CAT4104V-GT3 Datasheet, PDF (10/14 Pages) ON Semiconductor – 700 mA Quad Channel Constant Current LED Driver

CAT4104
VCC
5V
C2
1 µF
RPTC
C1
0.1 µF
VIN
LED1
CAT4104
LED2
EN/PWM LED3
R1
1436 Ω
RSET
LED4
GND
M1
R2
5 kΩ
350 mA
Figure 6. LED Current Derating
POWER DISSIPATION
The power dissipation (P_D) of the CAT4104 can be
calculated as follows:
( ) ( ) P_D = V_IN × I_IN + Σ 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 (θ_JC) thermal resistance which is
defined by the package style, and the second path is
the case to ambient (θ_CA) thermal resistance, which is
dependent on board layout. The overall junction to
ambient (θ_JA) thermal resistance is equal to:
θ_JA = θ_JC + θ_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 (θ_JC + θ_CA) = T_AMB + P_D θ_JA
When mounted on a double-sided printed circuit
board with two square inches of copper allocated for
“heat spreading”, the resulting θ_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
θ_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 7.
Figure 7. CAT4104 Recommended Layout
Doc. No. MD-5041 Rev. B
10
© 2009 SCILLC. All rights reserved.
Characteristics subject to change without notice