LM5118 datasheet(13/44 Pages) TI

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VOUT

CSG

LM5118

VIN

Q2 (OFF)

Buck Switch

Current

Diode Current

LM5118, LM5118-Q1

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SNVS566I – APRIL 2008 – REVISED AUGUST 2014

9.3 Feature Description

A buck-boost regulator can maintain regulation for input voltages either higher or lower than the output voltage.

The challenge is that buck-boost power converters are not as efficient as buck regulators. The LM5118 has been

designed as a dual mode controller whereby the power converter acts as a buck regulator while the input voltage

is above the output. As the input voltage approaches the output voltage, a gradual transition to the buck-boost

mode occurs. The dual mode approach maintains regulation over a wide range of input voltages, while

maintaining the optimal conversion efficiency in the normal buck mode. The gradual transition between modes

eliminates disturbances at the output during transitions. Figure 8 shows the basic operation of the LM5118

regulator in the buck mode. In buck mode, transistor Q1 is active and Q2 is disabled. The inductor current ramps

in proportion to the VIN - VOUT voltage difference when Q1 is active and ramps down through the recirculating

diode D1 when Q1 is off. The first order buck mode transfer function is VOUT/VIN = D, where D is the duty cycle

of the buck switch, Q1.

Figure 8. Buck Mode Operation

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LM5118 Datasheet(PDF) 13 Page - Texas Instruments