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參數(shù)資料
| 型號： | FM3808 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 元件分類： | DRAM |
| 英文描述： | 4Kb FRAM Serial 3V Memory |
| 中文描述： | 4Kb的鐵電串行3V的記憶 |
| 文件頁數(shù)： | 18/28頁 |
| 文件大?。?/td> | 191K |
| 代理商： | FM3808 |
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FM3808
Rev 1.1
May 2003
Page 18 of 28
Real-time Clock Design Considerations
The principal design issues in using the real time
clock are selection and specification of backup
energy source and the selection of the timekeeping
crystal. Selection of the backup source is primarily a
choice between a capacitor and a battery, and the
specifications needed for each. Selection of the
crystal is based on mechanical (surface mount versus
through-hole) considerations and the characteristic
capacitance. Each topic is discussed briefly.
Backup Power Source
The FM3808 is designed to accommodate either a
battery or a capacitor as a backup power source.
Unlike SRAM-based timekeepers that depend on the
battery to make data nonvolatile, the FM3808 is
unrestricted. Data stored in FRAM is not dependent
on the backup battery in any way. This means that
capacitor backup, which should be less expensive, is
a option. Selection of a capacitor is determined by the
expected
duration
of
timekeeping must be maintained, and the practical
difficulty in resetting the time should it be lost. If the
time is relatively easy to reset, or a typical power loss
is only a brownout, the capacitor may be a good, cost
effective choice. In addition, portable systems that
use a battery for primary power are good candidates
for capacitor backup. If the time is very difficult to
reset, or the power outage may be longer than a
capacitor can supply, then a small battery is best.
Each system and application can be evaluated for the
difficulty in setting the time. However, the expected
backup times for several capacitor choices are
illustrated below. These figures cannot be used as
guarantees due to the unknown leakage characteristics
in external components, but they provide guidelines
for realistic expectations about capacitor use. In the
power
outage
where
scenario using capacitor backup, the charging circuit
must also be considered. A typical representation is
shown below.
The backup times are based on a starting backup
voltage (fully charged) of 4.7V and minimum backup
voltage of 2.5V. A 0.3V forward drop from 5.0V
VDD might be expected from a schottky-diode. Note
the graph, which shows approximate backup current
as a function of backup voltage. Thus, the higher
voltages at the beginning of discharge provide less
incremental backup time than the lower voltages near
the end of discharge. However, the total backup time
depends on the capacitor size and the maximum, fully
charged voltage.
One important note about capacitor backup is that the
times are incremental. Each time power is restored
the capacitor is fully recharged. Rather than
examining the cumulative time without power in the
system over a 10-year period, the capacitor design is
only concerned with the maximum time without
power for one outage.
If the times available for a capacitor are not
sufficient, then a battery is the best selection. Most
users opt for a 3V lithium coin. Note that with non-
rechargeable batteries, the reservoir is not replenished
so the critical parameter is the total time without
power during the useful life of a system. For 1 year
without power (total) during a 10-year system life, the
battery capacity must be at least 9.25 mAhr. For 5
years without power during a 10-year period, it
becomes 46 mA*hr.
VBAK
VDD
x
2
x
1
FM3808
5V Supply
Schottky
Diode
Backup Capacitor or
Super/Golden Cap
Figure 9. Capacitor Backup Circuit
Capacitor Size
100
μ
F
1000
μ
F
10000
μ
F
0.47 F (super cap)
Backup Time
3 minutes
30 minutes
5.1 hours
240 hours
相關PDF資料 |
PDF描述 |
|---|---|
| FM3808-70-T | 4Kb FRAM Serial 3V Memory |
| FM3808DK | 4Kb FRAM Serial 3V Memory |
| FMA10A | General Purpose(Dual Digital Transistor)(通用(雙數(shù)字晶體管)) |
| FMA11A | Emitter Common (Dual Digital Transistors)(公共發(fā)射級(雙數(shù)字晶體管)) |
| FMA1 | General purpose (dual digital transistors) |
相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| FM3808_06 | 制造商:RAMTRON 制造商全稱:RAMTRON 功能描述:256Kb Bytewide FRAM w/ Real-Time Clock |
| FM3808-70-T | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:256Kb Bytewide FRAM w/ Real-Time Clock |
| FM3808DK | 制造商:RAMTRON 制造商全稱:RAMTRON 功能描述:256Kb Bytewide FRAM w/ Real-Time Clock |
| FM3808-S | 制造商:Ramtron International Corporation 功能描述:FRAM RTC 256K SMD 3808 TSOP32 |
| FM3808-SG | 制造商:Ramtron International Corporation 功能描述:FRAM RTC 256K SMD 3808 TSOP32 |
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