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Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 42
Keywords: supercap, battery backup, bbu, capacitor backup, backup battery, battery backup switchover
APPLICATION NOTE 42
Large Capacitor Replaces Backup Battery
Jul 09, 1998
Abstract: Use a supercap to reduce cost of simple battery backup circuits. In low-power systems that
require a battery backup switchover circuit during brownout conditions, a supercap may provide adequate
power to maintain memory operation prior to complete shutdown.
A large (0.1F) capacitor can replace your backup battery in certain applications. Though limited in
storage capacity, the capacitor offers sufficient backup for low-dissipation equipment in which typical
power outages last from a few seconds to several hours.
A simple implementation (Figure 1a) combines the capacitor with a battery-switchover IC—a device that
monitors the supply and switches the load to battery voltage when the supply fails or "browns out." As
the following explanation attests, however, the simple approach can be unreliable when applied to the
MAX690.
The capacitor charges rapidly to V
DD
-V
BE
. (V
BR
is the diode's forward-voltage drop, approximately
0.6V.) V
BE
and the charging current then decrease exponentially as the large capacitor continues
charging (toward V
DD
) with a time constant ranging from many hours to several days. When the
capacitor reaches full charge—typically within 0.2V of V
DD
—the diode leakage equals the sum of
leakages into the capacitor and the high-impedance input V
BATT
. Low-leakage circuits can charge the
capacitor nearly to V
DD
.
This proximity of battery voltage (V
BATT
) to V
DD
creates a problem, because the IC initiates a switchover
whenever V
DD
plus a small offset is less than V
BATT
. Swithovers imply power failures, but this circuit will
switch in response to the normal V
DD
fluctuations caused by variations in line, load, and temperature.
Adding a large resistor across the capacitor solves the problem by assuring the typical V
BE
drop of 0.6V,
but the resistor also accelerates discharge during the backup condition. Even a large (10MΩ) resistor
cuts the backup time in half.
An improved version of the backup circuit (Figure 1b) includes a blocking diode (D
2
) that prevents
discharge through the resistor during backup. The resistor can be smaller because its current comes only
from the main supply. As before, this current keeps the diode forward-biased, maintaining a safety
margin of one V
BE
against droop in the V
DD
supply. You can increase the margin by adding diodes in
series with D
1
.
A further improvement (Figure 1c) replaces D
1
with R
2
and D
2
with Q
1
. The resistor divider lets you set
the backup voltage (V
x
) as desired.
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