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AN2839 Datasheet(PDF) 6 Page - STMicroelectronics |
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AN2839 Datasheet(HTML) 6 Page - STMicroelectronics |
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6 / 40 page  Designing an application with L6228 AN2839 6/40 Doc ID 15104 Rev 1 Figure 4. Voltage at the two outputs at the beginning of the off-time 1.3 Choosing the bulk capacitor Since the bulk capacitor, placed between the VS and GND pins, is charged and discharged during IC operation, its AC current capability must be greater than the RMS value of the charge/discharge current. This current flows from the capacitor to the IC during the on time (tON) and from the IC (in fast decay; from the power supply in slow decay) to the capacitor during the off time (tOFF). The RMS value of the current flowing into the bulk capacitor depends on the peak output current, output current ripple, switching frequency, duty cycle and chopping style. It also depends on the characteristics of the power supply. A power supply with poor high-frequency performances (or long, inductive connections to the IC) will cause the bulk capacitor to be recharged slowly: the higher the current control switching frequency, the higher the current ripple in the capacitor; RMS current in the capacitor, however, does not exceed the RMS output current. The bulk capacitor value (C) and the ESR determine the amount of voltage ripple on the capacitor itself and on the IC. In slow decay, neglecting the dead-time and output current ripple, and assuming that during the on time the capacitor is not recharged by the power supply, the voltage at the end of the on time is: Equation 1 so the supply voltage ripple is: Equation 2 where IOUT is the output current. In fast decay, instead, the recirculating current recharges the capacitor, causing the supply voltage to exceed the nominal voltage. This can be very dangerous if the nominal supply voltage is close to the maximum recommended supply voltage (52 V). In fast decay the supply voltage ripple is about: V S I OUT – ESR t ON C ----------- + ⎝⎠ ⎛⎞ ⋅ I OUT ESR t ON C ----------- + ⎝⎠ ⎛⎞ ⋅ |
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Similar Description - AN2839 |
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