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CAT32
Table 8. R SET RESISTOR VALUES
I LED (mA)
40
30
25
20
15
10
5
R SET
562 W
750 W
909 W
1.13 k W
1.50 k W
2.26 k W
4.53 k W
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 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.
When a PWM control signal is used to drive the SHDN pin
of the CAT32, the LEDs are turned off and on at the PWM
frequency. The current through them alternates between full
current and zero current, so the average current changes with
R SET + 255
R MIN + 255
For other LED current values, use the following equation
to choose R SET .
0.1 V
I LED
Most white LEDs are driven at maximum currents of
15 mA to 20 mA. Some higher power designs will use two
parallel strings of LEDs for greater light output, resulting in
30 mA to 40 mA (two strings of 15 mA to 20 mA) flowing
into the LED pin.
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
duty cycle. This ensures that when the LEDs are on, they can
be driven at the appropriate current to give the purest white
light. LED brightness varies linearly with the PWM duty
cycle.
LED Dimming with a Logic Signal
For applications that need to adjust the LED brightness in
discrete steps, a logic signal can be used. RMIN sets the
minimum LED current value (when the NMOS is OFF):
0.1 V
I LED(MIN)
R INCR determines how much LED current increases when
the external NMOS switch is turned ON.
full current, but their average current changes with the PWM
signal duty cycle. Typically, a 5 kHz to 40 kHz PWM signal
is used. PWM dimming with the CAT32 can be
R INCR + 255
0.1 V
I LED(Increase)
R ADJ + 225
accomplished two different ways.
The SHDN pin can be driven directly or a resistor can be
added to drive the RSET pin. If the SHDN pin is used,
increasing the duty cycle will increase the LED brightness.
Using this method, the LEDs can be dimmed and turned off
completely using the same control signal. A 0% duty cycle
signal will turn off the CAT32, reducing the total quiescent
current to near zero.
If the RSET pin is used, increasing the duty cycle will
decrease the brightness. Using this method, the LEDs are
dimmed using RSET and turned off completely using
SHDN. If the RSET pin is used to provide PWM dimming,
the approximate value of R PWM should be calculated (where
V MAX is the “HIGH” value of the PWM signal):
LED Dimming with a DC Voltage
V MAX * 0.1 V
I LED(MAX) * I LEAD(MIN)
PCB Layout Guidelines
The CAT32 is a high ? frequency switching regulator and
therefore proper PCB board layout and component
placement can minimize noise and radiation and increase
efficiency. To maximize efficiency, the CAT32 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 switch, schottky output diode and output capacitor
R PWM + R SET
V MAX
0.15 V
* 1
signal path should be kept as short as possible. The ground
connection for the R SET resistor should be tied directly to the
GND pin and not be shared with other components.
CAT32
CAT32
CAT32
CAT32
CAT32
SHDN
5
PWM
RSET
4
R PWM
R SET
PWM
RSET
4
10 k W
R SET
R PWM
0.1 m F
PWM
RSET
4
R SET
R ADJ
RSET
4
V DC
R INCR
R MIN
Logic
Signal
Figure 15. LED Dimming Circuits
http://onsemi.com
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