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SC4505 Datasheet(PDF) 10 Page - Semtech Corporation |
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SC4505 Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 15 page  10 2007 Semtech Corp. www.semtech.com SC4505 POWER MANAGEMENT Applications Information An additional advantage of PWM dimming comes to customers who prefer to avoid inrush currents when filling the boost output capacitor – simply PWM the device at 12% duty for a millisecond or two, reducing inrush current to less than 50mA. This dim time will vary based on number of LED and size of output capacitor, but can be easily determined on the bench, and programmed into the uC firmware. For an example, suppose the current flowing through the I O1 channel is programmed at 20mA by RO1SET. A 1kHz PWM signal with the duty ratio of 20% is applied to the EN_BL pin of the SC4505. Then the average current flowing through the I O1 channel is 4mA 20mA 20% I O1AVG = × = A startup delay time between the enable signal goes high and the internal current regulator actually turns on is about 1.6 µs, which causes a small offset dependent on PWM frequency. As the PWM signal frequency goes higher, the effect of the delay will get more obvious to customers. However, since PWM is always linear, offset can be easily corrected in software. The offset correction factor can be described as: F PWM is in KHz. For an example, at 20kHz, D CORRECTION = 3%. So for 50% of the nominal LED current in I O1 channel, the PWM signal should have a duty ratio of 53%. Flash Timeout Programming When Channel IO2 is in flash mode, a timer is available to prevent LED overstress. The timer is only active in Flash mode – not active in torch mode. The capacitor tied between the FTO pin and the AGND sets the time duration of the flash mode. In flash mode, an external capacitor is charged with 1.2 µA. When the voltage on this capacitor reaches the 1.25V threshold Channel 2 is turned off. The timer can be reset by entering torch mode or turning off channel 2. The FTO pin can simply be grounded to disable this feature, as would be necessary when using I O2 for sub display. PWM CORRECTION F S µ 1.6 100 D × × = The relationship between the EN_FL, FL_TRB, FTO pin voltage and the current flowing through the IO2 pin is illustrated in Figure 4 below. Figure 4. Relationship between RN_FL, FL_TRB, FTO and I O2 Current To calculate the FTO capacitor needed for a desired timeout, a simple formula can be used as shown below: t 10 0.862 C -6 ⋅ ⋅ = Where C is in Farads, and t is in Seconds. EN_FL FL_TRB t t t t FTO I O2 I O2MAX 20% x I O2MAX 0 0 0 0 1.25V Over Voltage Protection (OVP) SC4505 includes a built-in overvoltage protection circuit to prevent damage to the IC and output capacitor in the event of open-circuit condition. The output voltage of the boost converter is detected at the VO pin, and divided down by an internal resistor strand of 500k Ω. If the voltage at the VO pin exceeds 28V, the boost converter will be shut down, and a strong pulldown will be applied to the VO pin to quickly discharge the output capacitor. This additional level of protection prevents a condition where the output capacitor and Schottky diode must endure high voltage for an extended period of time, which can pose a reliability risk for the user’s system. The boost OVP triggering point can be adjusted by adding an external resistor divider at VO pin as shown in Figure 5. |
Similar Part No. - SC4505_07 |
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Similar Description - SC4505_07 |
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