Data Sheet

Choosing SRAM AN1012
20/33 Doc ID 6395 Rev 4
7 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
V
CC
falls below V
PFD
(min).
● The SRAM should guarantee data retention when working at V
CC
= 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 (I
CCDR
) 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 I
BAT
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 I
BAT
value of 575 nA at 25 °C, and 800 nA at 70 °C.
The M40Z300W has an I
BAT
value of 5 nA at 25 °C, and 100 nA at 70 °C. Tabl e 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
I
BAT
(SRAM) (nA) I
BAT
(Total) (nA)
Lifetime in
years
(1)
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
1
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)
2. 3 V device
1000 10000 1005 10100 13.6 1.4
4 Renesas
R1LP0408CSB-5SC 800 8000 805 8100 17.0 1.7
R1LV0408CSB-5SC
(2)
500 8000 805 8100 > 20 1.7
8
Renesas HM62V8100LTTI-5SL 500 10000 505 10100 > 20 1.4
Samsung K6X8008T2B-UF5500 N/A 15000 N/A 15100 N/A 0.9