TPS54821 datasheet(26/42 Pages) TI2

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× p ×

REF

OUT

C9 =

R7 F

× p ×

C9 R7

× p ×

C9 R7

× p ×

C5 =

R5 F

× p ×

C4 =

PWRSTG

out

REF

R5 =

TPS54821

SLVSB14B – OCTOBER 2011 – REVISED FEBRUARY 2016

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For this design, the intended crossover frequency is 80 kHz. From the power stage gain and phase plots, the

gain at 80 kHz is -8.281 dB and the phase is -137 degrees. For 60 degrees of phase margin, additional phase

boost from a feed forward capacitor in parallel with the upper resistor of the voltage set point divider will be

required. R5 sets the gain of the compensated error amplifier to be equal and opposite the power stage gain at

crossover. The required value of R5 can be calculated from Equation 32.

(32)

To maximize phase gain, the compensator zero is placed one decade below the crossover frequency of 80 kHz.

The required value for C4 is given by Equation 33.

(33)

To maximize phase gain the high frequency pole is placed one decade above the crossover frequency of 80 kHz.

The required value for C5 can be calculated from Equation 34.

(34)

The feed forward capacitor C9, is used to increase the phase boost at crossover above what is normally

available from Type II compensation. It places an additional zero/pole pair located at Equation 35 and

Equation 36.

(35)

(36)

This zero and pole pair is not independent. Once the zero location is chosen, the pole is fixed as well. For

optimum performance, the zero and pole should be located symmetrically about the intended crossover

frequency. The required value for C9 can calculated from Equation 37.

(37)

For this design the calculated values for the compensation components are R5 = 4.68 k

Ω ,C4 = 4290 pF, C5 =

42.9 pF and C9 = 467 pF. Using standard values, the compensation components are R5 = 4.64 k

Ω ,C4 = 3900

pF, C5 = 39 pF and C9 = 470 pF.

8.2.2.11 Fast Transient Considerations

In applications where fast transient responses are important, Type III frequency compensation can be used

instead of the traditional Type II frequency compensation.

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