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 Custom Design with WEBENCH Tools

Click here to create a custom design using the LM5175 device with the WEBENCH® Power Designer.

1. Start by entering your VIN, VOUT and IOUT requirements.

2. Optimize your design for key parameters like efficiency, footprint and cost using the optimizer dial and

compare this design with other possible solutions from Texas Instruments.

3. WEBENCH Power Designer provides you with a customized schematic along with a list of materials with real

time pricing and component availability.

4. In most cases, you will also be able to:

–

Run electrical simulations to see important waveforms and circuit performance,

–

Run thermal simulations to understand the thermal performance of your board,

–

Export your customized schematic and layout into popular CAD formats,

–

Print PDF reports for the design, and share your design with colleagues.

5. Get more information about WEBENCH tools at www.ti.com/webench.

9.2.2.2 Frequency

The switching frequency of LM5175 is set by an RT resistor connected from RT/SYNC pin to AGND. The RT

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

of 84.5 k

Ω is selected for Fsw = 300 kHz.

9.2.2.3 VOUT

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)

9.2.2.4 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 ΔIL to ~40% of the maximum inductor

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 ΔIL to ~40% of the

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