AN2836 datasheet(12/57 Pages) STMICROELECTRONICS

12 / 57 page

Designing an application with L6225, L6226, L6227

AN2836

12/57

Doc ID 15084 Rev 1

4.3

Choosing the bulk capacitor

Since the bulk capacitor, placed between VS and GND pins, is charged and discharged

during the IC operation, its AC current capability must be greater than the RMS value of the

charge/discharge current. In case of PWM current regulation, the current flows from the

capacitor to the IC during the on-time (tON) and from the IC (implementing a fast decay

current recirculation technique) or from the power supply (implementing a slow decay

current recirculation technique) to the capacitor during the off-time (tOFF). The RMS value of

the current flowing into the bulk capacitor depends on peak output current, output current

ripple, switching frequency, duty cycle and chopping style. It also depends on power supply

characteristics. A power supply with poor high-frequency performances (or long, inductive

connections to the IC) causes 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. 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 deadtime 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. With fast decay, instead, 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:

Equation 3

always assuming that the power supply does not recharge the capacitor, and neglecting the

output current ripple and the deadtime. Usually (if C > 100 µF) the capacitance role is much

less than the ESR, then supply voltage ripple can be estimated as:

Equation 4

Equation 5

OUT

–

ESR

-----------

⎝⎠

⎛⎞

⋅

OUT

ESR

-----------

⎝⎠

⎛⎞

⋅

OUT

2ESR

⋅

OFF

--------------------------------

⎝⎠

⎛⎞

⋅

OUT

ESR

⋅

in slow decay

⋅

OUT

ESR

⋅

in fast decay

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