Datasheet

ADP1706/ADP1707/ADP1708
Rev. 0 | Page 13 of 20
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 zero. When the junction temperature drops below
135°C (typical), the output is turned on again and output
current is restored to its nominal value.
Consider the case where a hard short from OUT to ground
occurs. At first, the ADP1706/ADP1707/ADP1708 reach
current limit so that only 1.5 A is conducted into the short. If
self-heating of the junction becomes 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 1.5 A 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 1.5 A and 0 A 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 should be externally limited
so junction temperatures do not exceed 125°C.
THERMAL CONSIDERATIONS
To guarantee reliable operation, the junction temperature of
the ADP1706/ADP1707/ADP1708 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 contrib-
ute to junction temperature changes. These parameters include
ambient temperature, power dissipation in the power device,
and thermal resistance between the junction and ambient air (θ
JA
).
The θ
JA
value 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 θ
JA
values
of the 8-lead SOIC and 8-lead LFCSP for various PCB copper sizes.
Table 5. Typical θ
JA
Values
Copper Size (mm
2
) θ
JA
(°C/W), SOIC θ
JA
(°C/W), LFCSP
0
1
57.6 65.9
50 53.1 62.3
100 52.3 61.2
300 51.3 59.7
500 51.3 59.4
1
Device soldered to minimum size pin traces.
The junction temperature of the ADP1706/ADP1707/ADP1708
can be calculated by
T
J
= T
A
+ (P
D
× θ
JA
) (3)
where:
T
A
is the ambient temperature.
P
D
is the power dissipation in the die, given by
P
D
= [(V
IN
– V
OUT
) × I
LOAD
] + (V
IN
× I
GND
) (4)
where:
I
LOAD
is the load current.
I
GND
is the ground current.
V
IN
and V
OUT
are the 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 the following:
T
J
= T
A
+ {[(V
IN
– V
OUT
) × I
LOAD
] × θ
JA
} (5)
As shown in Equation 5, for a given ambient temperature,
input-to-output voltage differential, and continuous load
current, a minimum copper size requirement exists for the PCB
to ensure the junction temperature does not rise above 125°C.
Figure 36 to Figure 41 show junction temperature calculations
for different ambient temperatures, load currents, V
IN
to V
OUT
differentials, and areas of PCB copper.
140
0
0.5 5.0
V
IN
– V
OUT
(V)
T
J
(°C)
120
100
80
60
40
20
1.01.52.02.53.03.54.04.5
1mA
10mA
100mA
300mA
500mA
750mA
1A
(LOAD CURRENT)
MAX T
J
(DO NOT OPERATE ABOVE THIS POINT)
06640-035
Figure 36. 500 mm
2
of PCB Copper, T
A
= 25°C, SOIC