W-52 Datasheet, PDF (8/11 Pages) Copal Electronics – CMOS White LED Driver

W-52
Table 8. R_SET RESISTOR VALUES
I_LED (mA)
40
30
25
20
15
10
5
R_SET
562
750
909
1.13 k
1.50 k
2.26 k
4.53 k
For other LED current values, use the following equation
to choose R_SET.
0.1 V
R_SET 255 _ILED
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
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 W-52 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 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 W-52, 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):
R_PWM R_SET
V_{MAX 1}
0.15 V
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 W-52, 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):
0.1 V
R_MIN 255 _ILED(MIN)
R_INCR determines how much LED current increases when
the external NMOS switch is turned ON.
R_INCR
255
0.1 V
I_{LED(Increase)}
LED Dimming with a DC Voltage
R_ADJ
225
V_MAX
I_LED(MAX)
0.1 V
I_LEAD(MIN)
PCB Layout Guidelines
The W-52 is a highïfrequency switching regulator and
therefore proper PCB board layout and component
placement can minimize noise and radiation and increase
efficiency. To maximize efficiency, the W-52 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
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.
W-52
SHDN
5
W-52
RSET
4
PWM
R_SET
R_PWM
W-52
PWM
RSET
4
R_SET
10 k
W-52
RSET
R_PWM
4
PWM
0.1 F
R_SET
R_ADJ
W-52
RSET
V_DC
R_MIN
4
R_INCR
Logic
Signal
Figure 15. LED Dimming Circuits
8