SINGLE 12 V SUPPLY
The circuit shown in Figure 3 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 3. Single Supply Driving 12 LEDs
The circuit in Figure 4 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 4. Daylight Detection
The circuit shown in Figure 5 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 5. Night Detection
LED CURRENT DERATING
The circuit shown in Figure 6 provides LED tempe-
rature 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 70ºC 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.
© 2009 SCILLC. All rights reserved.
Characteristics subject to change without notice
Doc. No. MD-5041 Rev. B