TPS54341_15 datasheet(27/48 Pages) TI

27 / 48 page

VIN

GND

6 V to 42 V

2.2µF

DNP

GND

88.7k

365k

GND

TP1

TP2

GND

2.2µF

C10

DNP

C11

DNP

0.01µF

C13

PWRGD

TP10

BOOT

VIN

SS/TR

RT/CLK

COMP

GND

PWRGD

PAD

TPS54341DPR

162k

11.5k

47pF

5600pF

SS/TR

GND

SS/TR

GND

SS/TR

0.1µF

3.3V 3.5A

VOUT

GND

47µF

DNP

47µF

DNP

100µF

GND

49.9

31.6k

10.2k

TP6

TP8

TP4

TP7

TP5

TP3

5.6µH

7443552560

C12

DNP

PDS560-13

PWRGD PULL UP

TP9

1.00k

TPS54341

www.ti.com

SLVSC61 – NOVEMBER 2013

Figure 46. 3.3-V Output TPS54341 Design Example

Output Inductor Selection (LO)

To calculate the minimum value of the output inductor, use Equation 31.

KIND is a ratio that represents the amount of inductor ripple current relative to the maximum output current. The

inductor ripple current is filtered by the output capacitor. Therefore, choosing high inductor ripple currents

impacts the selection of the output capacitor because the output capacitor must have a ripple current rating equal

to or greater than the inductor ripple current. In general, the inductor ripple value is at the discretion of the

designer, however, the following guidelines may be used.

For designs using low ESR output capacitors such as ceramics, a value as high as KIND = 0.3 may be desirable.

When using higher ESR output capacitors, KIND = 0.2 yields better results. Because the inductor ripple current is

part of the current mode PWM control system, the inductor ripple current should always be greater than 150 mA

for stable PWM operation. In a wide input voltage regulator, it is best to choose relatively large inductor ripple

current. This provides sufficienct ripple current with the input voltage at the minimum.

For this design example, KIND = 0.3 and the minimum inductor value is calculated to be 4.8 μH. The nearest

standard value is 5.6

μH. It is important that the RMS current and saturation current ratings of the inductor not be

exceeded. The RMS and peak inductor current can be found from Equation 33 and Equation 34. For this design,

the RMS inductor current is 3.5 A and the peak inductor current is 3.95 A. The chosen inductor is a WE

7443552560, which has a saturation current rating of 7.5 A and an RMS current rating of 6.7 A.

As the equation set demonstrates, lower ripple currents will reduce the output voltage ripple of the regulator but

will require a larger value of inductance. Selecting higher ripple currents will increase the output voltage ripple of

the regulator but allow for a lower inductance value.

The current flowing through the inductor is the inductor ripple current plus the output current. During power up,

faults or transient load conditions, the inductor current can increase above the peak inductor current level

calculated above. In transient conditions, the inductor current can increase up to the switch current limit of the

device. For this reason, the most conservative design approach is to choose an inductor with a saturation current

rating equal to or greater than the switch current limit of the TPS54341 device which is nominally 5.5 A.

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