Application Notes: continued
A rule of thumb useful in determining if a protection diode
is required is to solve for current
Calculating Power Dissipation and Heat Sink Requirements
High power regulators such the CS5207A-1 usually operate
at high junction temperatures. Therefore, it is important to
calculate the power dissipation and junction temperatures
accurately to ensure that an adequate heat sink is used.
Since the package tab is connected to Vout on the
CS5207A-1, electrical isolation may be required for some
applications. Also, as with all high power packages, ther-
mal compound is necessary to ensure proper heat flow. For
added safety, this high current LDO includes an internal
thermal shutdown circuit
C ´ V
T
I =
, where
I is the current flow out of the load capacitance when
VIN is shorted,
C is the value of the load capacitance,
V is the output voltage, and
T is the time duration required for VIN to transition
from high to being shorted.
The thermal characteristics of an IC depend on the follow-
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:
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 including a pro-
tection diode.
TJ(max) = TA(max) + PD(max) ´ RQJA
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:
VIN
CS5207A-1
Adj
VOUT
PD(max) = (VIN(max) -VOUT(min))IOUT(max) + VIN(max) ´ IIN(max)
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
between the die junction and the surrounding air, RQJA
.
Figure 3.
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:
Current Limit
The internal current limit circuit limits the output current
under excessive load conditions, and protects the regula-
tor.
RQJA = RQJC + RQCS + RQSA
R
QJC is rated @ 1.4¡C/W for the CS5207A-1. For a high cur-
Short Circuit Protection
rent regulator such as the CS5207A-1 the majority of heat is
generated in the power transistor section. The value for
The device includes foldback short circuit current limit that
clamps the output current at approximately two amperes
less than its current limit value.
R
QSA depends on the heat sink type, while the RQCS
depends on factors such as package type, heat sink inter-
face (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
Thermal Shutdown
The thermal shutdown circuitry is guaranteed by design to
become activated above a die junction temperature of
150¡C and to shut down the regulator output. This circuit-
ry includes a thermal hysteresis circuit with 25¡C of typical
hysteresis, thereby allowing the regulator to recover from a
thermal fault automatically.
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.
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
