LM3478MASLASHNOPB datasheet(14/43 Pages) TI2

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LM3478

ISEN

RSEN

RSL

'In =

Sf - Se

Sn + Se

'In-1

LM3478

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Feature Description (continued)

(1)

Hence, if the quantity (Sf - Se)/(Sn + Se) is greater than 1, the inductor current diverges and sub-harmonic

oscillation results. This counts for all current mode topologies. The LM3478 has some internal slope

compensation VSL which is enough for many applications above 50% duty cycle to avoid sub-harmonic

oscillation .

For boost applications, the slopes Se, Sf and Sn can be calculated with Equation 2, Equation 3, and Equation 4.

Se = VSL x fs

(2)

Sf = Rsen x (VOUT - VIN)/L

(3)

Sn = VIN x Rsen/L

(4)

When Se increases, then the factor that determines if sub-harmonic oscillation will occur decreases. When the

duty cycle is greater than 50%, and the inductance becomes less, the factor increases.

For more flexibility, slope compensation can be increased by adding one external resistor, RSL, in the ISEN's path.

Figure 23 shows the setup. The externally generated slope compensation is then added to the internal slope

compensation of the LM3478. When using external slope compensation, the formula for Se becomes:

Se = (VSL + (K x RSL)) x fs

(5)

A typical value for factor K is 40 µA.

The factor changes with switching frequency. Figure 24 is used to determine the factor K for individual

applications and Equation 6 gives the factor K.

K = ΔVSL / RSL

(6)

It is a good design practice to only add as much slope compensation as needed to avoid sub-harmonic

oscillation. Additional slope compensation minimizes the influence of the sensed current in the control loop. With

very large slope compensation the control loop characteristics are similar to a voltage mode regulator which

compares the error voltage to a saw tooth waveform rather than the inductor current.

Figure 23. Adding External Slope Compensation

Figure 24. External Slope Compensation

ΔVSL vs RSL

LM3478MA/NOPB Datasheet(PDF) 14 Page - Texas Instruments

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