LM5177 datasheet(29/112 Pages) TI

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8.3.3 Reference System

V(VCC,SUP)

VSMART

SELECTION

VIN

BIAS

V(VCC,SUP)

VCC2

LDO

VT(VDET)

Digital

Core

Logic

VIO

LDO

VT(VIO_PG)

nRST

DVDD

LDO

VT(DVDD_PG)

nRST

VCC2

GATE

DRIVE

BAND

GAP

BG_GOOD

AVDD1

LDO

VCC1

LDO

PGND

BUCK-BOOST

AVDD2

LDO

VT(AVDD_PG)

VT(UVLO)

–

EN/

UVLO

VCC1

VT(VCC,UVLO)

Figure 8-10. Functional Block Diagram Reference System

8.3.3.1 VIO LDO and nRST-PIN

The VIO LDO supplies the IO pin buffers and comparators. Once the voltage on the VIN-pin or BIAS-pin is

above the positive going POR threshold VT+(POR) and the nRST-PIN is higher than VT+(nRST) the internal

bias is active and the device is in standby mode.

When the nRST - pin is below the standby threshold VT-(nRST), the device is held in a low power shutdown mode

to maintain a minimum input quiescent current of the device supply rails.

8.3.4 Supply Voltage Selection – VSMART Switch and Selection Logic

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

the device can reduce the power dissipation by ensuring a seamless operation at low input or output voltages as

well as in transient operating conditions like an output short. The VSMART 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 VOUT. During start-up, as long

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

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

• 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.

• If the BIAS pin is not used it is recommended to put BIAS to ground, the device always used the VIN LDO,

and the quiescent is minimized.

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