M48Z08 datasheet(8/16 Pages) STMICROELECTRONICS

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M48Z08, M48Z18

8/16

Data Retention Mode

With valid VCC applied, the M48Z08/18 operates

as a conventional BYTEWIDE™ static RAM.

Should the supply voltage decay, the RAM will au-

tomatically power-fail deselect, write protecting it-

self when VCC falls 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 con-

tent. 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 M48Z08/18 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 M48Z08/18 for an accumulated period

of at least 11 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 protec-

tion continues until VCC reaches VPFD (min) plus

trec (min). E should be kept high as VCC rises past

VPFD (min) to prevent inadvertent write cycles prior

to system stabilization. Normal RAM operation can

resume trec after VCC exceeds VPFD (max).

For more information on Battery Storage Life refer

to the Application Note AN1012.

VCC Noise And Negative Going Transients

ICC transients, including those produced by output

switching, can produce voltage fluctuations, re-

sulting in spikes on the VCC bus. These transients

can be reduced if capacitors are used to store en-

ergy 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 by-

pass capacitor value of 0.1µF (as shown in Figure

8.) is recommended in order to provide the needed

filtering.

In addition to transients that are caused by normal

SRAM operation, power cycling can generate neg-

ative 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, STMicroelectronics recom-

mends connecting 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.

Figure 8. Supply Voltage Protection

AI02169

VCC

0.1

µF

DEVICE

VCC

VSS

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