22
FN6451.0
March 30, 2007
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 F
OUT
pin is used as a clock, it should be routed
away from the RTC device as well. The traces for the VBAT
and VDD pins can be treated as a ground, and should be
routed around the crystal.
Super Capacitor Backup
The ISL12021 device provides a VBAT 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 ISL12021 is extremely low, it is possible to
get months of backup operation using a Super Capacitor.
Typical capacitor values are a few µF to 1F 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.
Following are some examples with equations to assist with
calculating backup times and required capacitance for the
ISL12021 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 13, 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 ISL12021 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 ISL12021 (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. 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 3 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 5 for this example, I
BATAVG
= 4.387E-7A
t
BACKUP
= 0.47*(2.9)/4.38E-7 = 3.107E6s
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:
I = C
BAT
*dV/dT
(EQ. 2)
dT = C
BAT
*dV/I
TOT
to solve for backup time.
(EQ. 3)
I
BAT
= 1.031E-7*(V
BAT
) + 1.036E-7A
(EQ. 4)
I
BATAVG
= 5.155E-8*(V
BAT2
+ V
BAT1
) + 1.036E-7A
(EQ. 5)
t
BACKUP
= C
BAT
*(V
BAT2
- V
BAT1
) / (I
BATAVG
+ I
LKG
)
(EQ. 6)
seconds
C
BAT
= 0.70*35.96 = 25.2 days
(EQ. 7)
FIGURE 13. SUPERCAPACITOR CHARGING CIRCUIT
2.7V to 5.5V
V
DD
V
BAT
GND
1N4148
C
BAT
ISL12021
