ADP123 Analog Devices, ADP123 Datasheet - Page 14

• Manufacturer Part Number: ADP123
• Description: 5.5 V Input, 300 mA, Low Quiescent Current, CMOS Linear Regulator, Adjustable Output Voltage
• Manufacturer: Analog Devices

ADP122/ADP123
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP122/ADP123 are protected from damage due to excessive
power dissipation by current and thermal overload protection
circuits. The ADP122/ADP123 are designed to limit the current
when the output load reaches 500 mA (typical). When the output
load exceeds 500 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 con-
ditions (that is, high ambient temperature and power dissipation),
when the junction temperature starts to rise above 150°C, the
output is turned off, reducing output current to zero. When the
junction temperature cools to less than 135°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 VOUT to GND occurs.
At first, the ADP122/ADP123 limit the current so that only 500 mA
is conducted into the short. If self-heating causes the junction
temperature to rise above 150°C, thermal shutdown activates,
turning off the output and reducing the output current to zero.
When the junction temperature cools to less than 135°C, the
output turns on and conducts 500 mA into the short, again
causing the junction temperature to rise above 150°C. This
thermal oscillation between 135°C and 150°C results in a current
oscillation between 500 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 from damage due to accidental overload conditions. For
reliable operation, the device power dissipation must be externally
limited so that the junction temperature does not exceed 125°C.
THERMAL CONSIDERATIONS
To guarantee reliable operation, the junction temperature of the
ADP122/ADP123 must not exceed 125°C. To ensure that the
junction temperature is less than 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
of θ
and the amount of copper to which the GND pins of the package
are soldered on the PCB. Table 6 shows typical θ
5-lead TSOT package and 6-lead LFCSP package for various
PCB copper sizes.
JA
is dependent on the package assembly compounds used
JA
JA
values of the
). The value
Rev. A | Page 14 of 24
Table 6. Typical θ
Copper Size (mm
0
50
100
300
500
1
The typical Ψ
44.1°C/W for LFCSP packages.
The junction temperature of the ADP122/ADP123 can be
calculated from the following equation:
where:
T
P
where:
I
I
V
The power dissipation due to ground current is quite small and
can be ignored. Therefore, the junction temperature equation
can be simplified as follows:
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 38
through Figure 44 show junction temperature calculations for
different ambient temperatures, load currents, V
differentials, and areas of PCB copper.
In cases where the board temperature is known, the thermal
characterization parameter, Ψ
ction temperature rise. The maximum junction temperature (T
calculated from the board temperature (T
(P
LOAD
GND
Device soldered to narrow traces.
1
D
A
IN
D
is the power dissipation in the die, given by
is the ambient temperature.
) using the formula
and V
is the ground current.
T
P
T
T
is the load current.
D
J
J
J
= T
= T
= T
= [(V
OUT
A
A
B
+ (P
+ (P
+ {[(V
IN
are input and output voltages, respectively.
JB
− V
values are 42.8°C/W for TSOT packages and
D
D
2
× Ψ
× θ
JA
)
IN
OUT
Values for Specified PCB Copper Sizes
− V
JA
) × I
JB
)
)
OUT
LOAD
) × I
JB
] + (V
, can be used to estimate the jun-
LOAD
] × θ
IN
TSOT
170
152
146
134
131
× I
B
JA
) and power dissipation
GND
}
)
θ
JA
IN
(°C/W)
to V
LFCSP
255
164
138
109
80
OUT
J
) is
(2)
(3)
(4)
(5)