LM5122ZPWPR datasheet(36/51 Pages) TI1

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OUT

RIPPLE _ MAX(COUT)

IN(MIN)

OUT

4.5A

24V

SLOPE

OUT

IN(MIN)

6 10

10 +

100 k

1 24V

K V

u u

: u

SLOPE

8 10

32 k

250 kHz

VIN

LM5122

CSN

CSP

CCS

LOSS(RS)

I R

(13.5 A 1.4)

4 m

1.43 W

CS TH1

PEAK(CL)

75 mV

3.97 m

13.5 A 1.4

LM5122, LM5122-Q1

SNVS954G – FEBRUARY 2013 – REVISED MAY 2016

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Product Folder Links: LM5122 LM5122-Q1

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8.2.2.4 Current Sense Resistor RS

The maximum peak input current capability should be 20~50% higher than the required peak current at low input

voltage and full load, accounting for tolerances. For this example, 40% is margin is chosen.

(28)

A closest standard value of 4 m

Ω is selected for RS. The maximum power loss of RS is calculated as follows.

(29)

8.2.2.5 Current Sense Filter RCSFP, RCSFN, CCS

The current sense filter is optional. 100 pF of CCS and 100 Ω of RCSFP, RCSFN are normal recommendations.

Because CSP and CSN pins are high impedance, CCS should be placed physically as close to the device.

Figure 40. Current Sense Filter

8.2.2.6 Slope Compensation Resistor RSLOPE

The K value is selected to be 1 at the minimum input voltage. RSLOPE should be carefully selected so that the

sum of sensed inductor current and slope compensation is less than COMP output high voltage.

(30)

(31)

A closest standard value of 100 k

Ω is selected for RSLOPE.

8.2.2.7 Output Capacitor COUT

The output capacitors smooth the output voltage ripple and provide a source of charge during transient loading

conditions. Also the output capacitors reduce the output voltage overshoot when the load is disconnected

suddenly.

Ripple current rating of output capacitor should be carefully selected. In boost regulator, the output is supplied by

discontinuous current and the ripple current requirement is usually high. In practice, the ripple current

requirement can be dramatically reduced by placing high quality ceramic capacitors earlier than the bulk

aluminum capacitors as close to the power switches.

The output voltage ripple is dominated by ESR of the output capacitors. Paralleling output capacitor is a good

choice to minimize effective ESR and split the output ripple current into capacitors.

In this example, three 330 µF aluminum capacitors are used to share the output ripple current and source the

required charge. The maximum output ripple current can be simply calculated at the minimum input voltage as

follows:

(32)

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