AN1012 datasheet(20/33 Pages) STMICROELECTRONICS

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Choosing SRAM

AN1012

20/33

Doc ID 6395 Rev 4

Choosing SRAM

Most low power SRAMs on the market today can be used with both ZEROPOWER® and

TIMEKEEPER® supervisors, although there are some issues that need addressing before

finally choosing which SRAM to use.

●

The chip enable input, when taken inactive, must disable all the other inputs to the

SRAM. This allows inputs to the external SRAMs to be treated as “Don’t care” once

VCC falls below VPFD(min).

●

The SRAM should guarantee data retention when working at VCC = 2.0 volts.

●

The chip-enable access time must be sufficient to meet the system needs, taking into

account propagation delays on chip enable and output enable.

Most SRAMs specify a data retention current (ICCDR) at 3.0 V. Manufacturers generally

specify a typical condition for room temperature along with a worst case condition (generally

at elevated temperatures). The system level requirements will determine the choice of which

value to use. The data retention current value of the SRAMs can then be added to the IBAT

value of the supervisor to determine the total current requirements for data retention. The

available battery capacity for the SNAPHAT® of your choice can then be divided by this

current to determine the data retention period (see Section 3.3: Capacity consumption on

page 10).

For example, the M48T201V/Y has an IBAT value of 575 nA at 25 °C, and 800 nA at 70 °C.

The M40Z300W has an IBAT value of 5 nA at 25 °C, and 100 nA at 70 °C. Table 5 indicates

typical data retention lifetimes for the M40Z300W ZEROPOWER supervisor when it is used

with a number of commercially available 1 Mbit and 4 Mbit SRAMs. Table6 on page21

shows the same kind of information for the M48T201V/Y TIMEKEEPER supervisors.

Table 5.

M40Z300W (120mAh SNAPHAT) data retention life vs. SRAM type

Size

(Mbit)

Product

IBAT (SRAM) (nA)

IBAT (Total) (nA)

Lifetime in

years(1)

According to the respective manufacturer’s datasheets at the time of writing.

25°C

70°C

25°C

70°C

25°C

70°C

Hynix

HY628100BLLT1-55

1000

10000

1005

10100

13.6

1.4

HY62V8100BLLT1-70(2)

1000

10000

1005

10100

13.6

1.4

Renesas

M5M51008DVP-55H

500

10000

505

10100

> 20

1.4

M5M5V108DVP-70H(2)

3 V device

1000

10000

1005

10100

13.6

1.4

Renesas

R1LP0408CSB-5SC

800

8000

805

8100

17.0

1.7

R1LV0408CSB-5SC(2)

500

8000

805

8100

> 20

1.7

Renesas

HM62V8100LTTI-5SL

500

10000

505

10100

> 20

1.4

Samsung

K6X8008T2B-UF5500

N/A

15000

N/A

15100

N/A

0.9

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