CAT37 Datasheet, PDF (8/13 Pages) Catalyst Semiconductor – CMOS White LED Driver Boost Converter

CAT37
LED DIMMING WITH DC SIGNAL
Dimming the LEDs can be done by applying a variable
DC voltage as shown on Figure 3. As the VDC increase
the 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 µA) is much smaller
than the LED current but much larger than the FB
leakage current (IFB). When adjusting VDC between 0V
and 2V, the resistors shown on Figure 3 will set the LED
current between 0mA and 15mA.
LED DIMMING WITH PWM SIGNAL
PWM brightness control provides the widest dimming
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 5kHz to
40kHz PWM signal is used.
A filtered PWM signal is used to dim the LEDs, as shown
in Figure 4.
In addition to providing the widest dimming range, PWM
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
15mA or 20mA. 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.
LED DIMMING WITH A LOGIC SIGNAL
For applications that need to adjust the LED brightness
in discrete steps, a logic signal can be used as shown in
Figure 5. R1 sets the minimum LED current value (when
the NMOS switch is OFF):
R1= ^0.095V
I_LED(MIN)
R_INCR determines how much LED current increases
when the external NMOS switch is turned ON.
R_INCR=
0.095V
I_{LED(INCREASE)}
PCB LAYOUT GUIDELINES
The CAT37 is a high-frequency switching regulator
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.
The SW pin, schottky diode and capacitor C2 signal
path 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.
VDC
CAT37
FB
R3
R2
18k
1k
10k
PWM
CAT37
FB
R3
R2
18k
1k
CAT37
FB
RINCR
LOGIC
SIGNAL
2N7002
R1
Figure 3. Dimming with a
DC Voltage
Figure 4. Dimming with a Filtered
PWM Signal
Figure 5. Dimming with a
Logic Signal
Doc. No. 5005, Rev. R
8