參數(shù)資料
型號： ISL1208IB8-TK
廠商： INTERSIL CORP
元件分類： XO, clock
英文描述： I2C Real Time Clock/Calendar
中文描述： 1 TIMER(S), REAL TIME CLOCK, PDSO8
封裝： SOIC-8
文件頁數(shù)： 19/21頁
文件大小： 393K
代理商： ISL1208IB8-TK
19
FN8085.3
July 29, 2005
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 20, use C
BAT
= 0.47F and V
CC
= 5.0V. With V
CC
=
5.0V, the voltage at V
BAT
will approach 4.7V as the diode
turns off completely. The ISL1208 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 ISL1208 (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:
where
C
BAT
= 0.47F
V
BAT2
= 4.7V
V
BAT1
= 1.8V
I
LKG
= 0 (assumed minimal)
Solving equation 4 for this example, I
BATAVG
= 4.387E-7 A
T
BACKUP
= 0.47 * (2.9) / 4.38E-7 = 3.107E6 sec
Since there are 86,400 seconds in a day, this corresponds to
35.96 days. If the 30% tolerance is included for capacitor
and supply current tolerances, then worst case backup time
would be:
C
BAT
= 0.70 * 35.96 = 25.2 days
Example 2. Calculating a capacitor value for a
given backup time
Referring to Figure 20 again, the capacitor value needs to be
calculated to give 2 months (60 days) of backup time, given
V
CC
= 5.0V. As in Example 1, the V
BAT
voltage will vary from
4.7V down to 1.8V. We will need to rearrange Equation 2 to
solve for capacitance:
Using the terms described above, this equation becomes:
where
T
BACKUP
= 60 days * 86,400 sec/day = 5.18 E6 sec
I
BATAVG
= 4.387 E-7 A (same as Example 1)
I
LKG
= 0 (assumed)
V
BAT2
= 4.7V
V
BAT1
= 1.8V
Solving gives
C
BAT
= 5.18 E6 * (4.387 E-7)/(2.9) = 0.784F
If the 30% tolerance is included for tolerances, then worst
case cap value would be
C
BAT
= 1.3 *.784 = 1.02F
FIGURE 20. SUPERCAPACITOR CHARGING CIRCUIT
2.7V to 5.5V
V
CC
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)
C
BAT
= dT*I/dV
(EQ. 6)
C
BAT
= T
BACKUP
* (I
BATAVG
+ I
LKG
)/(V
BAT2
– V
BAT1
)
(EQ. 7)
ISL1208
相關PDF資料
PDF描述
ISL1208 I2C Real Time Clock/Calendar(I2C實時時鐘/日歷)
ISL1218 Low Power RTC with Battery Backed SRAM(具有電池供電SRAM的低功率RTC)
ISL1219 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)
ISL1220 Low Power RTC with 8 Bytes of Battery Backed SRAM and Separate FOUT(具有電池供電的8位SRAM和獨立輸出的低功率RTC)
ISL1221 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)
相關代理商/技術參數(shù)
參數(shù)描述
ISL1208IB8Z 功能描述:實時時鐘 I2C REAL TIME CLOCK/ CALENDAR 8LD 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: 安裝風格:Through Hole 封裝 / 箱體:PDIP-8 封裝:Tube
ISL1208IB8ZR5291 功能描述:實時時鐘 I2CAL TIME CLK/CLNDR ALL BATRY MODE TESTS 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: 安裝風格:Through Hole 封裝 / 箱體:PDIP-8 封裝:Tube
ISL1208IB8Z-T7A 功能描述:實時時鐘 I2CAL TIME CLK/CLNDR 8LD 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: 安裝風格:Through Hole 封裝 / 箱體:PDIP-8 封裝:Tube
ISL1208IB8Z-TK 功能描述:實時時鐘 I2C REAL TIME CLOCK/ CALENDAR 8LD 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: 安裝風格:Through Hole 封裝 / 箱體:PDIP-8 封裝:Tube
ISL1208IB8Z-TKR5291 功能描述:實時時鐘 I2CAL TIME CLK/CLNDR BATRY MODE TESTS 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: 安裝風格:Through Hole 封裝 / 箱體:PDIP-8 封裝:Tube
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