Another consideration is systems with either ground bounce
or power supply transients that cause the VDD pin to drop
below ground for more than a few nanoseconds. This type of
power glitch can override the VBAT backup and reset or
corrupt the SRAM. If these transient glitches are present in a
system with the ISL1208, or the device is experiencing
unexplained loss of data when returning from VBAT mode, a
protection circuit should be added. Figure 20 shows a circuit
which effectively isolates the VDD input from negative
glitches. The Schottky diode is needed to for low voltage
drop and effective protection from the negative transient.
Note that this circuit will also help if the VDD fall time is less
than 50us as CIN holds up the VDD pin during the transient.
There is also a shunt shown between the battery and the
VBAT pin. This is for quick disconnect if there is a situation
where a transient has latched the device and it will not
communicate on the I2C bus. If ground bounce is a problem,
then a second Schottky diode should be added between the
battery and the VBAT pin.
2.7V TO 5.5V
FIGURE 20. POWER GLITCH PROTECTION CIRCUIT
Super Capacitor Backup
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 ISL1208 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.
Below are some examples with equations to assist with
calculating backup times and required capacitance for the
ISL1208 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
2.7V TO 5.5V
FIGURE 21. SUPERCAPACITOR CHARGING CIRCUIT
In Figure 21, use CBAT = 0.47F and VCC = 5.0V. With
VCC = 5.0V, the voltage at VBAT will approach 4.7V as the
diode turns off completely. The ISL1208 is specified to
operate down to VBAT = 1.8V. The capacitance
charge/discharge equation (Equation 4) is used to estimate
the total backup time:
I = CBAT * dV/dT
dT = CBAT * dV/ITOT to solve for backup time.
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 ISL1208 (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 on page 7). 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 in Equation 6:
IBAT = 1.031E-7*(VBAT) + 1.036E-7 Amps
Using this equation to solve for the average current given 2
voltage points gives Equation 7:
IBATAVG = 5.155E-8*(VBAT2 + VBAT1) + 1.036E-7 Amps
September 12, 2008