LM5145 datasheet(32/62 Pages) TI

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If the pole located at ωp1 cancels the zero located at ωESR and the pole at ωp2 is located well above crossover,

the expression for the loop gain, Tv(s) in Table 9-3, can be manipulated to yield the simplified expression given in

Equation 14.

RAMP

T (s)

(14)

Essentially, a multi-order system is reduced to a single-order approximation by judicious choice of compensator

components. A simple solution for the crossover frequency (denoted as fc in Figure 9-2) with Type-III voltage-

mode compensation is derived as shown in Equation 15 and Equation 16.

mid

RAMP

(15)

mid

FB1

(16)

-40

-20

100

1000

Frequency (kHz)

Gain

(dB)

Loop Gain

Filter Gain

Modulator

Gain

Compensator

Gain

Figure 9-3. Buck Regulator Constituent Gain Components

The loop crossover frequency is usually selected between one-tenth to one-fifth of switching frequency. Inserting

an appropriate crossover frequency into Equation 16 gives a target for the mid-band gain of the compensator,

Kmid. Given an initial value for RFB1, RFB2 is then selected based on the desired output voltage. Values for RC1,

RC2, CC1, CC2, and CC3 are calculated from the design expressions listed in Table 9-4, with the premise that the

compensator poles and zeros are set as follows: ωz1 = 0.5·ωo, ωz2 = ωo, ωp1 = ωSW/2, and ωp2 = ωESR.

Table 9-4. Compensation Component Selection

RESISTORS

CAPACITORS

FB1

FB2

OUT

REF

mid

FB1

LM5145-Q1

SNVSBU9 – JUNE 2021

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