
January 1999 TOKO, Inc.
Page 7
TK70001
The maximum operating current is:
I
OUT
= (D
PD
/ (V
IN(MAX)
-
V
OUT
)
BOARD LAYOUT
The copper pattern should be as large as possible.
+
GND
INPUT
+
+
PCB: CLASS EPOXY T=0.8 mM
OUTPUT 2
OUTPUT 1
CONT 1
CONT 2
DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)
SOT-26 BOARD LAYOUT
SOT-26 POWER DISSIPATION
measurements should allow for the ambient temperature
of the PCB. The value obtained from P
D
/(150
°
C - T
A
) is the
derating factor. The PCB mounting pad should provide
maximum thermal conductivity in order to maintain low
device temperatures. As a general rule, the lower the
temperature, the better the reliability of the device. The
thermal resistance when mounted is expressed as follows:
T
j
= 0
jA
x P
D
+ T
A
For Toko ICs, the internal limit for junction temperature is
150
°
C. If the ambient temperature (T
A
) is 25
°
C, then:
150
°
C = 0
jA
x P
D
+ 25
°
C
0
jA
= 125
°
C/ P
D
P
D
is the value when the thermal sensor is activated. A
simple way to determine P
D
is to calculate V
IN
x I
IN
when
the output side is shorted. Input current gradually falls as
temperature rises. You should use the value when thermal
equilibrium is reached.
The range of usable currents can also be found from the
graph below.
Procedure:
1)
2)
3)
4)
Find P
D
P
D1
is taken to be P
D
x (~ 0.8 - 0.9)
Plot P
D1
against 25
°
C
Connect P
D1
to the point corresponding to the 150
°
C
with a straight line.
In design, take a vertical line from the maximum
operating temperature (e.g., 75
°
C) to the derating
curve.
Read off the value of P
D
against the point at which the
vertical line intersects the derating curve. This is taken
as the maximum power dissipation, D
PD
.
5)
6)
APPLICATION INFORMATION
PD
DPD
25
50
75
150
(mW)
TA ( C)
3
6
5
4
0 50 100 150
TA ( C)
P
0
250
450
50
150
350
MOUNTED AS
SHOWN
FREE AIR