cat4101 ON Semiconductor, cat4101 Datasheet - Page 8

• Manufacturer Part Number: cat4101
• Description: 1 A Constant-current Led Driver With Pwm Dimming
• Manufacturer: ON Semiconductor

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
POWER DISSIPATION
The power dissipation (P
calculated as follows:
where V
Combinations of high V
temperature can cause the CAT4101 to enter
thermal shutdown. In applications where V
high, a resistor can be inserted in series with the
LED string to lower P
Thermal dissipation of the junction heat consists
primarily of two paths in series. The first path is the
junction to the case (θ
defined by the package style, and the second path is
the case to ambient (θ
is dependent on board layout. The overall junction to
ambient (θ
Doc. No. MD-5036, Rev. D
Figure 3. Single Supply Driving Three LEDs
1 µF
C2
LED
JA
) thermal resistance is equal to:
P
D
is the voltage at the LED pin.
6.2 V
=
5 kΩ
(
R2
V
D
θ
IN
JA
D
0.1 µF
×
.
JC
= θ
CA
I
IN
) thermal resistance which is
C1
LED
) thermal resistance, which
D
) (
JC
) of the CAT4101 can be
+
R1
voltage or high ambient
+ θ
V
LED
CA
M
×
I
RSET
VIN
EN/PWM
LED
CAT4101
)
GND
LED
LED
is
8
For a given package style and board layout, the
operating junction temperature T
power dissipation P
resulting in the following equation:
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
For example, at 60°C ambient temperature, the
maximum power dissipation is calculated as follow:
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.
JA
is about 30°C/W.
Figure 4. CAT4101 Recommended Layout
T
P
J
Dmax
= T
AMB
=
+ P
T
Jmax
D
D
θ
(θ
, and the ambient temperature,
JA
-
JC
T
Characteristics subject to change without notice
AMB
+ θ
CA
=
) = T
150
© 2009 SCILLC. All rights reserved.
J
30
is a function of the
AMB
-
60
+ P
=
3
D
W
θ
JA