JAJSLR9I February 2006 – May 2021 LM5009
PRODUCTION DATA
C3: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents false triggering of the VCC UVLO at the buck switch on and off transitions. For this reason, C3 must be no smaller than 0.1 µF.
C4: The recommended value is 0.022 µF for C4 because this value is appropriate in the majority of applications. A high-quality ceramic capacitor, with low ESR is recommended because C4 supplies the surge current to charge the buck switch gate at turn-on. A low ESR also ensures a quick recharge during each off-time. At minimum VIN when the on-time is at maximum, C4 can possibly not fully recharge at start-up during each 300-ns off-time. This failure to recharge results from the circuit being unable to complete the start-up and achieve output regulation. This condition can occur when the frequency is intended to be low (for example, RON = 500 kΩ). In this case, increase C4 to maintain sufficient voltage across the buck switch driver during each on-time.