Prevent Reverse Charging of a Lithium Battery to Meet UL Safety Requirement
APPLICATION NOTE
AN1535 Rev 0.00 Page 1 of 3
Jul 14, 2010
AN1535
Rev 0.00
Jul 14, 2010
Introduction
Lithium batteries are commonly used in many
applications with devices requiring backup power, such
as Real Time Clock (RTC) and memory devices.
Whenever lithium batteries are not the single power
source in a circuit, there is risk of fire or explosion if the
battery is accidentally connected with an electrical
power source that would charge the battery.
This application note provides information needed for
connecting Lithium batteries in backup power switching
circuits such that the circuit meets Underwriters
Laboratories (UL) standards. Specifically, UL standard
60950-1 describes the guidelines for Lithium batteries.
Protection Circuit
Protection Circuit for Intersil’s RTCs with
Battery Switchover Feature
All Intersil’s RTCs with a Battery Switchover feature,
such as the ISL12026 series, have internal protection
circuit to prevent reverse charging. Figure 1 shows the
internal switchover circuitry illustrating the
complementary control which disables one supply input
while enabling the other. The two series MOSFET
switches provide safe switching with redundancy. The
is also a path from V
DD
to V
BAT
through a MOSFET
(M1) used to pull-up the V
BAT
switch gate, and two
200resistors (R1 and R2). If the M1 gate is shorted,
the current through resistors R1 and R2 limits the
charging current to the Lithium battery (Up to 9.25mA
for V
DD
= 5.5V, V
BAT
= 1.8V).
(A detailed schematic of the battery switchover control
block is available upon request.
In order to meet full UL safety requirements, a series
Schottky diode needs to be added to the V
BAT
input for
these products. The external diode is used to provide
protection in the event that the internal MOSFET and
series resisters should fail. Figure 2 shows the actual
circuit with the external protection diode. (Note that a
high value series resistor would provide similar
protection, but would limit normal operating range.)
Only use a silicon diode or Schottky diode having a low
reverse current. A typical maximum reverse current of
1µA is recommended by UL. A few diodes that can be
used that exhibit low reverse current include, but are
not limited to, the BAS40, BAS70, and BAT54 diodes.
The reverse current can also be calculated for a specific
battery. The maximum reverse current of the diode for
a specific battery is given in Equation 1:
where Ir is the maximum reverse current in µA, Ic is
the total allowable charging amount of a battery in
mAh, and T is the total usage time in hour.
Ic is given in Equation 2:
where Q is the nominal capacity of the battery in mAh
and tp is the total allowable charging period in percent.
FIGURE 1. SIMPLIFIED BATTERY SWITCHOVER CIRCUIT
INTERNAL
RTC
POWER
-
+
-
+
R2
M1
R1
V
BAT
R0
2.2V
V
DD
BATTERY
V
DD
V
DD
V
BAT
GND
ISL12026
FIGURE 2. PROTECTION CIRCUIT WITH AN EXTERNAL
DIODE)
Ir
Ic
T
-----
1
6
10=
(EQ. 1)
Ic Qxtp=
(EQ. 2)
Prevent Reverse Charging of a Lithium
Battery to Meet UL Safety Requirement
AN1535 Rev 0.00 Page 2 of 3
Jul 14, 2010
For coin type battery, tp is 3%. For cylindrical battery, tp
is 1%.
Example: A 1000mAh coin-type battery is to be used for
five years.
With Equation 2, Ic is 30mAh (1000mAh x 3% (coin-type
battery) = 30mAh).
With Equation 1, a diode with a reverse current of 0.7μA
or less is required (30mAh ÷ usage period (5 years x 365
days x 24 hours) = 0.7μA).
Protection Circuit for Other Devices Without
Internal Protection
For circuits that require a battery backup feature but
either do not have a V
BAT
pin or the V
BAT
pin does not
offer reverse charging protection, then a classic UL
recommended three diode configuration is the best
protection against reverse charging.
For example, the ISL12058 does not offer a battery
switchover feature but an application requires it. The
battery backup function can be added simply by
connecting both main supply and backup battery to the
V
DD
pin. To have proper protection, a diode is placed in
series with the main supply and two diodes are placed in
series with the battery. The diode in series with the main
supply is to block current from the battery into the main
supply. Two diodes in series with the battery are to
prevent reverse charging. The second diode is used to
provide protection in the event that one should fail.
Figure 3 shows the actual circuit with three diodes
protection.
Testing
To insure the protection diodes are properly installed for
the reverse charging protection, a simple test sequence
is required after installation of the diodes.
A forced reverse charging test is done to insure the
diodes are placed correctly to prevent reverse charging
current to the battery. This is done by taking V
DD
to
3.6V. The battery input is then forced to 1.8V and the
charging current is check with an ampmeter capable of
measuring current as low as 100nA to insure that it does
not exceed the maximum reverse current of the
protection diode. The test circuit is shown in Figure 4.
Conclusion
Lithium batteries are commonly used in a wide range of
applications with devices requiring backup power, such as
RTCs and memory devices. In order to meet UL
standards for maximum charging (leakage) current,
extra circuitry is required such as low leakage diodes
even though a protected device has internal reverse
charging protection. For a system with supplies other
than the lithium battery, diode protection is not required
but may be put in place to prevent any reverse charging
from occurring.
Appendix
A) Intersil devices that require one diode in series
(Figure 2 on page 1) with the V
BAT
pin for UL
requirements: ISL1208, ISL1209, ISL1218,
ISL1219, ISL1220, ISL1221, ISL12020,
ISL12020M, ISL12022, ISL12022M, ISL12024,
ISL12025, ISL12026, ISL12027, ISL12028,
ISL12029.
B) Intersil devices that require two diode in series
(Figure 3) for UL requirements: ISL12057,
ISL12058, ISL12059.
BATTERY
V
DD
V
DD
GND
ISL12058
FIGURE 3. PROTECTION CIRCUIT WITH THREE
DIODES
FIGURE 4. TEST CIRCUIT FOR REVERSE CHARGING
PROTECTION
3.6V
V
DD
V
BAT
GND
ISL12026
1.8V
A