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參數(shù)資料
| 型號(hào)： | ISL1221IUZ-T |
| 廠商： | Intersil |
| 文件頁(yè)數(shù)： | 16/24頁(yè) |
| 文件大?。?/td> | 0K |
| 描述： | IC RTC LP BATT BACK SRAM 10MSOP |
| 產(chǎn)品培訓(xùn)模塊： | Solutions for Industrial Control Applications |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 類型： | 時(shí)間事件記錄器 |
| 特點(diǎn)： | 警報(bào)器，閏年，SRAM |
| 存儲(chǔ)容量： | 2B |
| 時(shí)間格式： | HH:MM:SS（12/24 小時(shí)） |
| 數(shù)據(jù)格式： | YY-MM-DD-dd |
| 接口： | I²C，2 線串口 |
| 電源電壓： | 2.7 V ~ 5.5 V |
| 電壓 - 電源，電池： | 1.8 V ~ 5.5 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 10-TFSOP，10-MSOP（0.118"，3.00mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 10-MSOP |
| 包裝： | 標(biāo)準(zhǔn)包裝 |
| 產(chǎn)品目錄頁(yè)面： | 1245 (CN2011-ZH PDF) |
| 其它名稱： | ISL1221IUZ-TDKR |
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23
FN6316.1
July 15, 2010
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 1A. A standard electrolytic capacitor
with DC leakage current in the microamps will have a
severely shortened backup time.
Following are some examples with equations to assist with
calculating backup times and required capacitance for the
ISL1221 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
5.0V, the voltage at VBAT will approach 4.7V as the diode
turns off completely. The ISL1221 is specified to operate
down to VBAT = 1.8V. The capacitance charge/discharge
equation is used to estimate the total backup time (see
Rearranging gives
CBAT is the backup capacitance and dV is the change in
voltage from fully charged to loss of operation. Note that
ITOT is the total of the supply current of the ISL1221 (IBAT)
plus the leakage current of the capacitor and the diode, ILKG.
In these calculations, ILKG 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 IBAT changes with VBAT almost linearly (see
Typical Performance Curves). This allows us to make an
approximation of IBAT, using a value midway between the
two endpoints. The typical linear equation for IBAT vs. VBAT
is shown in Equation 4.
Using this equation to solve for the average current given 2
voltage points gives (see Equation 5).
Combining with Equation 3 gives the equation for backup
time (see Equation 6).
where :
CBAT = 0.47F
VBAT2 = 4.7V
VBAT1 = 1.8V
ILKG = 0 (assumed minimal)
TBACKUP = 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:
CBAT = 0.70 * 35.96 = 25.2 days
Example 2. Calculating a capacitor value for a
given backup time
Referring to Figure 23 again, the capacitor value needs to be
calculated to give 2 months (60 days) of backup time, given
VDD = 5.0V. As in Example 1, the VBAT voltage will vary from
4.7V down to 1.8V. We will need to rearrange Equation 3 to
solve for capacitance (see Equation 7).
Using the terms described above, this equation becomes
(see Equation 8):
where:
TBACKUP = 60 days * 86,400 sec/day = 5.18 E6 sec
IBATAVG = 4.387 E-7 A (same as Example 1)
ILKG = 0 (assumed)
VBAT2 = 4.7V
VBAT1 = 1.8V
Solving gives:
CBAT = 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:
CBAT = 1.3 *.784 = 1.02F
FIGURE 23. SUPERCAPACITOR CHARGING CIRCUIT
2.7V to 5.5V
VDD
VBAT
GND
1N4148
CBAT
I = CBAT * dV/dT
(EQ. 2)
dT = CBAT * dV/ITOT to solve for backup time.
(EQ. 3)
IBAT = 1.031E-7*(VBAT) + 1.036E-7 Amps
(EQ. 4)
IBATAVG = 5.155E-8*(VBAT2 + VBAT1) + 1.036E-7 Amps
(EQ. 5)
TBACKUP = CBAT * (VBAT2 - VBAT1) / (IBATAVG + ILKG)
(EQ. 6)
seconds
CBAT = dT*I/dV
(EQ. 7)
CBAT = TBACKUP * (IBATAVG + ILKG)/(VBAT2 – VBAT1)
(EQ. 8)
ISL1221
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