TK715xxAS
DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)
determine the power dissipation for shutdown when
mounted, attach the device on the actual PCB and delib-
erately increase the output current (or raise the input
voltage) until the thermal protection circuit is activated.
Calculate the power dissipation of the device by subtract-
ing the output power from the input power. These mea-
surements 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:
2)
P
D1
is taken to be P
D
x (~ 0.8 - 0.9)
3)
Plot P
D1
against 25 °C
4)
Connect P
D1
to the point corresponding to the 150
°C with a straight line.
5)
In design, take a vertical line from the maximum
operating temperature (e.g., 75 °C) to the derating curve.
6)
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
.
The maximum operating current is:
I
OUT
= (D
PD
/ (V
IN(MAX)
- V
OUT
)
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.
(mW)
PD
3
500
400
PD (mW)
MOUNTED AS SHOWN
300
FREE AIR
200
100
0
0
50
TA (°C)
100
150
SOT23-3 POWER DISSIPATION CURVE
DPD
6
4
5
25
50
75
TA (°C)
150
Procedure:
1)
Find P
D
March 2001 TOKO, Inc.
Page 9
TOKO [ TOKO, INC ]
