f

CO +

f

LC

2

85 V

OUT

C

OUT +

1

3357

L

OUT

f

CO

V

OUT

ESR

MAX +

1

2p

C

OUT

f

CO

VPP(MAX)=

()

ESR

xV

V

V

MAX

OUT

IN(MAX)

OUT

x

-

NC

IN(MAX)

OUT

SW

xV

xL

xF

x0.8

1

x

ICOUT(RMS) =

Ö12

[

]

VOUT

IN(MAX)

OUT

xV

-V

VIN(MAX)

OUT

SW

xL

-F

x0.8xNC

()

TPS5410-EP

www.ti.com ......................................................................................................................................................................................... SLVS910 – SEPTEMBER 2009

(7)

and the desired output capacitor value for the output filter to:

(8)

For a desired crossover of 10 kHz and a 68-

μH inductor, the calculated value for the output capacitor is 36.5 μF.

The capacitor type should be chosen so that the ESR zero is above the loop crossover. The maximum ESR is:

(9)

The maximum ESR of the output capacitor also determines the amount of output ripple as specified in the initial

design parameters. The output ripple voltage is the inductor ripple current times the ESR of the output filter.

Check that the maximum specified ESR as listed in the capacitor data sheet results in an acceptable output

ripple voltage:

(10)

Where:

ΔVPP is the desired peak-to-peak output ripple.

NC is the number of parallel output capacitors.

FSW is the switching frequency.

The minimum ESR of the output capacitor should also be considered. For a good phase margin, if the ESR is

zero when the ESR is at its minimum, it should not be above the internal compensation poles at 24 kHz and

54 kHz.

The selected output capacitor must also be rated for a voltage greater than the desired output voltage plus one

half the ripple voltage. Any derating amount must also be included. The maximum RMS ripple current in the

output capacitor is given by Equation 11:

(11)

Where:

NC is the number of output capacitors in parallel.

FSW is the switching frequency.

For this design example, a single 47-

μF output capacitor is chosen for C3. This value is close to the calculated

value of 36.5

μF and yields an actual closed loop cross over frequency of 10.05 kHz. The calculated RMS ripple

current is 84.9 mA and the maximum ESR required is 339 m

Ω. A capacitor that meets these requirements is a

AVX TPSE476M020R0150, rated at 20 V with a maximum ESR of 150 m

Ω and a ripple current rating of 1.369 A.

This capacitor results in a peak-to-peak output ripple of 44 mV using equation 10. An additional small 0.1-

μF

ceramic bypass capacitor may also used, but is not included in this design.

Other capacitor types can be used with the TPS5410, depending on the needs of the application.

Output Voltage Setpoint

The output voltage of the TPS5410 is set by a resistor divider (R1 and R2) from the output to the VSENSE pin.

Calculate the R2 resistor value for the output voltage of 12 V using Equation 12:

Copyright © 2009, Texas Instruments Incorporated

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