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Calculating upper and lower DAC references for a hysteretic controller of PowerPSoC device

Last Updated: December 05, 2011
Question: 

How can one calculate upper and lower DAC references for a hysteretic controller of PowerPSoC device?

Answer: 

 Example: Let the current through an LED string be 300mA with a ripple of 40%.

 So Iavg = 300 mA.

 Hence Ipeak = 1.2 x Iavg = 360 mA and Ivalley = 0.8 x Iavg = 240 mA

 Let the,

sense resistance value Rsense = 0.22 W

gain of CSA be 20 and,

the 8-bit DAC be set at a resolution of 2.6V/10mV.

 Current sense voltage Vpeak = Ipeak x Rsense = 360 x 0.22 = 79.2 mV.

 Similarly Vvalley= Ivalley x Rsense = 240 x 0.22 = 52.8 mV.

 Corresponding to Vpeak and Vvalley, the output of CSA would be the gain multiplied by the voltage.

 Hence,

                        V’peak = Vpeak x 20 = 1.584 V and

                        V’valley= Vvalley x 20 = 1.056 V.

Now, the peak or upper limit DAC reference is calculated as,

REFupper = (V’peak/2.6) x 256 = 156


Similarly, valley or lower DAC reference is,

REFlower = (V’valley/2.6) x 256 = 104

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