CAT37_08 Datasheet, PDF (8/13 Pages) Texas Instruments – 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 V_DC 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 (I_FB). When adjusting V_DC 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):
0.095V
R1 =
I_LED(MIN)
R_INCR determines how much LED current increases
when the external NMOS switch is turned ON.
0.095V
R_INCR = _{ILED(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.
CAT37
FB
R3
R2
18kΩ
1kΩ
V_DC
R1
6.3Ω
PWM
10kΩ
CAT37
FB
R3
R2
18kΩ
1kΩ
0.1µF
R1
6.3Ω
Logic
Signal
CAT37
FB
R_INCR
2N7002
R1
6.3Ω
Figure 3. Dimming with a
DC Voltage
Figure 4. Dimming with a
Filtered PWM Signal
Figure 5. Dimming with a
Logic Signal
Doc. No. MD-5005 Rev. T
8
© Catalyst Semiconductor, Inc.
Characteristics subject to change without notice