MAX20745 datasheet(21/26 Pages) MAXIM

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For proper OCP operation of the regulator, it is important

that IPK never exceeds the saturation current rating of

the inductor (ISAT). It is recommended that a margin of at

least 20% is included between IPK and ISAT as shown in

Equation 15.

Equation 15:

SAT

1.2 I

>×

Also, note that during a hard VOUT short circuit,

IOUTRIPPLE increases because VOUT went to zero in

Equation 10.

Finally, the power dissipation of the inductor influences

the regulation efficiency. Losses in the inductor include

core loss, DC resistance loss and AC resistance loss.

For the best efficiency, use inductors with core material

exhibiting low loss in the range of 0.5MHz to 2MHz and

low-winding resistance.

Table 9 provides a summary of recommended inductor

suppliers and part numbers.

Output Capacitor Selection

The minimum recommended output capacitance for sta-

bility is given in the Control-Loop Stability section and is

normally implemented using several 100µF 1206 (or simi-

lar) MLCCs. For low slew rate transient loads, RGAIN_EFF

determines the VOUT_ERROR for a given load step per

the small-signal model as discussed above. In this case,

COUT has no effect on the VOUT_ERROR.

However, in the event that the slew rate of the load tran-

sient greatly exceeds the slew rate of the inductor current,

the transient VOUT error can be larger than predicted by

the small-signal model. In this case, the VOUT loading

and unloading transients can be approximated by taking

the larger result between Equation 7 and Equation 16.

Equation 16:

(

)

OUTRIPPLE

OUT

STEP

OUT

DDH

OUT

OUTRIPPLE

OUT

STEP

H_ ON

STEP

OUT

LOADINGTRANSIENT(V)

UNLOADINGTRANSIENT(V)







××

−







+×

××

In order to meet an aggressive transient specification,

COUT may have to be increased and/or LOUT may

have to be decreased. However, note that decreasing

LOUT results in larger inductor ripple current and thus

decreased efficiency and increased output ripple.

Output voltage ripple is another important consideration

in the selection of output capacitors. For a buck regula-

tor operating in CCM, the total voltage ripple across the

output capacitor bank can be approximated as the sum of

three voltage waveforms: 1) the triangle wave that results

from multiplying the AC ripple current by the ESR, 2) the

square wave that results from multiplying the ripple cur-

rent slew rate by the ESL and 3) the piece wise quadratic

waveform that results from charging and discharging the

output capacitor. Although the phasing of these three

components does impact the total output ripple, a com-

mon approximation is to ignore the phasing and to find

the upper bound of the peak-to-peak ripple by summing

all three components, as shown in Equation 17.

Equation 17:

OUTRIPPLE

DDH

OUTRIPPLE

OUT

ESR(I

) ESL













××







where:

ESR = Equivalent series resistance at the output

IOUTRIPPLE = Peak-to-peak inductor current ripple

ESL = High-frequency equivalent series inductance at

output

VDDH = Input voltage

LOUT = Output inductance

fSW = Switching frequency

COUT = Output capacitance

In a typical MAX20745 application with a bank of 0805,

X5R, 6.3V, and 22µF output capacitors, these three com-

ponents are roughly equal.

The ESL effect of an output capacitor on output voltage

ripple cannot be easily estimated from the resonant fre-

quency; the high-frequency (10MHz or above) impedance

of that capacitor should be used. PCB traces and vias in

the VOUT/GND loop contribute additional parasitic induc-

tance.

The final considerations in the selection of output capaci-

tors are ripple current rating and power dissipation. Using

a conservative design approach, the output capacitors

should be designed to handle the maximum peak-to-peak

AC ripple current experienced in the worst-case sce-

nario. Because the recommended output capacitors have

extremely low-ESR values, they are typically rated well

above the current and power stresses seen here. For the

triangular AC ripple current at the output, the total RMS cur-

rent and power is given by Equation 18 and Equation 19.

MAX20745

Integrated, Step-Down Switching Regulator

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