CAT4101_12 Datasheet, PDF (8/9 Pages) ON Semiconductor – 1 A Constant-Current LED Driver

CAT4101
Application Information
Single 12 V Supply
The circuit shown in Figure 16 shows how to power three
LEDs in series from a single 12 V supply using the
CAT4101. The CAT4101 can not be driven directly from
12 V, three components are needed to create a lower voltage
for the VIN pin (below 5.5 V). Resistor R2 and zener diode
D provide a regulated voltage while the quiescent current
runs through the N−Channel transistor M. Suitable parts for
this circuit are the ON Semiconductor MM3Z6V2 zener
diode and the 2N7002L N−channel transistor (SOT23
package).
12 V _C2
1 mF
R2
5 kW
D
6.2 V
C1
0.1 mF
M
VIN
LED
CAT4101
EN/PWM
RSET
R1
GND
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
The CAT4101 TO−263 5−lead package provides a thermal
resistance when the ground tab is soldered down to the PCB.
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 30°C/W.
For example, at 60°C ambient temperature, the maximum
power dissipation is calculated as follow:
P_Dmax
+
T_Jmax * T_AMB
q_JA
+
150 *
30
60
+
3
W
Recommended Layout
The board layout should provide good thermal dissipation
through the PCB. Multiple via can be used to connect the tab
of the CAT4101 to a large ground plane underneath the
package.
Input capacitor C1 should be placed as close to the driver
IC as possible. The RSET resistor should have a Kelvin
connection to the GND pin of the CAT4101.
Figure 16. Single Supply Driving Three LEDs
Power Dissipation
The power dissipation (P_D) of the CAT4101 can be
calculated as follows:
P_D + (V_IN I_IN) ) (V_LED I_LED)
where V_LED is the voltage at the LED pin. Combinations of
high V_LED voltage or high ambient temperature can cause
the CAT4101 to enter thermal shutdown. In applications
where V_LED is high, a resistor can be inserted in series with
the LED string to lower 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
Figure 17. CAT4101 Recommended Layout
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