CAT37TDI-TE7 ON Semiconductor, CAT37TDI-TE7 Datasheet - Page 8

• Manufacturer Part Number: CAT37TDI-TE7
• Description: LED Drivers White LED Boost
• Manufacturer: ON Semiconductor

Specifications of CAT37TDI-TE7

• Number Of Segments: 4
• High Level Output Current: 40 mA
• Operating Supply Voltage: 2.5 V to 7 V
• Maximum Supply Current: 0.7 mA
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: TSOT-23-5
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

LED Dimming with DC Signal
DC voltage as shown on Figure 14. As the V
voltage across R1 decreases and therefore lower the LED
current. The resistors R2 and R3 must be large enough so
that their current (tens of mA) is much smaller than the LED
current but much larger than the FB leakage current (I
When adjusting V
shown on Figure 14 will set the LED current between 0 mA
and 15 mA.
LED Dimming with PWM Signal
range (greater than 20:1). By turning the LEDs ON and OFF
using the control signal, the LEDs operate at either zero or
full current, but their average current changes with the PWM
signal duty cycle. Typically, a 5 kHz to 40 kHz PWM signal
is used.
in Figure 15.
brightness control also ensures the “purest” white LED color
over the entire dimming range. The true color of a white
LED changes with operating current, and is the “purest”
white at a specific forward current, usually 15 mA or
20 mA. If the LED current is less than or more than this
value, the emitted light becomes more blue. Applications
involving color LCDs can find the blue tint objectionable.
V
Dimming the LEDs can be done by applying a variable
PWM brightness control provides the widest dimming
A filtered PWM signal is used to dim the LEDs, as shown
In addition to providing the widest dimming range, PWM
Figure 14. Dimming with a
DC
180 kW
R3
DC Voltage
CAT37
FB
10 kW
R2
DC
between 0 V and 2 V, the resistors
R1
6.3 W
PWM
DC
10 kW
Figure 15. Dimming with a
increase the
Filtered PWM Signal
http://onsemi.com
180 kW
FB
0.1 mF
R3
).
CAT37
FB
8
10 kW
R2
LED Dimming with a Logic Signal
discrete steps, a logic signal can be used as shown in
Figure 16. R1 sets the minimum LED current value (when
the NMOS switch is OFF):
R
the external NMOS switch is turned ON.
PCB Layout Guidelines
where proper PCB board layout and component placement
can minimize noise and radiation and increase efficiency. To
maximize efficiency, the CAT37 design has fast switch rise
and fall times. To prevent radiation and high frequency
resonance problems minimize the length and area of all
traces connected to the SW pin and use a ground plane under
the switching regulator.
should be kept as short as possible. The ground connection
for the R1 resistor should be tied directly to the GND pin and
not be shared with other components.
INCR
For applications that need to adjust the LED brightness in
The CAT37 is a high−frequency switching regulator
The SW pin, schottky diode and capacitor C2 signal path
determines how much LED current increases when
R1
6.3 W
R
Signal
Logic
INCR
R1 +
+
Figure 16. Dimming with a
I
LED(INCREASE)
I
0.095 V
LED(MIN)
0.095 V
Logic Signal
2N7002
R
INCR
CAT37
FB
R1
6.3 W