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TS19450 Arkusz danych(PDF) 5 Page - Taiwan Semiconductor Company, Ltd |
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TS19450 Arkusz danych(HTML) 5 Page - Taiwan Semiconductor Company, Ltd |
5 / 8 page TS19450 AC/DC WLED Driver with External MOSFET Universal High Brightness 5/8 Version: A10 Application Information (Continue) The input current drawn from the VIN pin is a sum of the 1.0mA current drawn by the internal circuit and the current drawn by the GATE driver (which in turn depends on the switching frequency and the GATE charge of the external In the above equation, fS is the switching frequency and QGis the GATE charge of the external MOSFET (which can be obtained from the datasheet of the MOSFET). Current Sense The current sense input of the TS19450 goes to the non-inverting inputs of two comparators. The inverting terminal of one comparator is tied to an internal 250mV reference whereas the inverting terminal of the other comparator is connected to the LD pin. The outputs of both these comparators are fed into an OR GATE and the output of the OR GATE is fed into the reset pin of the flip-flop. Thus, the comparator which has the lowest voltage at the inverting terminal determines when the GATE output is turned off. The outputs of the comparators also include a 150~280ns blanking time which prevents spurious turn-offs of the exter- nal MOSFET due to the turn-on spike normally present in peak current mode control. In rare cases, this internal blanking might not be enough to filter out the turn-on spike. In these cases, an external RC filter needs to be added between the external sense resistor (RCS) and the CS pin. Please note that the comparators are fast (with a typical 80ns response time). Hence these comparators are more susceptible to be triggered by noise than the comparators of the TS19450. A proper layout minimizing external induc- tances will prevent false triggering of these comparators. Oscillator The oscillator in the TS19450 is controlled by a single resistor connected at the RT pin. The equation governing the oscillator time period tOSC is given by: If the resistor is connected between RT and GND, TS19450 operates in a constant frequency mode and the above equation determines the time-period. If the resistor is connected between RT and GATE, the TS19450 operates in a constant off-time mode and the above equation determines the off-time. Linear Dimming The Linear Dimming pin is used to control the LED current. There are two cases when it may be necessary to use the Linear Dimming pin. * In some cases, it may not be possible to find the exact value required to obtain the LED current when the RCS internal 250mV is used. In these cases, an external voltage divider from the VDD pin can be connected to the LD pin to obtain a voltage (less than 250mV) corresponding to the desired voltage across RCS. * Linear dimming may be desired to adjust the current level to reduce the intensity of the LEDs. In these cases, an external 0~250mV voltage can be connected to the LD pin to adjust the LED current during operation. To use the internal 250mV, the LD pin can be connected to VDD. Note: Although the LD pin can be pulled to GND, the output current will not go to zero. This is due to the presence of a mini- mum on-time (which is equal to the sum of the blanking time and the delay to output time) which is about 450ns. This will cause the MOSFET to be on for a minimum of 450ns and thus the LED current when LD = GND will not be zero. This current is also dependent on the input voltage, inductance value, forward voltage of the LEDs and circuit parasitic. To get zero LED current, the PWMD pin has to be used. |
Podobny numer części - TS19450 |
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Podobny opis - TS19450 |
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