ISL1219 Datasheet, PDF (23/24 Pages) Intersil Corporation – Low Power RTC with Battery Backed SRAM and Event Detection

ISL1219
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
1N4148
2.7V to 5.5V
VDD
V_BAT
GND
C_BAT
FIGURE 23. SUPERCAPACITOR CHARGING CIRCUIT
In Figure 23, use C_BAT = 0.47F and V_DD = 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. (Equation 2 and 3):
I = C_BAT * dV/dT
(EQ. 2)
Rearranging gives
dT = C_BAT * dV/I_TOT to solve for backup time.
(EQ. 3)
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 shown in Equation 4:
I_BAT = 1.031E-7*(V_BAT) + 1.036E-7 Amps
(EQ. 4)
Using this equation to solve for the average current given 2
voltage points gives (Equation 5):
I_BATAVG = 5.155E-8*(V_BAT2 + V_BAT1) + 1.036E-7 Amps
(EQ. 5)
Combining with Equation 3 gives the equation for backup
time (Equation 6):
T_BACKUP = C_BAT * (V_BAT2 - V_BAT1) / (I_BATAVG + I_LKG)
seconds
(EQ. 6)
where:
C_BAT = 0.47F
V_BAT2 = 4.7V
V_BAT1 = 1.8V
I_LKG = 0 (assumed minimal)
Solving Equation 5 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 23 again, the capacitor value needs to be
calculated to give 2 months (60 days) of backup time, given
V_DD = 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 3 to
solve for capacitance (Equation 7):
C_BAT = dT*I/dV
(EQ. 7)
Using the terms described above, this equation becomes
(Equation 8):
C_BAT = T_BACKUP * (I_BATAVG + I_LKG)/(V_BAT2 – V_BAT1)
(EQ. 8)
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
23
FN6314.2
July 15, 2010