LM5177DCPR datasheet(40/60 Pages) TI2

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Table 8-2. FB Pin Resistor Divider Examples with RFB,top = 71.5kΩ (continued)

VO – Target

RFB,bot – Calculation

RFB,bot – E48 Series

VO Nominal

Error from FB Resistor

28V

2.65kΩ

2.61kΩ

28.39V

1.4%

36V

2.04kΩ

2.05kΩ

35.88V

–0.3%

42V

1.74kΩ

1.78kΩ

41.17V

–2.0%

48V

1.5kΩ

47.43V

–1.2%

60V

1.2kΩ

60.09V

0.2%

8.2.2.4 Inductor and Current Sense Resistor Selection

For boost mode, the inductor selection is based on limiting the peak-to-peak current ripple, ΔIL, to approximately

20% of the maximum inductor current at the minimum input voltage. The target inductance for boost mode is:

LBOOST= VINMIN

2 × VOUT−VINMIN

0.2× IOUTMAX × fSW× VOUT2= 2.21μH

(19)

For this application, an inductor with 1.8 μH was selected.

When selecting the current sense resistor it needs to be ensured the peak inductor current will not hit the over

current limit at maximum output current. For that the peak inductor current needs to be calculated with the sum

of the average and ripple current through the inductor.

The maximum peak to peak inductor current occurs at minimum input voltage and is given by:

ILPEAK,PEAK = 1−VINMINVOUT ×VINMIN

L×fSW =5.23 A

(20)

The average input current at the maximum output current with an estimated efficiency of 95% is calculated by:

IIN,AVGMAX =VOUT×IOUTMAX

95%×VINMIN =22.5 A

(21)

For the current sense Resistor a margin of 20% is considered to have enough headroom for the dymamic

responses, e.g. load step regulation. To ensure the maximum output current can be delivered the mimium level

of the peak current limit threshold is used.

RCS=

38.5 mV

IIN,AVGMAX +12ILPEAK,PEAK ×1.2=1.28 mΩ

(22)

The standard value of RCS = 1 mΩ with 3 times 3 mΩ is selected. With the 3 resistors in parallel it also reduces

the parasitic inductance.

The maximum power dissipation in RCS happens at VIN(MAX):

PRCSMAX = 58.5 mVRCS 2× RCS× 1− VOUT

VINMAX = 1.90W

(23)

Therefore, for the 3 resistors in parallel a sense resistor with 1-W power rating is sufficient for this application.

A filter network to attenuate noise in the CSA and CSB sense lines should be added. For most applications it is

recommended to use a filter resistance RDIFF1 and RDIFF2 of 10 Ω. The capacitance CDIFF for the filter can be

calculated with Equation 15. In this configuration 180pF is used.

8.2.2.5 Slope Compensation

For stable current loop operation and to avoid subharmonic oscillations, the slope resistor must be selected

based on Equation 24:

LM5177

SNVSBU4F – JUNE 2022 – REVISED JANUARY 2025

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