DC-to-DC Step-Down Converter
SI-8008HD
5
Thermal Performance Characteristics
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–25 0 25 50 75 100 125
Power Dissipation versus Ambient Temperature
T
J(max) = 125°C; Mounted on glass-epoxy PCB (40 mm
×
40 mm),
with varying exposed copper areas
P
D
(W)
T
A
(°C)
Cu Area: 1600 mm2
RθJA = 33.3°C/W
Cu Area: 800 mm2
RθJA = 37°C/W
Cu Area: 400 mm2
RθJA = 44°C/W
Cu Area: 100 mm2
RθJA = 53°C/W
Device Thermal Resistance versus Exposed Copper Area on PCB
Glass-epoxy PCB, 40 mm × 40 mm
Copper Area (mm
2
)
55
50
45
40
35
30
0 800 1000 1200 1400 1600200 400 600 1800
RθJA(°C/W)
The application must be designed to ensure that the TJ(max)
of the device is not exceeded during operation. To do so, it is
necessary to determine values for maximum power dissipation,
PD(max), and ambient temperature, TA(max).
The relationships of TJ, PD, TA, and case temperature, TC, are as
shown in the following formulas:
TJ–T
C
RθJC
PD=TJ–T
A
RθJA
PD=
and .
PDcan be calculated from input values:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛−⋅−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛−⋅=
IN
O
OF
x
OOD
V
V
IVIVP 11
100
H
where:
VOis output voltage in V,
VIN is input supply voltage in V,
IOis output current in A,
ηxis IC efficiency in percent (varies with VIN and IO; refer to
efficiency performance curves for value), and
VFis forward voltage for the input diode, Di. In these tests, the
Sanken FMB-G16L was used, at 0.55 V. For application design,
obtain thermal data from the datasheet for the diode.
PDis substantially affected by the heat conductance properties of
the application, in particular any exposed copper area on the PCB
where the device is mounted. The relationships of PD, TA, and
copper area is represented in the Power Dissipation chart.
RθJA for a given copper area can be determined form the Device
Thermal Resistance chart. This can be substituted into the formula
above to determine the TJ (max) allowable in the application.
Generally, more than 10% to 20% derating is required.
Because the heat dissipation capacity of the copper area depends
substantively on how it is used in the actual application, thermal
characteristics of the application must be confirmed by testing.
TCis determined by connecting a thermocouple to the device as
shown here:
Thermocouple mount
at tab center
And analyzing the results using the following formula:
TJ=PD×RθJC +TC,
for this device, RθJC is 3 °C/W.
Power Dissipation versus Output Current
10
9
8
7
6
5
4
3
2
1
0
0123456
PD(W)
IO(A)
8
15
20
40
VIN (V)
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
