IN(MIN)

OUT

IN(MIN)

L(PEAK )

L(MAX)

sw

OUT

V

(V

V

)

I

I

14.4 A

2 L1 F

V

u

u

u

u

OUT

OUT(MAX)

L(MAX)

IN(MIN)

V

I

I

13.3 A

0.9 V

u

u

2

IN(MIN)

OUT

IN(MIN)

BOOST

2

OUT(MAX)

sw

OUT

V

(V

V

)

L

2.1 H

0.4 I

F

V

u

P

u

u

u

IN(MAX)

OUT

OUT

BUCK

OUT(MAX)

sw

IN(MAX)

(V

V

) V

L

11.1 H

0.4 I

F

V

u

P

u

u

u

OUT

FB2

FB1

V

0.8 V

R

R

280 k

0.8 V

u

:

FB1

R

20 k:

21

LM5175

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Typical Application (continued)

9.2.1 Design Requirements

For this design example, the following are used as the input parameters.

DESIGN PARAMETER

EXAMPLE VALUE

Input Voltage Range

6 V to 36 V

Output

12 V

Load Current

6 A

Switching Frequency

300 kHz

Mode

CCM, Hiccup

9.2.2 Detailed Design Procedure

9.2.2.1 Frequency

resistor required to set the desired frequency is calculated using Equation 5 or Figure 3 . A 1% standard resistor

of 84.5 k

The output voltage is set using a resistor divider to the FB pin. The internal reference voltage is 0.8 V. Normally

the bottom resistor in the resistor divider is selected to be in the 1 k

Ω to 100 kΩ range. Select

(11)

The top resistor in the feedback resistor divider is selected using Equation 12:

(12)

9.2.2.3 Inductor Selection

The inductor selection is based on consideration of both buck and boost modes of operation. For the buck mode,

inductor selection is based on limiting the peak to peak current ripple

current at the maximum input voltage. The target inductance for the buck mode is:

(13)

For the boost mode, the inductor selection is based on limiting the peak to peak current ripple

maximum inductor current at the minimum input voltage. The target inductance for the boost mode is:

(14)

In this particular application, the buck inductance is larger. Choosing a larger inductance reduces the ripple

current but also increases the size of the inductor. A larger inductor also reduces the achievable bandwidth of the

converter by moving the right half plane zero to lower frequencies. Therefore a judicious compromise should be

made based on the application requirements. For this design a 4.7-µH inductor is selected. With this inductor

The maximum average inductor current occurs at the minimum input voltage and maximum load current:

(15)

where a 90% efficiency is assumed. The peak inductor current occurs at minimum input voltage and is given by:

(16)