DN535 - Monolithic Linear Battery Charger Operates from Inputs Up to 60V

Monolithic Linear Battery Charger Operates from Inputs Up
to 60V
Design Note 535
Joshua Yee
Introduction
For charging relatively low capacity batteries, or
maintenance charging backup and keep-alive batteries,
linear topology battery chargers are valued for their
compact footprints, simplicity and affordability. Even
so, there is a dearth of linear chargers that accept a
10V or higher input voltage, leaving many industrial
and automotive systems underserved.
Some switch mode solutions can accept high input
voltages and switching topologies offer current and
efficiency advantages, but they also incur significant
costs in complexity and solution footprint. In the end,
a switch mode solution is usually overkill for the low
currents necessary in keep-alive systems or backup
battery chargers. Furthermore, few are suitable for
automotive and industrial applications up to 60V.
LTC ®4079 is a wide input range standalone charger
that can be powered by any DC source from 2.7V up
to 60V, enabling CC/CV charging directly from 12V and
24V DC system rails, or even 48V industrial supplies.
Its marriage of simplicity and robustness allow it to
easily satisfy the charging needs of keep-alive systems
or backup battery solutions in these environments.
Figure 1 is an example of a simple Li-Ion battery charger.
9V TO 60V
IN
1µF
2.55M
LTC4079
EN
8.4V
99mA
BAT
FB
412k
CHRG
FBG
PROG NTCBIAS
3.01k
10k
TIMER
NTC
GND
T 10k
+
Li-Ion
DN5JY F01
Figure 1. Wide Range Linear Standalone
Charger for 2-Cell Li-Ion Backup Battery
02/15/535
Fortitude and Flexibility, Neatly Packaged
The LTC4079’s charge voltage is resistor programmable,
matching the flexibility of its wide input voltage range
for practical purposes. The circuit is stable across
the entire input voltage range with minimal input and
output capacitance.
Using a single resistor on the PROG pin, charge current
is programmable up to 250mA and can be monitored
in proportion to the PROG voltage. Charge termination
functionality is familiar: timer-based, programmed via
TIMER pin capacitance, or, C/10 current detection by
connecting the TIMER pin to ground. CHRG status
signals termination by either method. The timer
capacitor is also used for bad-battery detection.
Temperature-qualified charging can be implemented
via the NTC and NTCBIAS sensing network to round
out the full charger circuit. The LTC4079’s thermally
enhanced 3mm × 3mm DFN package includes an
internal pass element, producing a compact and
comprehensive solution. The complete circuit featured
in Figure 2 shows its compact footprint.
Innovative Regulation for Utility and Facility
The LTC4079 includes a number of enhancements over
conventional chargers with several distinctive charge
current regulation methods. First, for wide range, but
current-limited or high impedance sources, the input
voltage can be regulated to at least 160mV above the
battery voltage (VIN(MIN) ≥ VBAT + 160mV). Charge
current is reduced to prevent the input voltage from
collapsing below this value, maximizing charge current.
No external components are required to exploit this
internal regulation scheme. Figure 3 shows an example
of temperature-compensated float charging a 12V sealed
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
lead acid battery stack from a solar panel, though any
combination of input and battery voltages are possible.
The final current regulation method—thermal
regulation—is important for a monolithic device
in general, but should be mandatory for a linear
regulator. This is especially useful in harsher ambient
environments and for high VIN/VBAT ratios, where the
charge voltage is much lower than the nominal input
voltage. Charge current is reduced until the IC junction
temperature is reduced below 118°C. See Figure 3 for
an example circuit with input voltage regulation that
prevents weak input sources from being overloaded.
The LTC4079's differential voltage regulation is
particularly useful when very low power sources such
as energy harvesters or small solar panels cannot
continuously provide the minimum 10mA charge
current. Instead of the somewhat arbitrary halt to
charging in the face of an undervoltage lockout (UVLO),
this feature allows charging to continue whenever
possible—a more efficient use of available input power.
Low Quiescent Current Draw
When charging, the LTC4079 consumes only 4µA,
maximizing energy transfer from the source to the
battery. This is particularly important when transferring
energy from a higher capacity battery to a smaller
backup battery. In battery backup systems, the voltage
feedback divider is taken out of the circuit to further
unload the battery, reducing shutdown current to 10nA
(typical) and ensuring that capacity is not unexpectedly
degraded in long-term standby or storage of the entire
battery system. This makes the LTC4079 especially
suited for low or zero-maintenance set-and-forget
designs with embedded charging capability.
For a more specific input voltage regulation setpoint,
the enable input pin, EN, can servo to a resistor divider.
As the input voltage reaches this setpoint, the charge
current is reduced to prevent loading the source any
further. In this manner, the enable input can be used
to set a minimum operating voltage for a given source.
Summary
The LTC4079’s compact and comprehensive design is
ideal for maintenance and keep-alive battery charging
solutions, but it is not limited to these applications.
Its rich feature set makes it easily adaptable to any
number of charging roles in industrial, automotive,
solar, medical, military/aerospace and consumer
electronics.
Figure 2. Complete Demo Board Circuit Footprint
Shown Actual Size
IN
1µF
LTC4079
EN
+
1µF
1.5M
TO
LOAD
FB
162k
SOLAR
PANEL
–
BAT
CHRG
PROG
VCHG = 12V
ICHG = 246mA
FBG
NTCBIAS
10k
1.21k
TIMER
NTC
+
GND
T
10k
BATTERY
PACK
4079 F10
Figure 3. Prevent Weak Sources from Being Overloaded with Input Voltage Regulation
Data Sheet Download
www.linear.com/LTC4079
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 3864
dn535f LT/AP 0215 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2014
Similar pages