M48Z512A datasheet(11/17 Pages) STMICROELECTRONICS

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M48Z512A, M48Z512AY

Figure 11. Supply Voltage Protection

AI02169

VCC

0.1

µF

DEVICE

VCC

VSS

DATA RETENTION MODE

With valid VCC applied, the M48Z512A/512AY op-

erates as a conventional BYTEWIDE

™ static

RAM. Should the supply voltage decay, the RAM

will automatically power-fail deselect, write pro-

tecting itself tWP after VCC falls below VPFD.All

outputs become high impedance, and all inputs

are treated as ”don’t care.”

If power fail detection occurs during a valid ac-

cess, the memory cycle continues to completion. If

the memory cycle fails to terminate within the time

tWP, write protection takes place. When VCC drops

below VSO, the control circuit switches power to

the internal energy source which preserves data.

The internal coin cell will maintain data in the

M48Z512A/512AY after the initial application of

VCC for an accumulated period of at least 10 years

when VCC is less than VSO. As system power re-

turns and VCC rises above VSO, the battery is dis-

connected, and the power supply is switched to

external VCC. Write protection continues for tER af-

ter VCC reaches VPFD to allow for processor stabi-

lization. After tER, normal RAM operation can

resume.

For more information on Battery Storage Life refer

to the Application Note AN1012.

POWER SUPPLY DECOUPLING

and UNDERSHOOT PROTECTION

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 0.1

µF (as shown in Figure

11) is recommended in order to provide the need-

ed 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 nega-

tive 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 con-

nected to VCC, anode to VSS). Schottky diode

1N5817 is recommended for through hole and

MBRS120T3 is recommended for surface mount.

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