DS273 Просмотр технического описания (PDF) - Maxim Integrated

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DS273

Cache-Memory Battery-Backup Management IC

Maxim Integrated 

Cache-Memory Battery-Backup Management IC

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

TPROTECT_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 VMIN, the charger proceeds to

CC mode. Precondition charge has a default 30-minute

timer. If the timer expires before the battery voltage

exceeds VMIN, 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: VAUX 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 < VMIN charge at a

reduced rate, 10% x ICHG, 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 VMIN within 30

minutes, the charger goes to the fault state. The charge

pin is active during precharge mode.

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