AN2419 datasheet(7/22 Pages) STMICROELECTRONICS

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AN2419

Power supplies

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1.2.4

Power scheme 4: dual external 5.0V and 1.8V power sources

In this configuration, the internal voltage regulators are switched off, by forcing the

VREG_DIS pin to high level. This scheme has the advantage of saving power consumption

when the 1.8V power supply is already available in the application and providing 5V I/O

capability. V18 and V18_BKP are provided externally through the V18REG, V18 and V18_BKP

power pins.

●

VREG_DIS pin is tied to high level which disables the Main Voltage Regulator and the

Low Power Voltage Regulator.

●

All digital power pins (V18REG, V18 and V18_BKP) must be externally shorted to the

same 1.8V power supply source.

In this scheme:

●

STANDBY Mode is not available

●

USB functionality is not available

Figure 4.

Power supply scheme 4 (5.0V and 1.8V supplies, VREGDIS=1)

1.3

Reset and power startup

1.3.1

Power startup specifications

To ensure the MCU starts-up cleanly, the rise time of the VDD_IO power supply must be

comprised between 20µs/V and 20ms/V.

In addition, you must provide an external RESET for at least 20µs after the VDD_IO power

supply has reached its minimum working value (3.0V). It is recommended to use an external

Power-On-Reset circuit monitoring VDD_IO to assert the RESET at power-up .

During VDD_IO power-up (from 0V to 3.3V or 5.0V), all I/Os are guaranteed to be in HiZ

state, assuming external RESET is asserted.

If you are using an external 1.8V power supply, the rise time of power supply V18 must be

comprised between 20µs/V and 20ms/V.

V18_BKP

VDD_ADC

VSS_ADC

VSS_PLL

VSS_IO

VDD_IO

V18REG

VDD_PLL

VSS18

VSS_BKP

VREG_DIS

V18

VSS18

STR750

+5.0V

+1.8V

+5.0V

NOTE : THE EXTERNAL 5.0V POWER SUPPLY MUST ALWAYS BE KEPT ON

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