14

LM5008A

SNVS583H – MARCH 2009 – REVISED OCTOBER 2018

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Copyright © 2009–2018, Texas Instruments Incorporated

Typical Application (continued)

Table 1. Bill of Materials

ITEM

DESCRIPTION

PART NUMBER

VALUE

C1

Ceramic Capacitor

TDK C4532X7R2A105M

1 µF, 100 V

C2

Ceramic Capacitor

TDK C4532X7R1E226M

22 µF, 25 V

C3

Ceramic Capacitor

Kemet C1206C474K5RAC

0.47 µF, 50 V

C4

Ceramic Capacitor

Kemet C1206C103K5RAC

0.01 µF, 50 V

C5

Ceramic Capacitor

TDK C3216X7R2A104M

0.1 µF, 100 V

D1

Schottky Power Diode

Diodes Inc. DFLS1100

100 V, 1 A

L1

Power Inductor

COILTRONICS DR125-221-R or

TDK SLF10145T-221MR65

220 µH

RFB2

Resistor

Vishay CRCW12063011F

3.01 kΩ

RFB1

Resistor

Vishay CRCW12061001F

1.0 kΩ

R3

Resistor

Vishay CRCW12063R00F

3.0 Ω

RT

Resistor

Vishay CRCW12063243F

324 kΩ

RCL

Resistor

Vishay CRCW12063323F

332 kΩ

U1

Switching Regulator

LM5008A

8.2.2 Detailed Design Procedure

8.2.2.1 Custom Design With WEBENCH® Tools

Click here to create a custom design using the LM5008A device with the WEBENCH® Power Designer.

1. Start by entering the input voltage (VIN), output voltage (VOUT), and output current (IOUT) requirements.

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

3. Compare the generated design with other possible solutions from Texas Instruments.

The WEBENCH Power Designer provides a customized schematic along with a list of materials with real-time

pricing and component availability.

In most cases, these actions are available:

•

Run electrical simulations to see important waveforms and circuit performance

•

Run thermal simulations to understand board thermal performance

•

Export customized schematic and layout into popular CAD formats

•

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

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

8.2.2.2 Selection Of External Components

RFB1, RFB2: VOUT = VFB × (RFB1 + RFB2) / RFB1, and because VFB = 2.5 V, the ratio of RFB2 to RFB1 calculates as

3:1. Standard values of 3.01 kΩ and 1 kΩ are chosen. Other values could be used as long as the 3:1 ratio is

maintained.

Fs and RT: The recommended operating frequency range for the LM5008A is 50 kHz to 1.1 MHz. Unless the

application requires a specific frequency, the choice of frequency is generally a compromise because it affects

the size of L1 and C2 and the switching losses. The maximum allowed frequency, based on a minimum on-time

of 400 ns, is calculated with Equation 7.

FMAX = VOUT / (VINMAX × 400 ns)

(7)

For this exercise, FMAX = 263 kHz. From Equation 2, RT calculates to 274 kΩ. A standard value 324-kΩ resistor is

used to allow for tolerances in Equation 2, resulting in a frequency of 223 kHz.

L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor

value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum

ripple current occurs at maximum VIN.

a. Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude