CAT4104
http://onsemi.com
9
Application Information
Single 12 V Supply
The circuit shown in Figure 20 shows how to power the
LEDs from a single 12 V supply using the CAT4104. Three
external components are needed to create a lower voltage
necessary for the VIN pin (below 5.5 V). The resistor R2 and
zener diode Z provide a regulated voltage while the
quiescent current runs through the N−Channel transistor M.
The recommended parts are ON Semiconductor MM3Z6V2
zener diode (in SOD−323 package), and 2N7002L
N−Channel transistor (in SOT23).
Figure 20. Single Supply Driving 12 LEDs
VIN
R2
5 kW
Z
6.2 V
M
CAT4104
12 V
R1
C1
0.1 mF
EN/PWM
RSET
LED1
LED2
LED3
LED4
GND
C2
1 mF
Daylight Detection
The circuit in Figure 21 shows how to use CAT4104 in an
automatic light sensor application. The light sensor allows
the CAT4104 to be enabled during the day and disabled
during the night. Two external components are required to
configure the part for ambient light detection and conserve
power. Resistor R1 sets the bias for the light sensor. The
recommended part is Microsemi LX1972 light sensor. For
best performance, the LED light should not interfere with
the light sensor.
Figure 21. Daylight Detection
VIN
VDD
VSS
Light
Sensor
R1
100 kW
CAT4104
5 V
R1
C1
0.1 mF
EN/PWM
RSET
LED1
LED2
LED3
LED4
GND
ON
OFF
C2
1 mF
Nightlight Detection
The circuit shown in Figure 22 illustrates how to use the
CAT4104 in an automatic night light application. The light
sensor allows the CAT4104 to be disabled during the day and
enabled during the night. Five external components are
needed to properly configure the part for night detection.
Resistor R3 limits the quiescent current through the
N−Channel transistor M. Resistors R1 and R2 act as a
voltage divider to create the required voltage to turn on
transistor M, which disables the CAT4104. The
recommended parts are ON Semiconductor 2N7002L
N−Channel transistor (in SOT23) and the Microsemi
LX1972 light sensor. For best performance, the LED light
should not interfere with the light sensor.
Figure 22. Nightlight Detection
VIN
VDD
VSS
Light
Sensor
R1
CAT4104
5 V
R4
M
C1
EN/PWM
RSET
LED1
GND
ON
OFF
C2
R2
R3
1 mF
1 MW
LED2
LED3
LED4
100 kW
100 kW
0.1 mF
LED Current Derating
The circuit shown in Figure 23 provides LED temperature
derating to avoid over−driving the LED under high ambient
temperatures, by reducing the LED current to protect the
LED from over−heating. The positive thermo coefficient
(PTC) thermistor RPTC is used for temperature sensing and
should be located near the LED. As the temperature of
RPTC increases, the gate voltage of the MOSFET M1
decreases. This causes the transistor M1 on−resistance to
increase which results in a reduction of the LED current. The
circuit is powered from a single VCC voltage of 5 V. The
recommended parts are Vishay 70°C thermistor
PTCSS12T071DTE and ON Semiconductor 2N7002L
N−Channel transistor (in SOT23).
The PCB and heatsink for the LED should be designed
such that the LED current is constant within the normal
temperature range. But as soon as the ambient temperature
exceeds a max threshold, the LED current drops to protect
the LEDs from overheating.
