STM802 datasheet(14/37 Pages) STMICROELECTRONICS

14 / 37 page

STM690A/692A/703/704/802/805/817/818/819

14/37

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 16 shows how to use a SuperCap as

a back-up power source. The SuperCap may be

connected through a diode to the 5V input. Since

VBAT can exceed VCC while VCC is above the reset

threshold, there are no special precautions when

using these supervisors with a SuperCap.

Negative-Going VCC Transients

The

STM690A/692A/703/704/802/805/817/818/

819 Supervisor are relatively immune to negative-

going

VCC

transients

(glitches).

Figure

38., page 25 shows typical transient duration ver-

sus reset comparator overdrive (for which the

STM690A/692A/703/704/802/805/817/818/819

will NOT generate a reset pulse). The graph was

generated 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.

Battery Freshness Seal (STM817/818/819)

The battery freshness seal disconnects the back-

up battery from internal circuitry and VOUT until it is

needed. This allows an OEM to ensure that the

back-up battery connected to VBAT will be fresh

when the final product is put to use. To enable the

freshness seal:

Connect a battery to VBAT;

Ground PFO;

Bring VCC above the reset threshold and hold

it there until reset is deasserted following the

reset timeout period; and

Bring VCC down again (Figure 17).

Use the same procedure for the STM818, but

ground ECON instead of PFO. Once the battery

freshness seal is enabled (disconnecting the

back-up battery from internal circuitry and any-

thing connected to VOUT), it remains enabled until

VCC is brought above VRST.

Figure 16. Using a SuperCap™

Figure 17. Freshness Seal Enable Waveform

AI08805

STMXXX

VBAT

VCC

VOUT

GND

RST

To external SRAM

To µP

AI08806

RST

PFO

(STM817/819)

VCC

VRST

ECON

(STM818)

trec

(Externally held at 0V)

ECON out state latched

at 1/2 trec, Freshness

Seal enabled

PFO out state latched

at 1/2 trec, Freshness

Seal Enabled

STM802 数据表(PDF) 14 Page - STMicroelectronics

STM802 数据表(HTML) 14 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

数据表下载