LM5118 datasheet(21/44 Pages) TI

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LM5118, LM5118-Q1

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SNVS566I – APRIL 2008 – REVISED AUGUST 2014

Device Functional Modes (continued)

9.4.2 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 16 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 16. Buck (HO) and Boost (LO) Switch Duty Cycle vs. Time,

Illustrating Gradual Mode Change with Decreasing Input Voltage

9.4.3 High Voltage Start-Up Regulator

The LM5118 contains a dual mode, high voltage linear regulator that provides the VCC bias supply for the PWM

controller and the MOSFET gate driver. The VIN input pin can be connected directly to input voltages as high as

75 V. For input voltages below 10 V, an internal low dropout switch connects VCC directly to VIN. In this supply

range, VCC is approximately equal to VIN. For VIN voltages greater than 10 V, the low dropout switch is disabled

and the VCC regulator is enabled to maintain VCC at approximately 7 V. A wide operating range of 4 V to 75 V

(with a startup requirement of at least 5 V) is achieved through the use of this dual mode regulator.

The output of the VCC regulator is current limited to 35 mA, typical. Upon power up, the regulator sources

current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds the VCC under-

voltage threshold of 3.7 V and the UVLO input pin voltage is greater than 1.23 V, the gate driver outputs are

enabled and a soft-start sequence begins. The gate driver outputs remain enabled until VCC falls below 3.5 V or

the voltage at the UVLO pin falls below 1.13 V.

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LM5118 Datasheet(PDF) 21 Page - Texas Instruments