LM5177DCPR.A datasheet(21/60 Pages) TI2

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In the buck-boost area where larger or smaller duty cycles is necessary, switching pulses are skipped. When

necessary, the control initiates switching activities with a minimum time of state I or state III to maintain the

inductor current as required by the voltage regulation loop.

VT+( Sleep)

Inducto r

Curren t

Output Voltage

Sing le Pulse

Slee p

VO(nom)

Figure 7-5. Timing Diagram for the Power Save Mode

7.3.4 Supply Voltage Selection – VMAX Switch

There are two pins to supply the LM5177 internal voltage regulators. Due to the internal supply voltage selection

circuit, the device can reduce the power dissipation through a seamless operation at low input or output voltages

as well as in transient operating conditions like an output short. The VMAX switch selects the pin with the lower

voltage from the VIN or BIAS pin once the voltage on both is above the switch-over threshold (VT(VCC, SUP)).

If one pin voltage is lower than the threshold, the other supply pin is selected. And if both pins are lower than

the switch-over threshold, the higher voltage of VIN or BIAS is selected as supply. The following are common

configurations for the supply pins:

• The VIN pin is connected to the supply voltage. The BIAS pin is connected to VO. During start-up, that is as

long as the output voltage is not higher than the supply switch-over threshold, the VIN supplies the internal

regulators. Once VO is high enough, the supply current comes from the BIAS pin.

• Both the VIN pin and the BIAS pin are connected together to the input supply voltage. This configuration is

often used in applications where the input supply voltage is usually lower or equal than the output voltage.

As the BIAS pin is connected to the input voltage, the device has the full current capability of the internal

regulators at low input voltages for start-up.

• The VIN is connected to the input supply voltage and the BIAS pin is connected to an auxiliary supply

(for example, an existing 12V DC/DC converter). This configuration is commonly used at high voltage

applications on the input and output voltages where the power dissipation over the integrated linear

regulators must be further minimized.

VIN

BIAS

Sup ply Vo ltag e S electio n

to Internal

Regula tors

Figure 7-6. VMAX Supply Scenario 1

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LM5177

SNVSBU4F – JUNE 2022 – REVISED JANUARY 2025

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LM5177DCPR.A Datasheet(PDF) 21 Page - Texas Instruments