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Power Supplies 101 Slide 35

Voltages other than the fixed options provided can be created by using a feedback network. It is recommended to use the 1V SC189 when requiring voltages other than the standard fixed voltages. Due to the internal compensation, the use of a capacitor across the top side of the feedback network is necessary to maintain stability. The equation for calculating the capacitor is found in this slide. VOUT is the desired output voltage and VOSTD is the voltage of SC189 with no feedback network. The feedback resistors are calculated via the formula found on slide 4. The VREF to be used in the equation is the VOSTD of the part, or 1V when the recommended SC189A is used. In this example, the desired voltage out is 2.2V. Here, a 10kΩ RFB1 is used and RFB2 is calculated to be 12kΩ. CFF is calculated to be approximately 390pF.

PTM Published on: 2011-09-27