CAT32_08 Datasheet, PDF (8/14 Pages) Catalyst Semiconductor – CMOS White LED Driver

CAT32
DIODE SELECTION
Schottky diodes, with their low forward voltage drop
and fast switching speed, are the ideal choice for high
efficiency applications. Table 3 shows several
different Schottky diodes that work well with the
CAT32. Make sure that the diode has a voltage rating
greater than the output voltage. The diode conducts
current only when the power switch is turned off
(typically less than one-third the time), so a 0.4A or
0.5A diode will be sufficient for most designs.
Table 3: Schottky Diode Suppliers
Part
MBR0520
MBR0530 s
MBR0540
ZHCS400
Supplier
ON Semiconductor
www.onsemi.com
800.282.9855
Zetex
LED CURRENT PROGRAMMING
The LED current is programmed with a single resistor
connected to the RSET pin The RSET pin is internally
regulated to 100mV, which sets the current flowing
out of this pin, ISET, equal to 100mV/RSET. The
CAT32 regulates the current into the LED pin, I_LED, to
225 times the value of I_SET. For the best accuracy, a
1% or better resistor is recommended. Table 4 shows
several typical 1% R_SET values.
Table 4: R_SET Resistor Values
I_LED (mA)
40
30
25
20
15
10
5
R_SET
562Ω
750Ω
909Ω
1.13kΩ
1.50kΩ
2.26kΩ
4.53kΩ
For other LED current values, use the following
equation to choose R_SET.
0.1V
R_SET = 255 × _ILED
Most white LEDs are driven at maximum currents of
15mA to 20mA. Some higher power designs will use
two parallel strings of LEDs for greater light output,
resulting in 30mA to 40mA (two strings of 15mA to
20mA) 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 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 ¯S¯H¯D¯N¯ pin can be driven directly or a resistor
can be added to drive the RSET pin. If the ¯S¯H¯D¯N¯
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.
R_PWM = R_SET ×
(
V_{MAX – 1 )}
0.15V
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 ¯S¯H¯D¯N¯. 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):
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
¯S¯H¯D¯N¯ 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.
Doc. No. MD-5001 Rev. AD
8
© Catalyst Semiconductor, Inc.
Characteristics subject to change without notice