Feed Forward Compensation For Zxsc300 Led Driver

Input voltage feed forward compensation for ZXSC300 to improve control of the LED current Introduction The ZXSC300 LED Drivers do not directly control the LED current. As a consequence the LED current is dependent of the input voltage. This application note describes a way of reducing the supply voltage dependency by a method of supply voltage feed forward compensation. The method CAN also be used to provide temperature compensation of the LED The ZXSC300 works on the PFM control scheme where the LED current is simply regulated by controlling the peak current through Transistor Q1. The internal voltage threshold of Current Sense pin is around 19mV and Transistor Q1 is switched off when its current reaches the preset threshold, thereby necessitating fewer external components required. However, this threshold value is invariant to the supply voltage level. In the event where input voltage increases, peak Q1 current will stay the same and current delivered to the LED creeps up which could potentially damage the LED if it exceeds the maximum rated current of the device. The circuit diagram in Figure 1 shows how to apply input voltage and thermal correction to a typical LED A simple design guide for a single LED Driver has also been put forward. The equations CAN generate a design capable of sourcing up to 200mA LED current, when used with the Zetex high current gain NPN transistor-ZXTN25012EFH.
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Circuit diagram of ZXSC300 with feed forward and thermal compensation
Circuit diagram of ZXSC300 with feed forward and thermal compensation

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