ISL1219 Intersil Corporation, ISL1219 Datasheet - Page 23

• Manufacturer Part Number: ISL1219
• Description: Real Time Clock/Calendar
• Manufacturer: Intersil Corporation

www.DataSheet4U.com
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
routed away from the RTC device as well. The traces for the
V
be routed around the crystal.
Super Capacitor Backup
The ISL1219 device provides a V
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.
BAT
and VDD pins can be treated as a ground, and should
OUT
pin is used as a clock, it should be
23
BAT
pin which is used for a
ISL1219
Example 1. Calculating Backup Time Given
Voltages and Capacitor Value
In Figure 23, use C
5.0V, the voltage at V
turns off completely. The ISL1219 is specified to operate
down to V
equation is used to estimate the total backup time:
Rearranging gives
C
voltage from fully charged to loss of operation. Note that
I
plus the leakage current of the capacitor and the diode, I
In these calculations, I
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
Typical Performance Curves). This allows us to make an
approximation of I
two endpoints. The typical linear equation for I
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:
dT = C
I
T
seconds
I = C
I
TOT
BAT
BATAVG
BAT
BACKUP
FIGURE 23. SUPERCAPACITOR CHARGING CIRCUIT
2.7V to 5.5V
BAT
is the total of the supply current of the ISL1219 (I
= 1.031E-7*(V
is the backup capacitance and dV is the change in
BAT
= 5.155E-8*(V
* dV/dT
BAT
BAT
= C
* dV/I
BAT
= 1.8V. The capacitance charge/discharge
changes with V
TOT
BAT
* (V
BAT
BAT
BAT
, using a value midway between the
to solve for backup time.
LKG
BAT2
= 0.47F and VDD = 5.0V. With V
) + 1.036E-7 Amps
VDD
BAT2
will approach 4.7V as the diode
is assumed to be extremely small
- V
1N4148
+ V
GND
BAT
BAT1
BAT1
almost linearly (see
V
BAT
) / (I
) + 1.036E-7 Amps
BATAVG
BAT
+ I
C
August 14, 2006
BAT
vs. V
LKG
(EQ. 5)
FN6314.1
(EQ. 1)
(EQ. 2)
(EQ. 3)
(EQ. 4)
BAT
DD
)
LKG
BAT
)
=
.