COILTRONICSDR125-221-R,or datasheet(13/16 Pages) TI1

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SM74301

Cff

RFB2

VOUT

RFB1

Cff =

3 x tON (max)

(RFB1//RFB2)

SM74301

www.ti.com

SNVS719A – JULY 2011

Figure 13. Efficiency vs. Load Current and VIN

Figure 14. Efficiency vs. VIN

LOW OUTPUT RIPPLE CONFIGURATIONS

For applications where low output ripple is required, the following options can be used to reduce or nearly

eliminate the ripple.

a) Reduced ripple configuration: In Figure 15, Cff is added across RFB2 to AC-couple the ripple at VOUT directly

to the FB pin. This allows the ripple at VOUT to be reduced to a minimum of 25 mVp-p by reducing R3, since the

ripple at VOUT is not attenuated by the feedback resistors. The minimum value for Cff is determined from:

(9)

where tON(max) is the maximum on-time, which occurs at VIN(min). The next larger standard value capacitor should

be used for Cff.

Figure 15. Reduced Ripple Configuration

b) Minimum ripple configuration: If the application requires a lower value of ripple (<10 mVp-p), the circuit of

Figure 16 can be used. R3 is removed, and the resulting output ripple voltage is determined by the inductor’s

ripple current and C2’s characteristics. RA and CA are chosen to generate a sawtooth waveform at their junction,

and that voltage is AC-coupled to the FB pin via CB. To determine the values for RA, CA and CB, use the

following procedure:

Calculate VA = VOUT - (VSW x (1 - (VOUT/VIN(min))))

(10)

where VSW is the absolute value of the voltage at the SW pin during the off-time (typically 1V). VA is the DC

voltage at the RA/CA junction, and is used in the next equation.

- Calculate RA x CA = (VIN(min) - VA) x tON/ΔV

(11)

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COILTRONICSDR125-221-R,or Datasheet(PDF) 13 Page - Texas Instruments