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SC441DEVB 数据表(PDF) 15 Page - Semtech Corporation |
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SC441DEVB 数据表(HTML) 15 Page - Semtech Corporation |
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15 / 21 page  SC441D 5 Applications Information (continued) depends on the PWM dimming frequency. Clearly, it is trivial to get 00% LED brightness by pulling PWM pin “High” constantly. When the user tries to dim the LED brightness using PWM signal from 00% down, he or she needs to observe the following. When the PWM dimming signal is actively switching from “High” to “Low” and to “High”, there is a minimal OFF time (T_off_min, 200ns, guaranteed by design) requirement of the PWM dimming signal with this IC. Such minimal OFF time sets the maximum PWM duty ratio before hitting to 00% in the following way. D max=-Toff_minfPWM For example, if the PWM dimming frequency f_ PWM=200Hz, the D_max=99.996%. If f_PWM=25kHz, the D_max=99.5%. With most practical dimming interfaces, theneededdimmingstepsandresolutions,itisuncommon to run into the above D_max before reaching 00%. While most applications will not run into D_max, the designer should be aware of possible parasitic elements from PWM dimming interface to the PWM pin of SC44D. Usually, simply checking signal D_max at PWM pin of SC44D is sufficient. Linear Dimming The linear dimming control is available for SC44D by applying an external control voltage on IOSET pin through an external resistor-like circuit (shown below). External environment brightness compensation can also be achieved when the control voltage is generated by a light sensor circuit. 5 5 4 4 3 3 2 2 1 1 D D C C B B A A V_EXT R_EXT R_IOSET IOSET The IOSET voltage is 0.5V when linear dimming is used and the minimum IOSET current must be higher than 27µA (i.e. 5mA per LED string). The external control voltage slew rate must slow at V/0ms. LED Strings Connection Generally, LED strings are connected to IO ~ IO4 pins through a mechanical connector which, generally, can- not support an electrical connection thereby resulting in significant noise. Consequently, the SC44D LED short- circuit protection may false trip when the noise level is large. Certain ceramic decoupling capacitor on pins IO ~ IO4 to GND are useful to prevent the SC44D from noise influence. As a general guideline, the decoupling capacitance should be limited as follows. o LED dcple V uS I C 6 . 0 * Where, I_LED is the LED current per string, Vo is the Boost output voltage and C_dcple is the suggested decoupling capacitor value. For example, if I_LED=0mA, Vo=3.5V, the calculated upper bound of C_dcple is about 444pF. One could use 390pF or less in the circuit. If I_LED=00mA, Vo=3.5V, the calculated upper bound of C_dcple is about 4.44nF. One may use 3.9nF or less in the circuit. In some appli- cations, circuit designers tend to select the decoupling capacitors in the range of (00pF ~ nF). For some low LED current (e.g. 0mA) applications, it is recommended to add M-0Mohm resistor from IO pin to GND in order to reduce IO pin voltage during PWM dimming. Parallel Operation When two or more SC44Ds are operating in parallel for a large-sized panel application, audible noise may be observed due to non-synchronous switching frequency. The ripple voltage on the input voltage rail will be mod- ulated by the beat frequency resulting in audible noise. This situation can be resolved by adding an input induc- tor between input voltage rail and the SC44D VIN pin. This situation can also be improved by adding more input decoupling capacitors. Inductor Selection The inductance value of the inductor affects the convert- er’s steady state operation, transient response, and its loop stability. Special attention needs to be paid to three specifications of the inductor, its value, its DC resistance and saturation current. The inductor’s inductance value also determines the inductor ripple current. The converter can operate in either CCM or DCM depending on its work- ing conditions. The inductor DC current or input current can be calculated as, |
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