ADP1706ARDZ-1.0-R7(RevA) Ver la hoja de datos (PDF) - Analog Devices
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ADP1706ARDZ-1.0-R7 Datasheet PDF : 20 Pages
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Data Sheet
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP1706/ADP1707/ADP1708 are protected against
damage due to excessive power dissipation by current and
thermal overload protection circuits. The ADP1706/
ADP1707/ADP1708 are designed to reach current limit
when the output load reaches 1.5 A (typical). When the
output load exceeds 1.5 A, 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 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.
ADP1706/ADP1707/ADP1708
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 (θJA).
The θJA 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 7 shows typical θJA values
of the 8-lead SOIC and 8-lead LFCSP for various PCB copper sizes.
Table 7. Typical θJA Values
Copper Size (mm2) θJA (°C/W), SOIC
01
57.6
50
53.1
100
52.3
300
51.3
500
51.3
1 Device soldered to minimum size pin traces.
θJA (°C/W), LFCSP
65.9
62.3
61.2
59.7
59.4
The junction temperature of the ADP1706/ADP1707/ADP1708
can be calculated by
TJ = TA + (PD × θJA)
(3)
where:
TA is the ambient temperature.
PD is the power dissipation in the die, given by
PD = ((VIN – VOUT) × ILOAD) + (VIN × IGND)
(4)
where:
ILOAD is the load current.
IGND is the ground current.
VIN and VOUT 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:
TJ = TA + (((VIN – VOUT) × ILOAD) × θJA)
(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, VIN to VOUT
differentials, and areas of PCB copper.
Rev. A | Page 15 of 20
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