PCB copper.
LOAD
GND
Device soldered to minimum size pin traces.
1
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
is the load current.
J
D
J
= T
= T
= [(V
OUT
A
A
+ (P
+ {[(V
IN
are input and output voltages, respectively.
− V
D
2
× θ
JA
)
IN
OUT
JB
Values
− V
Values
JA
) × I
JA
)
values of the 5-lead TSOT, 6-lead
TSOT
170
152
146
134
131
Ψ
58
28.3
OUT
43
LOAD
JB
JB
) × I
(°C/W)
values of the 5-lead TSOT, 6-lead
] + (V
JA
LOAD
). The θ
] × θ
IN
× I
WLCSP
260
159
157
153
151
θ
JA
JA
JA
GND
}
number is dependent
(°C/W)
)
IN
-to-V
ADP151
LFCSP
231.2
161.8
150.1
111.5
91.8
OUT
(2)
(3)
(4)