LTC4040_15 Datasheet, PDF (13/26 Pages) Linear Technology – 2.5A Battery Backup Power Manager

LTC4040
Operation
The LTC4040 is a complete battery backup system manager
for a 3.5V to 5.5V supply rail. The system has three prin-
cipal circuit components: a full-featured step-down (buck)
battery charger, a step-up (boost) backup converter with
automatic burst feature to deliver power to the system load
when external input power is lost and a power-fail com-
parator to decide which one to activate. The LTC4040 has
several other auxiliary components: an input current limit
(CLPROG) amplifier, an optional input overvoltage protec-
tion (OVP) circuit and a reset comparator.
The LTC4040 has three modes of operation: normal,
backup and shutdown. If the input supply is above an
externally programmable PFI threshold voltage, the part
is considered to be in normal mode in which power flows
from input to output (V_SYS) while the step-down switch-
ing regulator charges the battery to one of eight charge
voltage settings programmed by the F0, F1, and F2 digital
inputs. Please refer to the Block Diagram. The total system
load is monitored by the CLPROG amplifier via an external
series resistor, R_S, connected between the V_IN and CLN
pins. This amplifier can reduce the charge current from its
programmed value (set by the PROG pin external resis-
tor R_PROG) if the external load demand increases beyond
a programmable level set by R_S. When the input supply
falls below the PFI threshold, backup mode disconnects
the switches (MN1 and MN2) to isolate the system (V_SYS)
from the input while the boost converter powers the system
load from the battery using the same external inductor, L1.
THE BATTERY CHARGER
The LTC4040 includes a full-featured constant-current
(CC)/constant-voltage (CV) battery charger with automatic
recharge, automatic termination by safety timer, low volt-
age trickle charging, bad-battery detection and thermis-
tor sensor input for out-of-temperature charge pausing.
The battery charger is a high efficiency buck switching
converter used to transfer charge from V_SYS to BAT via
the SW pin. The charger can be disabled by pulling the
CHGOFF pin above 1.2V.
Buck Switching Charger
The LTC4040 battery charger is a constant frequency
(2.25MHz) synchronous buck converter capable of directly
charging the battery to its charge voltage with an externally
programmable charge current up to 2.5A from an input
supply as high as 5.5V. A zero current comparator monitors
the inductor current and shuts off the NMOS synchronous
rectifier once the current reduces to approximately 250mA.
This prevents the inductor current from reversing and
improves efficiency for low charging current.
Battery Preconditioning (Trickle Charge)
and Bad-Battery Fault
When a battery charge cycle begins, the battery charger
first determines if the battery is deeply discharged. If
the battery voltage is below V_LOWBAT, typically 2.85V, an
automatic trickle charge feature sets the charge current
to 1/8th or 12.5% of the programmed value. To improve
charge current accuracy at this low level, the buck switch-
ing charger is turned off and a secondary linear charger
is used to deliver charge to the battery. If the low voltage
persists for more than half an hour, the battery charger
automatically terminates and indicates, via the CHRG and
FAULT pins, that the battery is in bad-battery fault.
Constant-Current Mode Charging
Once the battery voltage is above V_LOWBAT, the charger
begins charging in full power constant-current mode. The
current delivered to the battery will try to reach 2000V/
R_PROG. Depending on the external load condition, the
battery charger may or may not be able to charge at the
full programmed rate. The external load will always be
prioritized over the battery charge current. The battery
charger will charge at the full programmed rate only if
the sum of the external load and the charger input current
is less than or equal to the input current limit set by R_S.
For more information www.linear.com/LTC4040
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