Datasheet

17
The value of 4.25 mA for I
CC
in the previous equation was
obtained by derating the I
CC
max of 5 mA (which occurs
at -40°C) to I
CC
max at 90°C (see Figure 7).
Since P
O
for this case is greater than P
O(MAX)
, Rg must be
increased to reduce the HCPL-3150 power dissipation.
P
O(SWITCHING MAX)
= P
O(MAX)
- P
O(BIAS)
= 154 mW - 85 mW
= 69 mW
P
O(SWITCHINGMAX)
E
SW(MAX)
=
f
69 mW
= = 3.45 µJ
20 kHz
For Qg = 500 nC, from Figure 27, a value of E
SW
= 3.45 µJ
gives a Rg = 41 Ω.
Thermal Model (HCPL-3150)
The steady state thermal model for the HCPL-3150 is
shown in Figure 28a. The thermal resistance values given
in this model can be used to calculate the temperatures
at each node for a given operating condition. As shown
by the model, all heat generated ows through θ
CA
which
raises the case temperature T
C
accordingly. The value of
θ
CA
depends on the conditions of the board design and
is, therefore, determined by the designer. The value of
θ
CA
= 83°C/W was obtained from thermal measurements
using a 2.5 x 2.5 inch PC board, with small traces (no
ground plane), a single HCPL-3150 soldered into the
center of the board and still air. The absolute maximum
power dissipation derating specications assume a
θ
CA
value of 83°C/W.
From the thermal mode in Figure 28a the LED and detec-
tor IC junction temperatures can be expressed as:
T
JE
= P
E
•
(θ
LC
||(θ
LD
+ θ
DC
) + θ
CA
)
θ
LC
•
θ
DC
+ P
D
•
( + θ
CA
) + T
A
θ
LC
+ θ
DC
+ θ
LD
θ
LC
•
θ
DC
T
JD
= P
E
( + θ
CA
)
θ
LC
+ θ
DC
+ θ
LD
+ P
D
•
(θ
DC
||(θ
LD
+ θ
LC
) + θ
CA
) + T
A
Inserting the values for θ
LC
and θ
DC
shown in Figure 28
gives:
T
JE
= P
E
•
(230°C/W + θ
CA
) + P
D
•
(49°C/W + θ
CA
) + T
A
T
JD
= P
E
•
(49°C/W + θ
CA
) + P
D
•
(104°C/W + θ
CA
) + T
A
For example, given P
E
= 45 mW, P
O
= 250 mW, T
A
= 70°C
and θ
CA
= 83°C/W:
T
JE
= P
E
•
313°C/W + P
D
•
132°C/W + T
A
= 45 mW
•
313°C/W + 250 mW
•
132°C/W + 70°C = 117°C
T
JD
= P
E
•
132°C/W + P
D
•
187°C/W + T
A
= 45 mW
•
132C/W + 250 mW
•
187°C/W + 70°C = 123°C
T
JE
and T
JD
should be limited to 125°C based on the board
layout and part placement (θ
CA
) specic to the applica-
tion.
T
JE
= LED junction temperature
T
JD
= detector IC junction temperature
T
C
= case temperature measured at the center of the package bottom
θ
LC
= LED-to-case thermal resistance
θ
LD
= LED-to-detector thermal resistance
θ
DC
= detector-to-case thermal resistance
θ
CA
= case-to-ambient thermal resistance
∗θ
CA
will depend on the board design and the placement of the part.
Figure 28a. Thermal Model.
HCPL-3150 fig 28
θ
LD
= 439°C/W
T
JE
T
JD
θ
LC
= 391°C/W θ
DC
= 119°C/W
θ
CA
= 83°C/W*
T
C
T
A
T
JE
= LED JUNCTION TEMPERATURE
T
JD
= DETECTOR IC JUNCTION TEMPERATURE
T
C
= CASE TEMPERATURE MEASURED AT THE
CENTER OF THE PACKAGE BOTTOM
θ
LC
= LED-TO-CASE THERMAL RESISTANCE
θ
LD
= LED-TO-DETECTOR THERMAL RESISTANCE
θ
DC
= DETECTOR-TO-CASE THERMAL RESISTANCE
θ
CA
= CASE-TO-AMBIENT THERMAL RESISTANCE
* θ
CA
WILL DEPEND ON THE BOARD DESIGN AND
THE PLACEMENT OF THE PART.