Application Notes: continued
A rule of thumb useful in determining if a protection diode is Also, as with all high power packages, thermal compound is
required is to solve for current
necessary to ensure proper heat flow. For added safety, this
high current LDO includes an internal thermal shutdown cir-
cuit.
C ´ V ,
I =
T
The thermal characteristics of an IC depend on the following
four factors: junction temperature, ambient temperature, die
power dissipation, and the thermal resistance from the die
junction to ambient air. The maximum junction temperature
can be determined by:
where
I
is the current flow out of the load capacitance
when VCONTROL is shorted,
C
V
T
is the value of load capacitance
is the output voltage, and
TJ(max) = TA(max) + PD(max) ´ RQJA
is the time duration required for VCONTROL to
transition from high to being shorted.
The maximum ambient temperature and the power dissipa-
tion are determined by the design while the maximum junc-
tion temperature and the thermal resistance depend on the
manufacturer and the package type. The maximum power
dissipation for a regulator is:
If the calculated current is greater than or equal to the typical
short circuit current value provided in the specifications, seri-
ous thought should be given to the use of a protection diode.
Current Limit
PD(max) = (VIN(max) -VOUT(min))IOUT(max) + VIN(max) ´ IIN(max)
The internal current limit circuit limits the output current
under excessive load conditions.
A heat sink effectively increases the surface area of the pack-
age 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 between the die
junction and the surrounding air, RQJA. This total thermal
resistance is comprised of three components. These resistive
terms are measured from junction to case (RQJC), case to heat
sink RQCS), and heat sink to ambient air (RQSA). The equation
is:
Short Circuit Protection
The device includes short circuit protection circuitry that
clamps the output current at approximately two amperes less
than its current limit value. This provides for a current fold-
back function, which reduces power dissipation under a
direct shorted load.
Thermal Shutdown
The thermal shutdown circuitry is guaranteed by design to
become activate above a die junction temperature of approxi-
mately 150¡C and to shut down the regulator output. This
circuitry has 25¡C of typical hysteresis, thereby allowing the
regulator to recover from a thermal fault automatically.
R
QJA = RQJC + RQCS + RQSA
The value for RQJC is 1.4ûC/watt for the CS52510-1 in a
TO-220 package. For a high current regulator such as the
CS52510-1 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 package
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 selec-
tion, see our Cherry application note ÒThermal Management
for Linear Regulators.Ó
Calculating Power Dissipation and Heat Sink
Requirements
High power regulators such as the CS52510-1 family usually
operate at high junction temperatures. Therefore, it is impor-
tant to calculate the power dissipation and junction tempera-
tures accurately to ensure that an adequate heat sink is used.
Since the package tab is connected to VOUT on the CS52510-1,
electrical isolation may be required for some applications.
8
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
