STM817M(2006) 查看數據表（PDF） - STMicroelectronics

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STM817M
(Rev.:2006)

5V Supervisor with Battery Switchover

STMicroelectronics 

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

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:

1. Connect a battery to VBAT;

2. Ground PFO;

3. Bring VCC above the reset threshold and hold

it there until reset is deasserted following the

reset timeout period; and

4. 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™

5V

VCC

VOUT

To external SRAM

STMXXX

VBAT

RST

GND

To µP

AI08805

Figure 17. Freshness Seal Enable Waveform

VRST

VCC

trec

RST

ECON (Externally held at 0V)

(STM818)

(Externally held at 0V)

PFO

(STM817/819)

ECON out state latched

at 1/2 trec, Freshness

Seal enabled

PFO out state latched

at 1/2 trec, Freshness

Seal Enabled

AI08806

14/37

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