CAT32 Datasheet, PDF (9/14 Pages) Catalyst Semiconductor – CMOS White LED Driver

CAT32
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. PWM dimming with the
CAT32 can be 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 bright-
ness. 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 RPWM should be calculated
(where VMAX is the “HIGH” value of the PWM signal):
R_PWM
=
R_SET
×

V_MAX
0.15V
− 1^{}
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.
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 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):
R_MIN
=
225 ×
 _0.1V 
 ^ILED(MIN) 
LED DIMMING WITH A DC VOLTAGE
R_ADJ
=
225
×


V_MAX − 0.1V
I_{LED(MAX)−ILEAD(MIN)}


R_INCR determines how much LED current increases
when the external NMOS switch is turned ON.
R_INCR
=
225
×


0.1V
I_{LED(Increase)}


CAT32
SHDN
5
PWM
CAT32
RSET
4 RPWM
PWM
RSET
CAT32
RSET
4 10kΩ
RPWM
PWM
RSET
0.1µF
CAT32
RSET
4 RADJ
RSET
VDC
CAT32
RSET
4 RINCR
RMIN
LOGIC
SIGNAL
Figure 3: LED Dimming Circuits
9
Doc No. 5001, Rev. AC