cat37tdi-t3 Catalyst Semiconductor, cat37tdi-t3 Datasheet - Page 8

• Manufacturer Part Number: cat37tdi-t3
• Description: Cmos White Led Driver Boost Converter
• Manufacturer: Catalyst Semiconductor

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
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
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.
Doc. No. MD-5005 Rev. T
Figure 3. Dimming with a
V
DC
18kΩ
R3
DC Voltage
CAT37
FB
1kΩ
R2
FB
). When adjusting V
R1
6.3Ω
PWM
Figure 4. Dimming with a
DC
10kΩ
DC
Filtered PWM Signal
increase
between
18kΩ
R3
0.1µF
CAT37
8
FB
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):
R
when the external NMOS switch is turned ON.
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.
1kΩ
R2
INCR
determines how much LED current increases
R1
6.3Ω
R
INCR
Signal
Logic
R
1
=
Figure 5. Dimming with a
=
I
LED
I
. 0
Characteristics subject to change without notice
LED
. 0
095
(
INCREASE
(
095
MIN
Logic Signal
2N7002
V
)
V
R
INCR
©
)
Catalyst Semiconductor, Inc.
CAT37
FB
R1
6.3Ω