Application Note (Continued)
10094816
FIGURE 6. Power Dissipation Diagram
Once the device power is determined, the maximum allow-
able (θJA(max)) is calculated as:
θJA (max) = TR(max)/PD = TJ(max) − TA(max))/PD
10094815
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 junc-
tion to case thermal resistances for each of these sections,
while the maximum junction temperatures (TJ(max)) for each
section is listed in the Absolute Maximum section of the
datasheet. TJ(max) is 125˚C for the control section, while
TJ(max) is 150˚C for the output section.
FIGURE 5. Regulator with Protection Diode
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 curve section.
θ
JA (max) should be calculated separately for each section as
follows:
θJA (max, CONTROL SECTION) = (125˚C for TA(max))/PD
θJA (max, OUTPUT SECTION) = (150˚C for TA(max))/PD
The required heat sink is determined by calculating its re-
quired thermal resistance (θHA(max)).
θHA(max) = θJA(max) − (θJC + θCH
θHA (max) should be calculated twice as follows:
θHA (max) = θJA(max, CONTROL SECTION) - (θJC (CON-
TROL SECTION) + θCH
θHA (max)= θJA(max, OUTPUT SECTION) - (θJC(OUTPUT
SECTION) + θCH
If thermal compound is used, θCH can be estimated at 0.2
C/W. If the case is soldered to the heat sink, then a θCH can
be estimated as 0 C/W.
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 lim-
iting 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 curve section for the short circuit
current vs. input differential voltage.
)
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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 (θJA).
Thermal resistance has units of temperature per power (C/
W). The higher the thermal resistance, the hotter the IC.
After, θHA (max) is calculated for each section, choose the
lower of the two θHA (max) values to determine the appropri-
ate heat sink.
If PC board copper is going to be used as a heat sink, then
Figure 7 can be used to determine the appropriate area
(size) of copper foil required.
The LM1086 specifies the thermal resistance for each pack-
age as junction to case (θJC). In order to get the total
resistance to ambient (θJA), two other thermal resistance
must be added, one for case to heat-sink (θCH) and one for
heatsink to ambient (θHA). The junction temperature can be
predicted as follows:
TJ = TA + PD (θJC + θCH + θHA) = TA + PD θJA
TJ is junction temperature, TA is ambient temperature, and
PD is the power consumption of the device. Device power
consumption is calculated as follows:
IIN = IL + IG
PD = (VIN−VOUT) IL + VIN G
I
Figure 6 shows the voltages and currents which are present
in the circuit.
10094864
FIGURE 7. Heat sink thermal Resistance vs. Area
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