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CM9153 Datasheet(PDF) 9 Page - California Micro Devices Corp |
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CM9153 Datasheet(HTML) 9 Page - California Micro Devices Corp |
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9 / 12 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 9 CM9153 PRELIMINARY CM9153 Design Examples Capacitor Selection The external bucket capacitors will affect the output impedance of the converter, so surface mount, low ESR ceramic capacitors are recommended. Tantalum and Aluminum capacitors should not be used because their ESR is too high. The ceramic dielectric must be stable over the operating temperature and voltage range. As a result, X7R or X5R type dielectric is recom- mended. In noise sensitive applications, output ripple can be further reduced by increasing the capacitance of the output capacitor. Reflected input ripple current depends on the impedance of the VIN source, which includes the PCB traces. Increasing the input capacitor will reduce this ripple. The input capacitor also affects the output voltage ripple. All the capacitors should be located close to the device for best performance. LED Brightness Control Changes in ambient light often require the backlight display intensity to be adjusted, usually to conserve battery life. There are simple solutions to lowering the LED brightness when using the CM9153. PWM Brightness Control, Lowered Quies- cent Current A PWM signal can be used to control the brightness, which is more efficient than other solutions that dissi- pate the unwanted LED current in the series resistors. It also maintains the white LED color fidelity by avoid- ing color temperature variations that come with bias current changes. The LED intensity is determined by the PWM duty cycle, which can vary from 0% to 100%. In the configuration shown in Figure , the brightness is controlled by the PWM signal applied to ENA. Decreased Duty Cycle will lower the LED brightness, See Figure 3 and Figure 4. Figure 3. High Brightness Waveforms The recommended PWM frequency is between 100 Hz and 20 kHz. If a frequency 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 bright- ness control linearity. In addition, higher frequency can cause chromaticity shifts because the fixed rise and fall times of the PWM signal will shift the forward current. Figure 4. Low Brightness Waveforms Vin C1P VOUT VIN C1N C2P GND ENA C2N PWM input CM9153 20 uSec/div Iin, 200 mA/div C2N, 5V/div Vout ripple, 200 mV/div PWM, 20 kHz, 60% D.C., 5V/div Vin=3.8V Iin, 200 mA/div C2N, 5V/div Vout ripple, 200 mV/div PWM, 20 kHz, 10% D.C., 5V/div 20 uSec/div Vin=3.8V Application Information (cont’d) |
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