NCV8509_06 Datasheet, PDF (15/19 Pages) ON Semiconductor – Sequenced Linear Dual−Voltage Regulator

NCV8509 Series
4.5 V
V_IN1
Max V_IN Delta
I(V_IN2) × R_EX
Shunt Off
V_IN2
Mode 1
VIN1 t VREF ) VSAT
VIN2 + VIN1 * VSAT
Shunt On
Mode 3
VIN1 u VREF ) (IOUT2 REX)
Mode 2
VIN2 + VIN1 * (IOUT2 REX)
VREF ) VSAT t VIN1 t VREF ) (IOUT2 REX)
VIN2 + VREF
Figure 50. V_IN Shunt
100
90
80
70
60
50
40
0
200
400
600
800
Copper Area (mm²)
Figure 51. 16 Lead SOW (Exposed Pad), qJA as a
Function of the Pad Copper Area (2 oz. Cu
Thickness), Board Material = 0.0625, G−10/R−4
Once the value of P_IC(max) is known, the maximum
permissible value of R_qJA can be calculated:
R_qJA
+
150°C *
PIC
TA
(8)
The value of R_qJA can then be compared with those in the
package section of the data sheet. Those packages with
R_qJA’s less than the calculated value in equation 2 will keep
the die temperature below 150°C.
In some cases, none of the packages will be sufficient to
dissipate the heat generated by the IC, and an external
heatsink will be required.
Heat Sinks
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 will have a thermal resistance. Like
series electrical resistances, these resistances are summed to
determine the value of R_qJA:
R_qJA + R_qJC ) R_qCS ) R_qSA
(9)
where:
R_qJC = the junction−to−case thermal resistance,
R_qCS = the case−to−heatsink thermal resistance, and
R_qSA = the heatsink−to−ambient thermal resistance.
R_qJC appears in the package section of the data sheet. Like
R_qJA, it too is a function of package type. R_qCS and R_qSA are
functions of the package type, heatsink and the interface
between them. These values appear in heat sink data sheets
of heat sink manufacturers.
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