LM5177DCPR datasheet(43/60 Pages) TI2

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The rise (tr) and the fall (tf) times can be based on the MOSFET data sheet information or measured in the lab.

Typically, a MOSFET with smaller RDSON (lower conduction loss) has longer rise and fall times (larger switching

loss).

The power loss in QH2 in the boost mode of operation is shown in Equation 38:

PCONDQH2 = VINVOUT× IOUT×VOUTVIN 2×RDS,OnQH2

(38)

8.2.2.12 Frequency Compensation

This section presents the control loop compensation design procedure for the LM5177 buck-boost controller. The

LM5177 operates mainly in buck or boost modes, separated by a transition region, and therefore, the control

loop design is done for both buck and boost operating modes. Then, a final selection of compensation is made

based on the mode that is more restrictive from a loop stability point of view. Typically, for a converter designed

to go deep into both buck and boost operating regions, the boost compensation design is more restrictive due to

the presence of a right half plane zero (RHPZ) in boost mode.

The boost power stage output pole location is given by:

fp1boost = 12π 2

ROUT× COUT = 1.22 kHz

(39)

where

• ROUT = 2.0 Ω corresponds to the maximum load of 8.0 A.

The boost power stage ESR zero location is given by:

fz1= 12π 1

RESR× COUT = 61.2 kHz

(40)

The boost power stage RHP zero location is given by:

fRHP= 12π ROUT×1−DMAX2

L1 = 24.87 kHz

(41)

where

• DMAX is the maximum duty cycle at the minimum VIN.

The buck power stage output pole location is given by:

fp1buck = 12π 1

ROUT× COUT =612 Hz

(42)

The buck power stage ESR zero location is the same as the boost power stage ESR zero.

It is clear from Equation 43 that RHP zero is the main factor limiting the achievable bandwidth. For a robust

design, the crossover frequency must be less than 1/3 of the RHP zero frequency. Given the position of the RHP

zero, a reasonable target bandwidth in boost operation is around 5 kHz:

fbw= 5 kHz

(43)

For some power stages, the boost RHP zero may not be as restrictive, which happens when the boost maximum

duty cycle (DMAX) is small, or when a really small inductor is used. In those cases, compare the limits posed

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.

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LM5177

SNVSBU4F – JUNE 2022 – REVISED JANUARY 2025

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