CM9142 Datasheet, PDF (11/14 Pages) California Micro Devices Corp – Two Group, 4 and 2, WLED Driver, Different Current Settings

PRELIMINARY
CM9142
Application Information (cont’d)
operate either at the full ISET current, or at zero cur-
rent. Only the time averaged current changes. Above a
minimum frequency, the human eye will perceive the
change in duty cycle as a change in brightness.
V_BATT
VIN
off
PWM Group2
on
ISET2
R_SET2
ISET1
VOUT
LED1
LED2
LED3
LED4
Group1
R_SET1
on ^Enable
CM9142
LED5
LED6
EN
off
GND
Display
Group1
Display
Group2
Figure 7. PWM applied to Display 2
The recommended frequency is between 100 Hz and
200 Hz, with a duty cycle greater than 20%. If a fre-
quency of less then 100 Hz is used, flicker might be
seen in the LEDs. The frequency should also be
greater than the refresh rate of the TFT display. Higher
frequencies will cause a loss of brightness control lin-
earity. In addition, higher frequency can cause chroma-
ticity shifts because the fixed rise and fall times of the
PWM signal will shift the forward current.
The PWM signal will cause the average LED current to
be reduced. The average current is determined by the
PWM duty cycle, which can vary from 0% to 100%.
Decreased Duty Cycle will linearly lower LED bright-
ness, 0% Duty Cycle will turn off the display LEDs.
EN
VOUT
I_{LED (1,2,3)}
PWM signal
I_SET
V_BATT
on
PWM
off
on
R_SET2
PWM
off
R_SET1
on ^Enable
off
VIN
VOUT
ISET2 LED1
LED2
LED3
ISET1 LED4
CM9142
EN
GND
LED5
LED6
Display
Group1
Display
Group2
Figure 9. Separate PWM signals for each group
CM9142 Design Examples
Two Display Cell Phone
Typically, the mobile phone LCD displays (both STN
and mini-TFT) require three to four white LEDs for
backlighting, but as few as two of the newer high-
brightness LEDs can be used. Light guides are used to
distribute the light uniformly behind the LCD. In this
application, four white LEDs are used for the larger
main display (inside the clamshell) and two for the sub-
display.
A typical application for the CM9142 is a dual display
clamshell phone, with an internal main display and an
external sub-display typically used for caller ID and
time of day, backlighting only when there is an incom-
ing call. When the clamshell is opened, the sub-display
backlight goes off and the main display backlight goes
on. See Figure 10.
Either display’s intensity can be lowered by a PWM sig-
nal applied to R_SET resistors for the host controller, as
determine by ambient light conditions.
on
PWM
off
or
Analog
Enable
on ^{Display 1}
off
V_BATT
R_SET2
R_SET1
on ^Enable
off
VIN
ISET2
ISET1
VOUT
LED1
LED2
LED3
LED4
CM9142
EN
GND
LED5
LED6
Main
Display
MENU
Sub
Display
Caller ID
Figure 8. PWM Signal Dimming
Figure 10. Clamshell Phone Application
© 2006 California Micro Devices Corp. All rights reserved.
04/26/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
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