TPS54386PWPR datasheet(23/52 Pages) TI

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Example: TPS54386 Buck Converter Operating at 12-V Input, 3.3-V Output and 400-mA(P-P) Ripple Current

OUT

DIODE

+ V

+ VDIODE

3.3

0.5

d =

=30%

(10)

OUT

3.3

0.3

10.9 H

0.4

600000

´ d ´

(11)

(

)

(

)

RES

70 F

10 10

3.14

6000

´ p ´

(12)

(

)

ESR

RES

40 m

3.14 10

6000

´ p ´

(13)

100

100k

10k

-20

-180

-90

-45

-135

180

135

Frequency

Gain

Phase

-10

TPS54383,, TPS54386

SLUS774B – AUGUST 2007 – REVISED OCTOBER 2007

First, the steady state duty cycle is calculated. Assuming the rectifier diode has a voltage drop of 0.5 V, the duty

cycle is approximated using Equation 10.

The filter inductor is then calculated; see Equation 11.

A custom-designed inductor may be used for the application, or a standard value close to the calculated value

may be used. For this example, a standard 10-

µH inductor is used. Using Figure 28, find the 30% duty cycle

curve. The 30% duty cycle curve has a down slope from low frequency and rises at approximately 6 kHz. This

curve is the resonant frequency that must be compensated. Any frequency wthin an octave of the peak may be

used in calculating the capacitor value. In this example, 6 kHz is used.

A 68-

µF capacitor should be used as a bulk capacitor, with up to 10 µF of ceramic bypass capacitance. To

ensure the ESR zero does not significantly impact the loop response, the ESR of the bulk capacitor should be

placed a decade above the resonant frequency.

The resulting loop gain and phase are shown in Figure 31. Based on measurement, loop crossover is 45 kHz

with a phase margin of 60 degrees.

GAIN AND PHASE

FREQUENCY

Figure 31. Example Loop Result

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