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Part Number: LMZ14201H
Tool/software: WEBENCH® Design Tools
Using WEBENCH POWER DESIGNER created a design using Part # LMZ14201H.
Dc input 12-13V , Output -5V , Output current 0.5A.
My question is Cout capactiance value. Is the simulation implying Qty 3, 22uf caps in parallel ?
Is the ESR of 3.71mOhm for one cap or all 3 in parallel ?
My concern is getting the correct about of ripple because this device requires some ripple for feedback.
Is there a formula for calculating the Required ripple ?
Below is the suggested values from simulation :
Cap = 22 µF Total Derated Cap = 14 µF VDC = 10 V ESR = 3.71 mΩ Package = 0603
One last question , what does Total Derated cap = 14uF imply ?
I've generated the IBB LMZ14201 WEBENCH design for 12Vin, -5Vout, 0.5A loading and attached it at the end of this response.
The schematic shows a Cout of 3 x 22uF with each capacitor having 3.71mOhms. This will result in a total ESR of 1.24mOhms. If you go down towards the bottom of the page, you can see operating values for IC,capacitor, inductor, etc. This should provide information on the power dissipation and ripple current.
When voltage is applied across a capacitor, the effective capacitance will not be the same as the typical capacitance. This is because the capacitor experiences DC bias effects. For an output of -5V the capacitor will see a total of 5V in terms on derating. Attached is a simulation of the effects of 5V bias on the C1608X5R1A226M080AC capacitor used in the WEBENCH design. With three of these capacitors at the output of the converter, the effective Cout will be 3 x 4.4uF which is roughly 13 -14uF.
For more information on the process of configuring a buck converter to IBB, here is app note that should act as good reference material. For the inductor ripple current calculation please refer to the second term in equation 5.
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