cat4104 ON Semiconductor, cat4104 Datasheet - Page 10

• Manufacturer Part Number: cat4104
• Description: 700 Ma Quad Channel Constant Current Led Driver
• Manufacturer: ON Semiconductor

CAT4104
POWER DISSIPATION
The power dissipation (P
calculated as follows:
where V
the LED current. Combinations of high V
and high ambient temperature can cause the
CAT4104 to enter thermal shutdown. In applications
where V
series with the LED string to lower the power
dissipation P
Thermal dissipation of the junction heat consists
primarily of two paths in series. The first path is the
junction to the case (θ
defined by the package style, and the second path is
the case to ambient (θ
dependent on board layout. The overall junction to
ambient (θ
For a given package style and board layout, the
operating junction temperature T
Doc. No. MD-4051 Rev. A
VCC
5 V
LEDN
LEDN
5 kΩ
R2
Figure 6. LED Current Derating
JA
P
C2
1 µF
) thermal resistance is equal to:
D
D
is the voltage at the LED pin, and I
.
is high, a resistor can be inserted in
=
0.1 µF
(
C1
V
1436 Ω
IN
θ
×
JA
R1
I
CA
JC
IN
= θ
) thermal resistance which is
) thermal resistance, which is
)
M1
+
D
JC
) of the CAT4104 can be
RSET
Σ
VIN
EN/PWM
+ θ
(
V
CAT4104
LEDN
CA
GND
J
is a function of the
×
RPTC
LED1
LED2
LED3
LED4
I
LEDN
)
LEDN
voltage
350 mA
LEDN
is
10
power dissipation P
resulting in the following equation:
When mounted on a double-sided printed circuit
board with two square inches of copper allocated for
“heat spreading”, the resulting θ
the TDFN-8 package, and 160°C/W for the SOIC-8
package.
For example, at 60°C ambient temperature, the
maximum power dissipation for the TDFN-8 is
calculated as follow:
RECOMMENDED LAYOUT
A small ceramic capacitor should be placed as close
as possible to the driver VIN pin. The RSET resistor
should have a Kelvin connection to the GND pin of
the CAT4104.
The board layout should provide good thermal
dissipation through the PCB. In the case of the
CAT4104VP2 in the TDFN package, a via can be
used to connect the center tab to a large ground
plane underneath as shown on figure 7.
Figure 7. CAT4104 Recommended Layout
P
Dmax
=
T
T (
J
= T
Jmax
= T
D
θ
AMB
, and the ambient temperature,
JA
-
AMB
T
+ P
AMB
Characteristics subject to change without notice
+ P
D
)
(θ
=
D
JC
(
θ
JA
150
© 2009 SCILLC. All rights reserved.
JA
+ θ
is about 90°C/W for
90
-
CA
60
)
)
=
1
W