CAT4101TV-T75 Datasheet, PDF (8/11 Pages) ON Semiconductor – 1 A Constant-Current LED Driver with PWM Dimming

CAT4101
APPLICATION INFORMATION
SINGLE 12 V SUPPLY
The circuit shown in Figure 3 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 µF
R2
5 kΩ
D
6.2 V
C1
0.1 µF
R1
M
VIN
LED
CAT4101
EN/PWM
RSET
GND
Figure 3. 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 (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
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
θ_JA is about 30°C/W.
For example, at 60°C ambient temperature, the
maximum power dissipation is calculated as follow:
P_Dmax
= ^{TJmax - TAMB}
θ _JA
= ^{150 - 60}
30
= 3W
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 4. CAT4101 Recommended Layout
Doc. No. MD-5036, Rev. D
8
© 2009 SCILLC. All rights reserved.
Characteristics subject to change without notice