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參數(shù)資料
| 型號： | ISL1219 |
| 廠商： | Intersil Corporation |
| 英文描述： | Low Power RTC with Battery Backed SRAM and Event Detection(具有電池供電的SRAM和事件檢測功能的低功率RTC) |
| 中文描述： | 低功耗RTC與電池供電的SRAM和事件檢測（具有電池供電的SRAM的和事件檢測功能的低功率實時時鐘） |
| 文件頁數(shù)： | 23/25頁 |
| 文件大?。?/td> | 373K |
| 代理商： | ISL1219 |
23
FN6314.1
August 14, 2006
In addition, it is a good idea to avoid a ground plane under
the X1 and X2 pins and the crystal, as this will affect the load
capacitance and therefore the oscillator accuracy of the
circuit. If the ~IRQ/F
OUT
pin is used as a clock, it should be
routed away from the RTC device as well. The traces for the
V
BAT
and VDD pins can be treated as a ground, and should
be routed around the crystal.
Super Capacitor Backup
The ISL1219 device provides a V
BAT
pin which is used for a
battery backup input. A Super Capacitor can be used as an
alternative to a battery in cases where shorter backup times
are required. Since the battery backup supply current
required by the ISL1219 is extremely low, it is possible to get
months of backup operation using a Super Capacitor.
Typical capacitor values are a few μF to 1 Farad or more
depending on the application.
If backup is only needed for a few minutes, then a small
inexpensive electrolytic capacitor can be used. For extended
periods, a low leakage, high capacity Super Capacitor is the
best choice. These devices are available from such vendors
as Panasonic and Murata. The main specifications include
working voltage and leakage current. If the application is for
charging the capacitor from a +5V ±5% supply with a signal
diode, then the voltage on the capacitor can vary from ~4.5V
to slightly over 5.0V. A capacitor with a rated WV of 5.0V
may have a reduced lifetime if the supply voltage is slightly
high. The leakage current should be as small as possible.
For example, a Super Capacitor should be specified with
leakage of well below 1μA. A standard electrolytic capacitor
with DC leakage current in the microamps will have a
severely shortened backup time.
Below are some examples with equations to assist with
calculating backup times and required capacitance for the
ISL1219 device. The backup supply current plays a major
part in these equations, and a typical value was chosen for
example purposes. For a robust design, a margin of 30%
should be included to cover supply current and capacitance
tolerances over the results of the calculations. Even more
margin should be included if periods of very warm
temperature operation are expected.
Example 1. Calculating Backup Time Given
Voltages and Capacitor Value
In Figure 23, use C
BAT
= 0.47F and VDD = 5.0V. With V
DD
=
5.0V, the voltage at V
BAT
will approach 4.7V as the diode
turns off completely. The ISL1219 is specified to operate
down to V
BAT
= 1.8V. The capacitance charge/discharge
equation is used to estimate the total backup time:
Rearranging gives
C
BAT
is the backup capacitance and dV is the change in
voltage from fully charged to loss of operation. Note that
I
TOT
is the total of the supply current of the ISL1219 (I
BAT
)
plus the leakage current of the capacitor and the diode, I
LKG
.
In these calculations, I
LKG
is assumed to be extremely small
and will be ignored. If an application requires extended
operation at temperatures over 50°C, these leakages will
increase and hence reduce backup time.
Note that I
BAT
changes with V
BAT
almost linearly (see
Typical Performance Curves). This allows us to make an
approximation of I
BAT
, using a value midway between the
two endpoints. The typical linear equation for I
BAT
vs. V
BAT
is:
Using this equation to solve for the average current given 2
voltage points gives:
Combining with Equation 2 gives the equation for backup
time:
FIGURE 23. SUPERCAPACITOR CHARGING CIRCUIT
2.7V to 5.5V
VDD
V
BAT
GND
1N4148
C
BAT
I = C
BAT
* dV/dT
(EQ. 1)
dT = C
BAT
* dV/I
TOT
to solve for backup time.
(EQ. 2)
I
BAT
= 1.031E-7*(V
BAT
) + 1.036E-7 Amps
(EQ. 3)
I
BATAVG
= 5.155E-8*(V
BAT2
+ V
BAT1
) + 1.036E-7 Amps
(EQ. 4)
T
BACKUP
= C
BAT
* (V
BAT2
- V
BAT1
) / (I
BATAVG
+ I
LKG
)
seconds
(EQ. 5)
ISL1219
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| ISL1219IUZ-T | 功能描述:實時時鐘 REAL TIME CLKRTC IN RoHS:否 制造商:Microchip Technology 功能:Clock, Calendar. Alarm RTC 總線接口:I2C 日期格式:DW:DM:M:Y 時間格式:HH:MM:SS RTC 存儲容量:64 B 電源電壓-最大:5.5 V 電源電壓-最小:1.8 V 最大工作溫度:+ 85 C 最小工作溫度: 安裝風(fēng)格:Through Hole 封裝 / 箱體:PDIP-8 封裝:Tube |
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