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AN47 Datasheet(PDF) 2 Page - Zetex Semiconductors |
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AN47 Datasheet(HTML) 2 Page - Zetex Semiconductors |
2 / 6 page AN47 www.zetex.com 2 Issue 1 - August 2006 © Zetex Semiconductors plc 2006 If the PWM frequency is higher than approximately 10kHz and the duty cycle above the specified minimum value, the device will remain active and the output will be continuous, with a nominal output current given by: I OUT 0.1 D PWM R S -------------------------- ≈ [for 0.16< DPWM <1] PWM GND ZXLD1350 ADJ GND Input buffer transistor For PWM dimming an input bipolar transistor with open collector output is recommended. This will ensure the 200mV input shutdown threshold is achieved. It is possible to PWM directly without a buffer transistor. This must be done with caution. Doing this will overdrive the internal 1.25V reference. If a 2.5V input level is used at 100% PWM (DC) the output current into the LED will be 2X the normal current which may destroy the ZXLD1350. Overdriving with a 5V logic signal is very likely to damage the device as it exceeds the ADJ pin voltage rating. Soft start and decoupling capacitors Any extra capacitor on the ADJ pin will affect the leading and falling edge of the PWM signal. Take this into account as the rise time will be increased by approximately 0.5ms/nF. Compare this with a 100Hz PWM. 50% duty cycle Ton and Toff are 5ms at 1% duty cycle Ton is 0.1ms. 1nF on the ADJ pin will cause 0.5ms rise time which result in an error and limitation in dimming at low duty cycles. |
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