SMB139 datasheet(11/27 Pages) SUMMIT

11 / 27 page

SMB139

Summit Microelectronics, Inc

2121 3.0 6/19/2008

EN (ENABLE)

EN is a logic input pin for enabling/disabling the device

and/or restarting a charge cycle. The EN pin can be

configured for different polarity (logic high or logic low).

When the EN is at the correct state, the charger is

enabled for normal operation. When the EN is at the

wrong state, the charger is disabled. A programmable

option also exists that allows the user to initiate battery

charging only when an I

2C command has been issued.

Automatic Battery Recharge

The SMB139 allows the battery to be automatically

recharged (topped off) when the battery voltage falls by

a value of VRECH (125mV typical) below the

programmed float voltage. Provided that the input

power supply is still present, charging remains enabled

(ENABLE input or I

2C command) and all the pre-

qualification parameters are still met, a new charging

cycle will be initiated. This ensures that the battery

capacity remains high, without the need to manually re-

start a charging cycle. The automatic battery recharging

can be disabled if not required by the application

(Register 03h).

Safety Timers

The integrated safety timers provide protection in case

of a defective battery pack. The pre-charge timer starts

after the pre-qualification check is completed and

resets when the transition to the constant current mode

happens. At that point, the fast charge timer is initiated.

The fast charge timer expires and charge cycle is

terminated if the termination current level is not reached

within the pre-determined duration. Each safety timer

has three programmable timeout periods, which

eliminates the need for external timing capacitors and

allows for maximum design flexibility. In addition, each

timer can be disabled by the appropriate bit selection in

Thermal Monitor

A temperature sensing I/O (THERM) is provided to

prevent

excessive

battery

temperatures

during

charging. The battery temperature is measured by

sensing the voltage between the THERM pin and

ground. The voltage is created by injecting a current

into the parallel combination of Negative Temperature

Coefficient (NTC) thermistor and a resistor. This

voltage is then compared to two predetermined

voltages representing the maximum and minimum

temperature settings of the battery. The purpose of the

resistor in parallel to the NTC thermistor is to linearize

the resistance of the thermistor. Table 1, shows the 1%

resistor that should be placed in parallel with the

corresponding thermistor.

If the temperature limits are exceeded, battery charging

will be suspended until the temperature level has fallen

within the safe operating range. The over-temperature

limit is programmable from 30

°C to 65°C, and the

under-temperature limit is programmable from

−20°C to

°C, each in 5°C increments using Register 04h. In

addition, the user can easily select the required bias

current, based on the value of the negative temperature

coefficient (NTC) thermistor located in the battery pack:

10k, 25k, 100k (Register 04h). Disabling the thermal

monitor is also possible by selecting the appropriate

bits in Register 04h.

As the temperature changes, the resistance of the

thermistor changes creating a voltage proportional to

temperature. The temperature coefficient or Beta (

Β) of

the thermistor must be as close to 4400 as possible to

achieve the maximum temperature accuracy.

NTC THERMISTOR

RESISTANCE

10K

24.9K

25K

61.9K

100K

249K

Table 1: NTC values and associated parallel

resistances.

STAT Output

The STAT is an open-drain output that indicates battery

charge status. STAT has two modes of operation, as

determined by Register 05h[7]: in Mode 0, STAT is

asserted low whenever the battery is charging and de

asserted at all other times; in Mode 1, STAT is de-

asserted when the charger is disabled, blinks during

charging, and remains continuously asserted when the

charge cycle has completed. A pull-up resistor should

be applied on this pin for interfacing to a microcontroller

or other logic IC.

Programmable Battery Charging

A unique feature of the SMB139 is the ability to modify

all of the important charger parameters via internally

programmable EEPROM, found in Registers 00-07.

Once the device has been configured correctly, the

EEPROM may be locked, preventing any further

changes. Additionally, these registers may also be

configured so that they may be updated in RAM

(volatile), even if the underlying EEPROM is locked.

This feature is useful if it is desired to actively manage

the charging profile without making changes to the non-

volatile defaults. Use Register 0E to control locking

and volatile access. Before writing to Registers 00-07

APPLICATIONS INFORMATION (CONTINUED)

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