Feed Forward Compensation For Zxsc300 Led Driver

 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.
Input voltage feed forward compensation
Normally, IPK is set by the output current threshold voltage VISENSE divided by RSENSE. As the
input voltage increases, the inductor ripple current level I decreases because the Transistor off
time, T  is fixed by the ZXSC300
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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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