STM805 datasheet(12/31 Pages) STMICROELECTRONICS

12 / 31 page

STM690/704/795/802/804/805/806

12/31

Using a SuperCap™ as a Backup Power

Source

SuperCaps™ are capacitors with extremely high

capacitance values (e.g., order of 0.47F) for their

size. Figure 15 shows how to use a SuperCap as

a back-up power source. The SuperCap may be

connected through a diode to the VCC supply.

Since VBAT can exceed VCC while VCC is above

the reset threshold, there are no special precau-

tions when using these supervisors with a Super-

Cap.

Figure 15. Using a SuperCap™

Negative-Going VCC Transients

The STM690/704/795/802/804/805/806 Supervi-

sors are relatively immune to negative-going VCC

transients (glitches). Figure 32., page 20 was gen-

erated using a negative pulse applied to VCC,

starting at VRST + 0.3V and ending below the reset

threshold by the magnitude indicated (comparator

overdrive). The graph indicates the maximum

pulse width a negative VCC transient can have

without causing a reset pulse. As the magnitude of

the transient increases (further below the thresh-

old), the maximum allowable pulse width decreas-

es. Any combination of duration and overdrive

which lies under the curve will NOT generate a re-

set signal. Typically, a VCC transient that goes

100mV below the reset threshold and lasts 40µs or

less will not cause a reset pulse. A 0.1µF bypass

capacitor mounted as close as possible to the VCC

pin provides additional transient immunity.

AI08805

STMXXX

VBAT

VCC

VOUT

GND

RST

To external SRAM

To µP

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