CAT4104
http://onsemi.com
10
VIN
CAT4104
5 V
VCC
R1
1436 W
M1
C1
0.1 mF
EN/PWM
RSET
LED1
LED2
LED3
LED4
GND
350 mA
C2
1 mF
R2
5 kW
RPTC
Figure 23. LED Current Derating
Power Dissipation
The power dissipation (P
D
) of the CAT4104 can be
calculated as follows:
P
D
+ (V
IN
I
IN
) ) S(V
LEDN
I
LEDN
)
where V
LEDN
is the voltage at the LED pin, and I
LEDN
is the
LED current. Combinations of high V
LEDN
voltage and high
ambient temperature can cause the CAT4104 to enter
thermal shutdown. In applications where V
LEDN
is high, a
resistor can be inserted in series with the LED string to lower
the power dissipation P
D
.
Thermal dissipation of the junction heat consists
primarily of two paths in series. The first path is the junction
to the case (q
JC
) thermal resistance which is defined by the
package style, and the second path is the case to ambient
(q
CA
) thermal resistance, which is dependent on board
layout. The overall junction to ambient (q
JA
) thermal
resistance is equal to:
q
JA
+ q
JC
) q
CA
For a given package style and board layout, the operating
junction temperature T
J
is a function of the power
dissipation P
D
, and the ambient temperature, resulting in the
following equation:
T
J
+ T
AMB
) P
D
(q
JC
) q
CA
) + T
AMB
) P
D
q
JA
When mounted on a double−sided printed circuit board
with two square inches of copper allocated for “heat
spreading”, the resulting q
JA
is about 90°C/W for 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:
P
Dmax
+
T
Jmax
* T
AMB
q
JA
+
150 * 60
90
+ 1W
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 24.
Figure 24. CAT4104 Recommended Layout
