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PAM2845 Datasheet(PDF) 11 Page - Power Analog Micoelectronics |
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PAM2845 Datasheet(HTML) 11 Page - Power Analog Micoelectronics |
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11 / 14 page  , Power Analog Microelectronics Inc 11 www.poweranalog.com 08/2008 Rev 1.0 Method 1: LED Current Setting with External Resistor R Method 2: LED Current Setting Using PWM Signal to PWMD Pin Method 3: LED Current Setting with single wire logic to ENA Pin LED Short Protection ISET The most basic means of setting the LED current is connecting a resistor between R and GND. The LED current is decided by I Resistor. I =228/ R This circuit uses resistor R to set the on state current and the average LED current, then proportional to the percentage of on-time when the PWMD pin is logic low. Average LED current is approximately equal to: I= ( *I)/ (t + t ) Also, the recommended PWM frequency is between 100Hz and 1kHz. Frequency <100Hz can cause the LEDs to blink visibly. When the LEDs are enabled by high level, the LED current initially goes to I . Dimming is done by pulsing ENA low (500ns to 10 s pulse width). Each pulse reduces the LED current by 1/32, so after one pulse the LED current is 31/32*I . The 32th pulse sets the LED current back to . Figure 1 shows a timing diagram for EN. ISET SET LED ISET ISET on off LED LED t I on LED μ Setting the Output Voltage The FB pin is connected to the center tap of a resistive voltage divider (R1 and R2 in Figure 1) from the high-voltage output. V =V (1+R2/R1) The r ecommend procedure i s t o c hoose R2=300k and R1=9.2k to set V =40V. Generally the Vout must be higher than total LED voltage. For 10-LED application, R1=9.1k R2=270k Vout=36.8V, higher than V =33V. One or more of the LED1-6 pins could be floating if not used because the PAM2845 uses external resistor to set the output voltage. One or more LEDx pins floating is just like change of output loadings. OUT FB OUT LED ΩΩ Ω, Ω, MAX The PAM2845 uses LED FB function to protect devices when one or more LED(s) is/are shorted. V= V – V N Normally is around 0.4V and is decided by LED numbers. When one or more LED(s) is/are shorted, the PAM2845 will clamp to make sure all LED pins’ voltage is less then 5V. With this function will be clamped at (5V+ N ). Note: : LED pin voltage : Output voltage V : LED forward voltage : The minimum LED numbers among all strings. LED OUT f MIN f VV V V V N LED OUT f LED OUT MIN V V OUT OUT PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source |
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