Detailed Description
The DS2731 is a complete power-management solution
for modular backup applications. It is well suited for
2.5V and below memory bus voltages. It has a 12V
supply input for battery charging and a 3.3V aux supply
input for power failure circuitry and memory voltage
regulation. The DS2731 includes an internal MOSFET-
switching power stage for charging a lithium-ion (Li+)
cell. It has a fully integrated synchronous-buck regula-
tor capable of supplying up to 450mA of cache-backup
supply current. It also handles switching from system
power to battery power.
The battery-charging method is CCCV. Charging can
be broken into three different modes: precondition,
constant-current (CC) charge, and constant-voltage
(CV) charge. Precondition mode charges at a reduced
rate for a severely depleted cell. CC mode charge rate
is user selectable from 0.5A to 1.5A. In CV mode,
charge is terminated when the charging current falls
below 5% of CC mode charge rate. The CV mode out-
put is also user selectable from 3.8V to 4.6V. Charge
status is signaled to the user through three open-drain
pins that can be used to drive LEDs. A thermistor input
is provided to prevent charging outside of temperature
specifications and a safety timer prevents the contin-
ued charging of a damaged cell.
Switchover to battery backup is initiated by an internal
comparator and occurs automatically when a sensed
input voltage drops below 2.93V. The power failure
switching circuitry can be disabled by the user.
Memory voltage is user selectable from 0.9V to 2.5V. At
light loads, the 2MHz internal synchronous buck regu-
lator operates in burst mode for maximum efficiency. All
nonessential functions of the DS2731 are disabled dur-
ing a power failure while supplying holdup current to
the cache memory. To prevent damaging the battery,
the regulator shuts down and the IC goes into a very
low-current dormant mode when the battery voltage
drops below a user-settable threshold (LO_BATT).
Li+ Charger
The CCCV charger circuitry is powered by the 12V
input supply. When the charger circuitry is enabled,
battery voltage is continuously monitored at the BATT+
pin. Charging begins when the battery voltage drops
below the charge restart threshold. Accurate charge
current measurements are achieved by the Kelvin
remote-sense connections at the SNS and BATT+ pins.
Measuring the voltage through a Kelvin remote-sense
connection eliminates offset error caused by small
trace resistances at high current.
Charging Algorithm
From initiate, CC charging proceeds if ENC is high, the
UVLO-CHG is false, the die temperature is below
T
PROTECT_CCCV
, and the battery is above the minimum
voltage. If the battery is below the minimum voltage, the
charger goes into preconditioning charge. Once the
battery voltage exceeds V
MIN
, the charger proceeds to
CC mode. Precondition charge has a default 30-minute
timer. If the timer expires before the battery voltage
exceeds V
MIN
, the charger goes to fault. For CC charg-
ing, the charge safety timer starts. CC charging pro-
ceeds until the output voltage reaches the CV set point.
The charger then proceeds to CV mode. Charging ter-
minates when the current drops below 5% of the CC
charge rate. The charge-safety timer duration is the
sum of the CC and CV charge times and should be set
to about 15% above the expected maximum charge
time. If the charge-safety timer expires, the charger
goes to fault. After charge termination, the charger
monitors the battery voltage for self-discharge. When it
drops 200mV below the CV charge threshold, the
charger enters Initiate and a new charge cycle begins.
Note: V
AUX
must be above 3.08V for the charger to
operate.
Precharge Mode
Precharge mode is intended to restore severely deplet-
ed cells. Batteries with a voltage < V
MIN
charge at a
reduced rate, 10% x I
CHG
, to prevent damaging the
cell. Precharge mode has a fixed safety timer of 30
minutes. This timer is independent of the STMR pin. If
the battery voltage has not exceeded V
MIN
within 30
minutes, the charger goes to the fault state. The charge
pin is active during precharge mode.
DS2731
Cache-Memory Battery-Backup Management IC
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