JAJSAH6H January 2006 – October 2017 LM3100
PRODUCTION DATA.
CIN’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at VIN, assume the voltage source feeding VIN has an output impedance greater than zero. If the source’s dynamic impedance is high (effectively a current source), CIN supplies the average input current, but not the ripple current.
At maximum load current, when the buck switch turns on, the current into VIN suddenly increases to the lower peak of the inductor’s ripple current, ramps up to the peak value, then drop to zero at turn-off. The average current during the on-time is the load current. For a worst case calculation, CIN must supply this average load current during the maximum on-time. CIN is calculated from:
where IOUT is the load current, tON is the maximum on-time, and ΔV is the allowable ripple voltage at VIN.
CIN3’s purpose is to help avoid transients and ringing due to long lead inductance at VIN. A low ESR, 0.1 µF ceramic chip capacitor is recommended, located close to the LM3100.