CAT4104 Datasheet, PDF (10/13 Pages) ON Semiconductor – 700 mA Quad Channel Constant Current LED Driver

CAT4104
VCC
5V
C2
1 mF
RPTC
C1
0.1 mF
VIN
LED1
CAT4104
LED2
EN/PWM LED3
R1
1436 W
RSET LED4
GND
M1
R2
5 kW
350 mA
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 *
90
60
+
1
W
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
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