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CM9133 Datasheet(PDF) 11 Page - California Micro Devices Corp |
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CM9133 Datasheet(HTML) 11 Page - California Micro Devices Corp |
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11 / 14 page  © 2006 California Micro Devices Corp. All rights reserved. 04/26/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: 408.263.3214 l Fax: 408.263.7846 l www.cmd.com 11 CM9133 PRELIMINARY 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. Figure 7. PWM applied to Group 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 the brightness control linearity. In addition, higher frequency can cause chro- maticity 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 the LED brightness, 0% Duty Cycle will turn off the display LEDs. Figure 8. PWM Signal Dimming Figure 9. Separate PWM signals for each group CM9133 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. Lightguides 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 CM9133 is a two-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. The either display’s intensity can be lowered by a PWM signal applied to RSET resistors for the host controller, as determine by the ambient light conditions. Figure 10. Clamshell Phone Application Display Group1 Display Group2 LED4 VOUT LED1 LED2 LED3 LED5 LED6 CM9133 VIN VBATT GND R SET1 ISET2 EN ISET1 R SET2 on off Enable Group2 Group1 PWM on off EN ILED (4,5,6) ILED (1,2,3) VOUT I SET1 Low < 10 mV Group2 I SET2 LED4 CM9133 VOUT LED1 VIN LED2 LED3 LED5 V BATT Display Group1 Display Group2 GND RSET1 ISET2 EN ISET1 PWM on off R SET2 PWM on off LED6 on off Enable Main Display MENU CM9133 VOUT LED1 LED2 LED3 LED4 LED5 Sub Display Caller ID Enable Display 1 VIN V BATT GND R SET1 ISET2 EN ISET1 RSET2 Enable on off on off LED6 PWM on off Analog or Application Information (cont’d) |
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