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IR3725 Datasheet(PDF) 10 Page - International Rectifier |
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IR3725 Datasheet(HTML) 10 Page - International Rectifier |
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10 / 19 page  IR3725 Data Sheet Page 10 of 19 www.irf.com 2008_12_09 INDUCTOR DCR CURRENT SENSING APPLICATION Referring to the Functional Description Diagram, it can be seen that the shunt function can be accomplished by the DC resistance of the inductor that is already present. Omitting the resistive shunt reduces BOM cost and increases efficiency. In exchange for these two significant advantages two easily compensated design complications are introduced, a time constant and a temperature coefficient. The inductor voltage sensed between the Rcs1 resistors is not simply proportional to the inductor current, but rather is expressed in the Laplace equation below. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⋅ Ι = DCR L s 1 DCR L L V This inductor time constant is canceled when CS1 CS2 CS1 CS2 CS1 C R R R R 2 DCR L ⋅ + ⋅ ⋅ = . Let eq CS2 CS1 CS2 CS1 R R R R R = + ⋅ . A second equation is used to set the full-scale inductor current. ( ) DCR R R R V I CS2 CS1 T IG FS + ⋅ = . Let sum CS2 CS1 R R R = + and solve for Rsum. Select a standard value CCS1 that is larger than sum R DCR L 4 ⋅ ⋅ . Solve for Req. We now know Req and Rsum, but we do not know the individual resistor values RCS1 or RCS2. The next step is to solve for them simultaneously. By substituting Rsum into the Req equation the following can be written: sum CS2 CS1 eq R R R R ⋅ = , which can then be rearranged to 0 R R R R R sum eq sum CS1 2 CS1 = ⋅ + ⋅ − . Note that this equation is of the form 0 c bx ax2 = + + where a=1, b=-Rsum, and c=Req•Rsum. The roots of this quadratic equation will be RCS1 and RCS2. Use the higher value resistor as RCS1 in order to minimize ripple current in CCS1. 2 R R 4 1 1 R R SUM eq SUM CS1 ⋅ − + ⋅ = 2 R R 4 1 1 R R SUM eq SUM CS2 ⋅ − − ⋅ = |
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