May 22, 1997
Buck-converter
charger
also provides system power
Robert Hanrahan, National Semiconductor,
Woodcliff Lake, NJ
Many systems
require long-time operation during periods of power loss.
Often, a gel or wet-cell lead-acid battery is the best
choice because of high capacity and relatively low cost.
The battery charges during normal operation and powers
the system during power loss. These systems require a
circuit to charge the battery as well as to regulate the
system's VCC. The design must provide a
current-limited voltage to the battery for charging and
still develop system VCC in the charge or the
discharge condition.
Many older designs use
inefficient linear regulators to provide these functions.
These designs require a large heat sink for regulating
the battery voltage to the system VCC, which
is typically 5V. However, you can use switching
regulators for a more efficient design at about the same
relative cost as a linear regulator. Many of these
designs use low-voltage ac power from a low-cost wall
transformer. Switching technology provides a wide
input-voltage range, including power-line voltages from
100 to 240V.
One of the best designs is
a current-limited voltage source that sources current
into the battery until the voltage reaches a setpoint.
The charger then operates in a constant-voltage mode,
supplying the current required to maintain the voltage.
Most lead-acid batteries have a voltage setpoint of 13.8V
at 25°C. You set the current limit depending on the
exact battery and charge-time requirement.
The design in Figure 1 uses two simple-switcher buck
converters. The first regulator, IC1,
efficiently steps down the unregulated input voltage from
the rectifier's output. This buck converter generates the
input voltage for the battery and provides voltage to the
second regulator. The second regulator, IC2,
is a small DIP or SOIC capable of providing as much as
0.5A of system VCC. You must consider the
system-current requirements when setting the charger's
current-limit value. You must increase the current-limit
value (set by the gain of IC2) by the amount
of current necessary to supply system power.
IC1 regulates
the battery charge voltage using the feedback network of
R1 and R2. You determine these
resistor values according to the equation VOUT=1.23(1+R2/R1).
D1 provides current switching between IC1
and the battery during power loss.
IC3 and
associated components measure and regulate the current
flow into the battery during charge. A shunt resistor
measures the battery current, which IC3
amplifies. IC3's CMOS op amp provides an
output voltage inversely proportional to current and has
a bandwidth of 1 MHz. Other op amps, such as the LMC6482,
can also do the job. With the gain of 10 that the op amp
provides, D2 forward-biases and pulls up the
feedback voltage when the output current is approximately
1.6A (VREF+diode drop). During normal
regulation, the diode is reverse-biased.
IC2 provides 5V
to the system. This buck regulator efficiently provides
system power when the input is at its highest voltage of
about 13.8V or at the lower voltage when IC2
is current-limiting.
Some systems may need a
charge-complete indicator. In a system that uses a µC
with an on-chip ADC, you can connect IC3's
output into the ADC's input and directly read the charge
current. Depending on the ADC's voltage reference and the
required accuracy, you may need to add another op-amp
stage in front of the ADC. Figure 1 also shows an optional voltage
comparator. This circuit compares the output of IC3
to the voltage set by R3. You can set this
voltage to represent the current flow that takes place at
the end of charge. (DI #2032)
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