LM1086
www.ti.com
SNVS039H –JUNE 2000–REVISED MAY 2013
OVERLOAD RECOVERY
Overload recovery refers to regulator's ability to recover from a short circuited output. A key factor in the recovery
process is the current limiting used to protect the output from drawing too much power. The current limiting circuit
reduces the output current as the input to output differential increases. Refer to short circuit curve in the Typical
Performance Characteristics section.
During normal start-up, the input to output differential is small since the output follows the input. But, if the output
is shorted, then the recovery involves a large input to output differential. Sometimes during this condition the
current limiting circuit is slow in recovering. If the limited current is too low to develop a voltage at the output, the
voltage will stabilize at a lower level. Under these conditions it may be necessary to recycle the power of the
regulator in order to get the smaller differential voltage and thus adequate start up conditions. Refer to Typical
Performance Characteristics section for the short circuit current vs. input differential voltage.
THERMAL CONSIDERATIONS
ICs heats up when in operation, and power consumption is one factor in how hot it gets. The other factor is how
well the heat is dissipated. Heat dissipation is predictable by knowing the thermal resistance between the IC and
ambient (θ
JA
). Thermal resistance has units of temperature per power (C/W). The higher the thermal resistance,
the hotter the IC.
The LM1086 specifies the thermal resistance for each package as junction to case (θ
JC
). In order to get the total
resistance to ambient (θ
JA
), two other thermal resistance must be added, one for case to heat-sink (θ
CH
) and one
for heatsink to ambient (θ
HA
). The junction temperature can be predicted as follows:
T
J
= T
A
+ P
D
(θ
JC
+ θ
CH
+ θ
HA
) = T
A
+ P
D
θ
JA
where
• T
J
is junction temperature
• T
A
is ambient temperature
• P
D
is the power consumption of the device
Device power consumption is calculated as follows:
I
IN
= I
L
+ I
G
P
D
= (V
IN
−V
OUT
) I
L
+ V
IN
I
G
Figure 23 shows the voltages and currents which are present in the circuit.
Figure 23. Power Dissipation Diagram
Once the device power is determined, the maximum allowable (θ
JA(max)
) is calculated as:
θ
JA (max)
= T
R(max)
/P
D
= T
J(max)
− T
A(max)
)/P
D
The LM1086 has different temperature specifications for two different sections of the IC: the control section and
the output section. The Electrical Characteristics table shows the junction to case thermal resistances for each of
these sections, while the maximum junction temperatures (T
J(max)
) for each section is listed in the Absolute
Maximum section of the datasheet. T
J(max)
is 125°C for the control section, while T
J(max)
is 150°C for the output
section.
θ
JA (max)
should be calculated separately for each section as follows:
θ
JA
(max, CONTROL SECTION) = (125°C for T
A(max)
)/P
D
θ
JA
(max, OUTPUT SECTION) = (150°C for T
A(max)
)/P
D
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