ƒ

LC +

1

2p

L

OUT

C2

^ ƒZ1

(17)

C2 ^

1

Z1

L

OUT

(18)

ƒ

P(ESR) +

1

2pR

(C2ESR)

C5

+ ƒZ2

(19)

ƒ

CO +

1.125

3

ƒ

P(ESR)

ƒ

LC

(20)

Capacitor ESR and Output Ripple

TPS54352-EP

,, TPS54353-EP,, TPS54354-EP

TPS54355-EP

, TPS54356-EP, TPS54357-EP

The TPS54356 contains a compensation network with the following nominal characteristics:

For a stable design, the closed-loop crossover frequency should be set less than one-fifth of the switching

frequency, and the phase margin at crossover must be greater than 45 degrees. The general procedure outlined

here produces results consistent with these requirements, without going into great detail about the theory of loop

compensation.

In this case, the output filter LC corner frequency should be selected to be near the first compensation zero

frequency, as described by equation 17:

Solving for C2 using equation 18:

The desired value for C2 is calculated as 184

µF. A close standard value of 330 µF is chosen, with a resulting

LC corner frequency of 1.9 kHz. As shown, this value is not critical as long as it results in a corner frequency in

Next, when using a large ceramic capacitor in parallel with a high-ESR electrolytic capacitor, there is a pole in

Now, the actual C2 capacitor must be selected based on the ESR and the value of capacitor C5, so that the

capacitors, by choosing a Panasonic EEVFKOJ331XP aluminum electrolytic capacitor with a nominal ESR of

0.34

Ω yields a calculated value for C5 of 98

µF. The closest standard value is 100 µF. As the actual ESR of the

capacitor can vary by a large amount, this value also is not critical.

frequency. Also, the crossover frequency should not exceed 70 kHz, as the error amplifier may not provide the

this design, the crossover frequency can be estimated by:

This simplified equation is valid for this design because the output filter capacitors are mixed technology.

Compare this result to the actual measured loop response plot of Figure 5. The measured closed-loop crossover

frequency of 19.95 kHz differs from the calculated value because the actual output filter capacitor component

parameters differed slightly from the specified data-sheet values.

The amount of output ripple voltage, as specified in the initial design parameters, is determined by the maximum

ESR of the output capacitor and the input ripple current. The output ripple voltage is the inductor ripple current

times the ESR of the output filter, so the maximum specified ESR as listed in the capacitor data sheet is given by

equation 21:

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