LM51772 datasheet(24/112 Pages) TI

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8.3 Feature Description

8.3.1 Buck-Boost Control Scheme

The LM251772 buck-boost control algorithm ensure a seamless transition between the different operating

modes, fixed frequency operation, and power stage protection features. The internal state machine controls the

current flow using three active switching states:

State I: Transistors Q1 and Q3 are conducting. Q2 and Q4 are not conducting (boost mode magnetization state).

State II: Transistors Q1 and Q4 are conducting. Q2 and Q3 are not conducting (boost demagnetization or buck

magnetization state).

State III: Transistors Q2 and Q4 are conducting. Q1 and Q3 are not conducting (buck demagnetization state).

Switch

State I

State II

State III

OFF

IL(peak)

III

Figure 8-2. Buck-Boost Active Switching States

8.3.1.1 Buck Mode

In buck mode operation, the converter starts a buck magnetization cycle (state II) with the internal clock signal.

When the inductor reaches its peak current, the converter proceeds to the buck demagnetization (state III). With

the next clock signal, the converter changes back to a buck magnetization cycle and starts a new switching

cycle with sampling the peak current. As long as the duty cycle does not reach the minimum off-time, the current

control remains in buck operating mode.

IL(peak)

III

Main Oscil lato r

Clock

Switchi ng

State

Inducto r

Curren t

Figure 8-3. Inductor Current in Continuous Current Buck Operation

LM251772

SNVSCT9 – OCTOBER 2024

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LM51772 Datasheet(PDF) 24 Page - Texas Instruments