LM5176PWPR datasheet(23/45 Pages) TI2

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IN(MIN)

OUT

IN(MIN)

BOOST

OUT(MAX)

OUT

)

2.8 H

0.3 I

(14)

In this particular application, the buck inductance is larger. Choosing a larger inductance reduces the ripple

current but also increases the size of the inductor. A larger inductor also reduces the achievable bandwidth of

the converter by moving the right half plane zero to lower frequencies. Therefore, a judicious compromise should

be made based on the application requirements. For this design a 4.7-µH inductor is selected. With this inductor

selection, the inductor current ripple is 6.5 A, 4.3 A, and 2.1 A, at VIN of 50 V, 24 V, and 6 V, respectively.

The maximum average inductor current occurs at the minimum input voltage and maximum load current:

OUT

OUT(MAX)

L(MAX)

IN(MIN)

13.3 A

0.9 V

(15)

where a 90% efficiency is assumed. The peak inductor current occurs at minimum input voltage and is given by:

IN(MIN)

OUT

IN(MIN)

L(PEAK )

L(MAX)

OUT

)

14.4 A

2 L1 F

(16)

To ensure sufficient output current, the current limit threshold must be set to allow the maximum load current in

boost operation. The inductor peak current during overload depends on the current limit resistor, RSENSE, (refer

to the subsection on selecting RSENSE). The peak inductor current in current limit when in boost mode is given

by:

L(PEAK, ILIMIT, BOOST)

SENSE

120 mV

(17)

The peak inductor current in current limit when in buck mode happens at high input voltage and is given by:

u ¨

IN(MAX)

OUT

L(PEAK, ILIMIT, BUCK)

SENSE

IN(MAX)

80 mV

L1 F

(18)

The peak inductor current in current limit is 15 A and 16.5 A in boost mode and buck mode, respectively. The

inductor should be selected to handle this current.

8.2.2.5 Output Capacitor

In the boost mode, the output capacitor conducts high ripple current. The output capacitor RMS ripple current is

given by Equation 19 where the minimum VIN corresponds to the maximum capacitor current.

OUT

COUT(RMS)

OUT

(19)

In this example, the maximum output ripple RMS current is ICOUT(RMS) = 6 A. A 5-mΩ output capacitor ESR

causes an output ripple voltage of 60 mV as given by:

OUT

RIPPLE(ESR)

IN(MIN)

ESR

(20)

A 400-µF output capacitor causes a capacitive ripple voltage of 25 mV as given by:

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LM5176

SNVSAI1D – JUNE 2017 – REVISED AUGUST 2021

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