C2 and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage resulting from the C2 ESR, ripple voltage resulting from the C2 capacitance, and the nature of the load.
- ESR and R3: A low ESR for C2 is generally desirable to minimize power losses and heating within the capacitor. However, this regulator requires a minimum amount of ripple voltage at the feedback input for proper loop operation. For the LM5009, the minimum ripple required at pin 5 is 25 mV peak-to-peak, requiring a minimum ripple at VOUT1 of 100 mV. The minimum ESR required at VOUT1 is 3 Ω because the minimum ripple current (at minimum VIN) is 33 mA peak-to-peak. R3 is inserted as illustrated in the Section 7.2 section because quality capacitors for SMPS applications have considerably less ESR. The value of R3, along with the ESR of C2, must result in at least a 25-mV peak-to-peak ripple at pin 5. Generally, R3 is 0.5 Ω to 5.0 Ω.
- Nature of the load: The load can be connected to VOUT1 or VOUT2. VOUT1 provides good regulation, but with a ripple voltage that ranges from 100 mV (at VIN = 12 V) to 580 mV (at VIN = 90 V). Alternatively, VOUT2 provides low ripple (3 mV to 13 mV) but lower regulation resulting from R3.
C2 generally must be no smaller than 3.3 µF. Typically, the value of C2 is 10 µF to 20 µF, with the optimum value determined by the load. If the load current is fairly constant, a small value suffices for C2. If the load current includes significant transients, a larger value is necessary. For each application, experimentation is needed to determine the optimum values for R3 and C2.
- Ripple reduction: The ripple amplitude at VOUT1 can be reduced by reducing R3 and by adding a capacitor across R1 to transfer the ripple at VOUT1 directly to the FB pin without attenuation. The new value of R3 is calculated by Equation 8:
Equation 8. R3 = 25 mV / IOR(min)
where
- IOR(min) is the minimum ripple current amplitude—33 mAp-p in this example
The added capacitor value is calculated by Equation 9:
Equation 9. C = TON(max) / (R1 // R2)
where
- TON(max) is the maximum on-time (at minimum VIN)
The selected capacitor must be larger than the value calculated in Equation 9.