LM5145 datasheet(33/60 Pages) TI2

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7.2.1.2 Detailed Design Procedure

7.2.1.2.1 Custom Design With WEBENCH® Tools

Click here to create a custom design using the LM25139 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 gives 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.

7.2.1.2.2 Buck Inductor

1. Use Equation 30 to calculate the required buck inductance based on a 30% inductor ripple current at

nominal input voltages.

LO= VOUT

∆ILO×FSW× 1− VOUT

VINnom = 12V

3A×1MHz× 1−12V24V =2μH

(30)

2. Select a standard inductor value of 2.2μH. Use Equation 31 to calculate the peak inductor current at

maximum steady-state input voltage. Subharmonic oscillation occurs with a duty cycle greater than 50%

for peak current-mode control. For design simplification, the LM25139 has an internal slope compensation

ramp proportional to the switching frequency that is added to the current sense signal to damp any tendency

toward subharmonic oscillation.

ILOPK =IOUT+∆ILO2=IOUT+ VOUT

2×LO×FSW× 1− VOUT

VINmax =10+ 12V

2×2.2μH×1MHz× 1−12V36V =11.8A (31)

7.2.1.2.3 Current-Sense Components

This design uses a series resistor to sense the output current ripple and set the current ripple. The following

procedure details how to select the current sense resistance.

1. Calculate the current-sense resistance based on a maximum peak current capability of at least 25% higher

than the peak inductor current at full load to provide sufficient margin during start-up and load-on transients.

Calculate the current sense resistances using Equation 32.

RS= VCS−TH

1.25×ILOPK = 60mV

1.25×11.8A=4.1mΩ

(32)

2. In this example, 3mΩ is selected for a higher current limit. An 0508 footprint component with wide aspect

ratio termination design provides 1W power rating, low parasitic series inductance, and compact PCB layout.

Carefully adhere to the layout guidelines in Section 7.4.1 to make sure that noise and DC errors do not

corrupt the differential current-sense voltages measured at the ISNS+ and VOUT pins.

3. Place the shunt resistor close to the inductor.

4. Use Kelvin-sense connections, and route the sense lines differentially from the shunt to the LM25139.

5. The CS-to-output propagation delay (related to the current limit comparator, internal logic, and power

MOSFET gate drivers) causes the peak current to increase above the calculated current limit threshold.

For a total propagation delay tDELAY-ISNS of 70ns, use Equation 33 to calculate the worst-case peak inductor

current with the output shorted.

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LM25139

SLVSJ80 – OCTOBER 2025

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LM5145 Datasheet(PDF) 33 Page - Texas Instruments

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