LM67660 datasheet(39/66 Pages) TI2

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RRTkΩ = 16.4

FSWMHz −0.633=16.40.4−0.633=40.36kΩ

(27)

8.2.1.2.3 Buck Inductor Selection

1. Use Equation 28 to calculate the buck inductance based on an inductor peak-to-peak ripple current of 30%

to 40% of the converter maximum-rated current. Select a standard value for LO of 3.3µH.

LO= VOUT

FSW×∆ILO× 1− VOUT

VINnom = 5V

400kHz×3.2A× 1− 5V48V =3.5µH

(28)

2. Use Equation 29 to calculate the peak inductor current at maximum steady-state input voltage. Subharmonic

oscillation occurs with peak current-mode control at a duty cycle greater than 50%. For design simplification,

the LM65680 has an internal slope compensation ramp proportional to the switching frequency that adds to

the current-sense signal to damp any tendency toward subharmonic oscillation.

ILOpk =IOUT+ VOUT

2×FSW×LO× 1− VOUT

VINmax =8A+ 5V

2×400kHz×3.3µH× 1− 5V65V =9.75A

(29)

3. To avoid subharmonic oscillation, choose a buck inductance greater than that given by Equation 30. An

effective maximum inductance value sets the minimum ripple current amplitude required for current-mode

control to perform correctly. As a general rule, the minimum inductor ripple current must be no less than

approximately 10% of the converter maximum-rated current under nominal conditions. This limit applies

to applications where the switch duty cycle becomes greater than or equal to 50% under any operating

condition.

LO,min≥M×VOUTFSW

(30)

where

• M = 0.16, 0.21 and 0.29 for the LM65680, LM65660 and LM65640, respectively.

8.2.1.2.4 Input Capacitor Selection

A power supply input typically has a relatively high source impedance at the switching frequency. Good-quality

input capacitors are necessary to limit the input ripple voltage. In general, the ripple current splits between the

input capacitors based on the relative impedance of the capacitors at the switching frequency.

1. Select the input capacitors with sufficient voltage and RMS current ratings. Use Equation 31 to calculate

the RMS current in the input capacitors, where the worst-case operating point is at an input voltage of 10V,

corresponding to 50% duty cycle.

ICINrms = IOUT× D× 1−D =8A × 0.5 × 1−0.5 =4A

(31)

2. Use Equation 32 to find the required input capacitance, assuming an approximate 10% duty cycle for

48V-to-5V conversion:

CIN ≥ D× 1−D ×IOUT

FSW× ΔVIN−RESR,Cin×IOUT = 0.1× 1−0.1 ×8A

400kHz × 480mV−2mΩ × 8A = 4.8µF

(32)

where

• ΔVIN is the specification for the peak-to-peak input ripple voltage.

• RESR,Cin is the effective ESR of the input capacitors.

3. Recognizing the voltage coefficient of ceramic capacitors, select four 4.7µF, 100V, X7R, 1210 ceramic input

capacitors. Each capacitor has an effective capacitance value of approximately 1.3µF at 48VDC. Place

these capacitors adjacent to the input pin pairs, [VIN1, PGND1] and [VIN2, PGND2].

4. Use Equation 33 to calculate the peak-to-peak ripple voltage amplitude.

∆VIN=IOUT×D× 1−D

CIN×FSW +RESR,Cin×IOUT=8A×0.1× 1−0.1

4.2µF×400kHz +2mΩ×8A=0.44V

(33)

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LM65680

SNVSCF2 – NOVEMBER 2025

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