L9788 datasheet(171/264 Pages) STMICROELECTRONICS

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DS12308 Rev 4

171/264

L9788

Built in self test

263

Built in self test

14.1

Power supply independency and voltage monitors

In order to guarantee independence between monitor functions of the L9788 and monitored

functions of the same L9788 two independent band-gap references are used; a simple field

of application of this concept is the under/over voltage detection of MCU power supply

generated by the same L9788.

The basic schematic of the band-gap used to generate the main function and of the band-

gap used to monitor it, including their own supply architecture, is shown in Figure 79.

Two band-gaps VBG1 and VBG2 are generated starting from protected battery line: VBG1

is used as reference voltage to generate internal analog and digital supply lines and to

generate external power supply voltages; VBG2 is used as reference for the uv/ov monitors

of these supplies.

The concept to guarantee independence between the two references is that both of them

have independent supply and ground architectures to ensure that a single point failure in

one of the two structures will not affect the other; independence is achieved by means of

physical isolation between the two band-gaps and their supply circuits (A and B areas),

isolation is done layouting the circuits in different floor plan areas and by usage of deep

trench isolation in between.

Figure 79. Band-gap supply architecture

A cross-referenced monitor is used to detect eventual failures in any of the band-gap supply

and enter a safe state: VBG1 is used as reference for the monitor of the supply of VBG2 and

VBG2 is used as monitor of the supply of VBG1.

The concept is needed to solve dependencies of band-gap voltage from the supply line: let

us suppose that V3V3PRE_mon in the above picture has a fault impacting VBG2 voltage

reference, since V3V3PRE_mon is also the supply for monitor circuits a failure in this region

V3V3PRE

VDD2V7

monitor

VINT3V3

monitor

V3V3PRE

(monitor)

VBG2

GND_ANA_2

GND_ANA_1

(reference)

VBG1

V3V3PRE_mon

monitor

VBatt

VChargePump

ANALOG blocks

DIGITAL blocks

GND_DIG

VDD2V7

VINT3V3

GADG1212161531PS

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