LM2746MXAX datasheet(19/24 Pages) NSC

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Application Information (Continued)

In practice, a good trade off between phase margin and

bandwidth can be obtained by selecting the closest ±10%

capacitor values above what are suggested for C

C1 and CC2,

the closest ±10% capacitor value below the suggestion for

C3, and the closest ±1% resistor values below the sugges-

tions for R

C1,RC2. Note that if the suggested value for RC2 is

less than 100

Ω, it should be replaced by a short circuit.

Following this guideline, the compensation components will

be:

C1 = 27pF ±10%, CC2 = 820pF ±10%

C3 = 2.7nF ±10%, RC1 = 39.2k

Ω ±1%

C2 = 2.55k

Ω ±1%

The transfer function of the compensation block can be

derived by considering the compensation components as

impedance blocks Z

F and ZI around an inverting op-amp:

As with the generic equation, G

EA-ACTUAL must be modified

to take into account the limited bandwidth of the error ampli-

fier. The result is:

The total control loop transfer function H is equal to the

power stage transfer function multiplied by the error amplifier

transfer function.

H=G

PS xHEA

The bandwidth and phase margin can be read graphically

from Bode plots of H

EA are shown in Figure 15.

The bandwidth of this example circuit is 59kHz, with a phase

margin of 60˚.

EFFICIENCY CALCULATIONS

The following is a sample calculation.

A reasonable estimation of the efficiency of a switching buck

controller can be obtained by adding together the Output

Power (P

OUT) loss and the Total Power (PTOTAL) loss:

The Output Power (P

OUT) for theTypical Application Circuit

design is (1.2V x 4A) = 4.8W. The Total Power (P

TOTAL), with

an efficiency calculation to complement the design, is shown

below.

The majority of the power losses are due to low and high

side of MOSFET’s losses. The losses in any MOSFET are

group of switching (P

SW) and conduction losses(PCND).

FET =PSW +PCND = 61.38mW + 270.42mW

FET = 331.8mW

FET Switching Loss (P

SW)

SW =PSW(ON) +PSW(OFF)

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FIGURE 15. Overall Loop Gain and Phase

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