Datasheet

LM5118, LM5118-Q1

SNVS566H –APRIL 2008–REVISED JANUARY 2014

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OPERATION MODES

Figure 11 illustrates how duty cycle effects the operational mode and is useful for reference in the following

discussions. Initially, only the buck switch is active and the buck duty cycle increases to maintain output

regulation as VIN decreases. When VIN is approximately equal to 15.5V, the boost switch begins to operate with

a low duty cycle. If VIN continues to fall, the boost switch duty cycle increases and the buck switch duty cycle

decreases until they become equal at VIN = 13.2 V.

Buck Mode Operation: VIN > VOUT

The LM5118 buck-boost regulator operates as a conventional buck regulator with emulated current mode control

while VIN is greater than VOUT and the buck mode duty cycle is less than 75%. In buck mode, the LO gate drive

output to the boost switch remains low.

Buck-Boost Mode Operation: VIN ≊≊ VOUT

When VIN decreases relative to VOUT, the duty cycle of the buck switch will increase to maintain regulation.

Once the duty cycle reaches 75%, the boost switch starts to operate with a very small duty cycle. As VIN is

further decreased, the boost switch duty cycle increases until it is the same as the buck switch. As VIN is further

decreased below VOUT, the buck and boost switch operate together with the same duty cycle and the regulator

is in full buck-boost mode. This feature allows the regulator to transition smoothly from buck to buck-boost mode.

It should be noted that the regulator can be designed to operate with VIN less than 4 V, but VIN must be at least

5 V Figure 12 presents a timing illustration of the gradual transition from buck to buck-boost mode when the input

voltage ramps downward over a few switching cycles.

Figure 12. Buck (HO) and Boost (LO) Switch Duty Cycle vs. Time,

Illustrating Gradual Mode Change with Decreasing Input Voltage

Product Folder Links: LM5118 LM5118-Q1