PLM5177DCPR datasheet(40/54 Pages) TI

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by the RHP zero (fRHP / 3) with 1/20 of the switching frequency and use the smaller of the two values as the

achievable bandwidth.

The compensation zero can be placed at 1.5 times the boost output pole frequency. Keep in mind that this

locates the zero at three times the buck output pole frequency, which results in approximately 30 degrees of

phase loss before crossover of the buck loop and 15 degrees of phase loss at intermediate frequencies for the

boost loop:

fZC= 2.16 kHz

(47)

If the crossover frequency is well below the RHP zero and the compensation zero is placed well below the

crossover, the compensation gain resistor, Rc1, is calculated using the approximation:

RC1= 2π×fbw

gmEA ×RFB1+RFB2

RFB2 ×ACS×RSENSE×COUT

1−DMAX × 11+fbwfRHP2=1.1 kΩ

(48)

where

• DMAX is the maximum duty cycle at the minimum VIN in boost mode.

• ACS is the current sense amplifier gain.

The compensation capacitor, Cc1, is then calculated from:

CC1= 1

2π×fZC× Rc1= 97nF

(49)

The standard values of compensation components are selected to be Rc1 = 768 Ω and Cc1 = 100 nF.

A high frequency pole (fpc2) is placed using a capacitor (Cc2) in parallel with Rc1 and Cc1. Set the frequency of

this pole at seven to ten times of fbw to provide attenuation of switching ripple and noise on COMP while avoiding

excessive phase loss at the crossover frequency. For a target fpc2 = 10 kHz, Cc2 is calculated using Equation 50:

CC2= 1

2π×fpc2×Rc1= 20.9 nF

(50)

Select a standard value of 22 nF for Cc2. These values provide a good starting point for the compensation

design. Each design must be tuned in the lab to achieve the desired balance between stability margin across the

operating range and transient response time.

9.2.3 Application Curves

Output Current (A)

0.4

0.8

1.2

1.6

2.4

2.8

3.2

3.6

Figure 9-2. Efficiency Versus IO in Boost Mode

(VIN = 5 V)

Output Current (A)

0.6

1.2

1.8

2.4

3.6

4.2

4.8

5.4

77.5

82.5

87.5

92.5

97.5

100

Figure 9-3. Efficiency Versus IO in Buck-Boost

Mode

(VIN = 16 V)

LM5177

SNVSBU4A – JUNE 2022 – REVISED OCTOBER 2022

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