LM5175PWPR.A datasheet(22/43 Pages) TI2

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CIN(RMS)

OUT

D (1 D)

IN(MIN)

OUT

RIPPLE(COUT)

OUT

u ¨

OUT

RIPPLE(ESR)

IN(MIN)

ESR

OUT

COUT(RMS)

OUT

L(PEAK)

L(SAT)

1.2 I

21.6 A

0.8

IN(MIN)

OUT

IN(MIN)

L(PEAK )

L(MAX)

OUT

)

14.4 A

2 L1 F

OUT

OUT(MAX)

L(MAX)

IN(MIN)

13.3 A

0.9 V

IN(MIN)

OUT

IN(MIN)

BOOST

OUT(MAX)

OUT

)

2.1 H

0.4 I

LM5175

SNVSA37A – OCTOBER 2015 – REVISED MAY 2016

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(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 5.7 A, 4.3 A, and 2.1 A, at VIN of 36 V, 24 V, and 6 V respectively.

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

(15)

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

(16)

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

boost operation. To ensure that the inductor does not saturate in current limit, the peak saturation current of the

inductor should be higher than the maximum current limit. Adjusting for a ±20% current limit threshold tolerance,

the peak inductor current limit is:

(17)

Therefore, the inductor saturation current should be greater than 21.6 A. If hiccup mode protection is not

enabled, the RMS current rating of the inductor should be sufficient to tolerate continuous operation in cycle-by-

cycle current limiting.

9.2.2.5 Output Capacitor

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

given by Equation 18 where the minimum VIN corresponds to the maximum capacitor current.

(18)

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:

(19)

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

(20)

Typically a combination of ceramic and bulk capacitors is needed to provide low ESR and high ripple current

capacity. The complete schematic in Figure 24 at the end of this section shows a good starting point for COUT for

typical applications.

9.2.2.6 Input Capacitor

In the buck mode, the input capacitor supplies high ripple current. The RMS current in the input capacitor is given

by:

(21)

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