SNVSC42A September 2023 – July 2024 LMQ64480-Q1 , LMQ644A0-Q1 , LMQ644A2-Q1
PRODUCTION DATA
A feedforward capacitor, CFF, on the order of tens of picofarads and no bigger than 15 nF, can be used to improve phase margin and transient response of circuits which have output capacitors with low ESR and tight voltage margin requirements. Because this CFF capacitor can conduct noise from the output of the circuit directly to the FB node of the IC, a 4.99-kΩ resistor, RFF, can be placed in series with CFF to lower high frequency susceptibility when it is used. If the ESR zero of the output capacitor is below 200 kHz, no CFF must be used. In general the Cff capacitor can be sized to add phase boost slightly above the desired crossover frequency.
If output voltage is less than 2.5 V, CFF has little effect, so it can be omitted. If output voltage is greater than 14 V, CFF must be used cautiously because it can easily introduce too much gain at higher frequencies.
If 1 MΩ is selected for RFBT, then a feedforward capacitor CFF is usually required to offset parasitic capacitance in PCB construction.