M48Z58MH datasheet(9/17 Pages) STMICROELECTRONICS

9 / 17 page

DATA RETENTION MODE

With valid VCC applied, the M48Z58/58Y operates

as a conventional BYTEWIDE

™ static RAM.

Should the supply voltage decay, the RAM will

automatically power-fail deselect, write protecting

itself when VCC f alls within the VPFD(max),

VPFD(min) window. All outputs become high imped-

ance, and all inputs are treated as "don’t care."

Note: A power failure during a write cycle may

corrupt data at the currently addressed location,

but does not jeopardize the rest of the RAM’s

content. At voltages below VPFD(min), the user can

be assured the memory will be in a write protected

state, provided the VCC fall time is not less than tF.

The M48Z58/58Y may respond to transient noise

spikes on VCC that reach into the deselect window

during the time the device is sampling VCC. There-

fore, decoupling of the power supply lines is rec-

ommended.

When VCC drops below VSO, the control circuit

switches power to the internal battery which pre-

serves data. The internal button cell will maintain

data in the M48Z58/58Y for an accumulated period

of at least 10 years when VCC is less than VSO.

As system power returns and VCC rises above VSO,

the battery is disconnected, and the power supply

is switched to external VCC. Write protection con-

tinues until VCC reaches VPFD(min) plus tREC(min).

Normal RAM operation can resume tREC after VCC

exceeds VPFD(max).

For more information on Battery Storage Life refer

to the Application Note AN1012.

POWER SUPPLY DECOUPLING and UNDER-

SHOOT PROTECTION

ICC transients, including those produced by output

switching, can produce voltage fluctuations, result-

ing in spikes on the VCC bus. These transients can

be reduced if capacitors are used to store energy,

which stabilizes the VCC bus. The energy stored in

the bypass capacitors will be released as low going

spikes are generated or energy will be absorbed

when overshoots occur. A ceramic bypass capaci-

tor value of 0.1

µF (as shown in Figure 9) is recom-

mended in order to provide the needed filtering.

In addition to transients that are caused by normal

SRAM operation, power cycling can generate

negative voltage spikes on VCC that drive it to

values below VSS by as much as one Volt. These

negative spikes can cause data corruption in the

SRAM while in battery backup mode. To protect

from these voltage spikes, it is recommeded to

connect a schottky diode from VCC to VSS (cathode

connected to VCC, anode to VSS). Schottky diode

1N5817 is recommended for through hole and

MBRS120T3 is recommended for surface mount.

AI02169

VCC

0.1

µF

DEVICE

VCC

VSS

Figure 9. Supply Voltage Protection

9/17

M48Z58, M48Z58Y

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