Application Notes: continued
A rule of thumb useful in determining if a protection diode The thermal characteristics of an IC depend on the follow-
is required is to solve for current:
ing four factors: junction temperature, ambient tempera-
ture, die power dissipation, and the thermal resistance
from the die junction to ambient air. The maximum junc-
tion temperature can be determined by:
C ´ V
I =
,
T
where
I
TJ(max) = TA(max) + PD(max) ´ RQJA
is the current flow out of the load capacitance
when VCONTROL is shorted,
The maximum ambient temperature and the power dissi-
pation are determined by the design while the maximum
junction temperature and the thermal resistance depend on
the manufacturer and the package type. The maximum
power dissipation for a regulator is:
C
V
T
is the value of load capacitance
is the output voltage, and
is the time duration required for VCONTROL
to transition from high to being shorted.
PD(max) = (VIN(max) -VOUT(min))IOUT(max) + VIN(max) ´ IIN(max)
If the calculated current is greater than or equal to the typi-
cal short circuit current value provided in the specifica-
tions, serious thought should be given to the use of a pro-
tection diode.
A heat sink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air. Each material in the heat flow
path between the IC and the outside environment has a
thermal resistance which is measured in degrees per watt.
Like series electrical resistances, these thermal resistances
are summed to determine the total thermal resistance
Current Limit
The internal current limit circuit limits the output current
under excessive load conditions.
between the die junction and the surrounding air, RQJA
.
This total thermal resistance is comprised of three compo-
nents. These resistive terms are measured from junction to
case (RQJC), case to heat sink (RQCS), and heat sink to ambi-
ent air (RQSA). The equation is:
Short Circuit Protection
The device includes short circuit protection circuitry that
clamps the output current at approximately 500mA less
than its current limit value. This provides for a current
foldback function, which reduces power dissipation under
a direct shorted load.
R
QJA = RQJC + RQCS + RQSA
The value for RQJC is 2.5ûC/watt for the CS5253-1 in the D2
PAK package. For a high current regulator such as the
CS5253-1 the majority of heat is generated in the power
transistor section. The value for RQSA depends on the heat
sink type, while the RQCS depends on factors such as pack-
age type, heat sink interface (is an insulator and thermal
grease used?), and the contact area between the heat sink
and the package. Once these calculations are complete, the
maximum permissible value of RQJA can be calculated and
the proper heat sink selected. For further discussion on
heat sink selection, see our Cherry application note
ÒThermal Management for Linear Regulators.Ó
Thermal Shutdown
The thermal shutdown circuitry is guaranteed by design to
activate above a die junction temperature of approximately
150¡C and to shut down the regulator output. This circuit-
ry has 25¡C of typical hysteresis, thereby allowing the reg-
ulator to recover from a thermal fault automatically.
Calculating Power Dissipation and Heat Sink
Requirements
High power regulators such as the CS5253-1 usually oper-
ate at high junction temperatures. Therefore, it is impor-
tant to calculate the power dissipation and junction tem-
peratures accurately to ensure that an adequate heat sink is
used. Since the package tab is connected to VOUT on the
CS5253-1, electrical isolation may be required for some
applications. Also, as with all high power packages, ther-
mal compound in necessary to ensure proper heat flow.
For added safety, this high current LDO includes an inter-
nal thermal shutdown circuit
8
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
