LM5145 datasheet(34/60 Pages) TI2

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ILO−PKSC =VCS−THRS+VINmax×tDELAY−ISNS

LO =60mV3mΩ+36V×70ns

2.2μH =21A

(33)

6. Based on this result, select an inductor with saturation current greater than 21A across the full operating

temperature range.

Alternatively, the current can be sensed using the DCR of the output inductor. The following steps are used to

achieve accurate DCR sensing:

1. Calculate the sense resistor by matching the inductor time constant to the sensing time constant using

Equation 34.

RCS= LO

CCS×RDCR= 2.2μH

0.1μF×4.3mΩ=5.1kΩ

(34)

2. Calculate the current limit given the DCR of the inductor using Equation 35.

ICL=VCS−TH

RDCR =60mV4.3mΩ=13.9A

(35)

where

• VCS-TH is the 60mV current limit threshold.

3. Place the sense resistor and capacitor close to the inductor.

4. Use Kelvin-sense connections, and route the sense lines differentially from the sense network 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 36 to calculate the worst-case peak inductor

current with the output shorted.

ILO−PKSC =VCS−TH

RDCR +VINmax ×tDELAY−ISNS

LO =60mV4.3mΩ+36V×70ns

2.2μH =15.1A

(36)

6. Based on the result, choose an inductor with a saturation current of 16A or greater across the full operating

temperature range.

7.2.1.2.4 Output Capacitors

1. Use Equation 37 to estimate the output capacitance required to manage the output voltage overshoot during

a load-off transient (from full load to no load) assuming a load transient deviation specification of 1.5%

(180mV for a 12V output).

COUT≥ LO×∆IOUT2

VOUT+∆VOVERSHOOT2−VOUT2= 2.2μH×10A2

12V+180mV2−12V2=50.5μF

(37)

2. Noting the voltage coefficient of ceramic capacitors where the effective capacitance decreases significantly

with applied voltage, select seven 22-µF, 50V, X5R, ceramic output capacitors. Generally, when sufficient

capacitance is used to satisfy the load-off transient response requirement, the voltage undershoot during a

no-load to full-load transient is also satisfactory.

3. Use Equation 38 to estimate the peak-peak output voltage ripple at nominal input voltage.

∆VOUT= ILO

8×FSW×COUT 2+ RESR×∆ILO2= 3A

8×1MHz×133μF 2+ 1mΩ×3A2=4.1mV (38)

where

• RESR is the effective equivalent series resistance (ESR) of the output capacitors.

• 133 µF is the total effective (derated) ceramic output capacitance at 12V.

4. Use Equation 39 to calculate the output capacitor RMS ripple current using and verify that the ripple current

is within the capacitor ripple current rating.

LM25139

SLVSJ80 – OCTOBER 2025

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