SMB135_08 datasheet(13/34 Pages) SUMMIT

13 / 34 page

SMB135

Summit Microelectronics, Inc

2106 3.1 11/4/2008

APPLICATIONS INFORMATION

DEVICE OPERATION

The SMB135 is a fully programmable battery charger

for single-cell Li-Ion and Li-Polymer battery packs. The

device’s

high-efficiency,

switch-mode

operation

reduces heat dissipation and allows for higher current

capability for a given package size.

The SMB135

provides four main charging phases: trickle-charge,

pre-conditioning (pre-charge), constant (fast-charge)

current and constant voltage.

The overall system

accuracy of the SMB135 is 1%, allowing for a higher

capacity utilization versus other conventional solutions.

Furthermore, the main battery charging parameters are

programmable, allowing for high design flexibility and

sophisticated battery management.

Power Supply

The SMB135 can be powered from an input voltage

between +4.35 and +6.5 Volts applied between the IN

pin and ground. The voltage on the IN pin is monitored

by Under-Voltage (UVLO) and Over-Voltage Lockout

(OVLO) circuits, which prevent the charger from turning

on when the voltage at this node is less than the UVLO

threshold (+3.5V), or greater than the OVLO threshold

(+7.0V). The IN pin also supplies an internal +2.5V

VDD regulator, filtered by an external capacitor

attached between the VDDCAP pin and ground; this

filtered voltage is then used as an internal VDD supply.

When the input supply is removed, the SMB135 enters

a low-power shutdown mode, exhibiting a very low

discharge leakage current (2µA), thereby extending

battery life.

Pre-qualification Mode

When an external wall adaptor or a USB cable is

connected, the SMB135 performs a series of pre-

qualification tests before initiating the first charge cycle.

The input voltage level needs to be higher than the

UVLO threshold, lower than the OVLO threshold and

130mV greater than the battery voltage; the ENABLE

input needs to be asserted or the appropriate I2C

command needs to be asserted; and the cell

temperature

needs

within

the

specified

temperature limits for the charging cycle to start.

The

pre-qualification parameters are continuously monitored

and charge cycle is suspended when one of them is

outside the limits.

Trickle-charge Mode

Once all pre-qualification conditions are met, the device

checks the battery voltage to decide if trickle-charging

is required (Figure 3). If the battery voltage is below

approximately 2.0V, a charging current of 10mA

(typical) is applied on the battery cell. This allows the

SMB135 to reset the protection circuit in the battery

pack and bring the battery voltage to a higher level

without compromising safety.

Pre-charge Mode

Once the battery voltage crosses the 2.0V level, the

SMB135 pre-charges the battery to safely charge the

deeply discharged cells (Figure 3).

The pre-charge

(pre-conditioning) current is programmable from 25mA

to 212.5mA in 12.5mA steps, assuming a sense

resistor of 100m

Ω (Register 00h).

The SMB135

remains in this mode until the battery voltage reaches

the pre-charge to fast-charge voltage threshold

(programmable from +2.4V to +3.1V in 100mV steps).

If the pre-charge to fast-charge voltage threshold is not

exceeded before the pre-charge timer expires, the

charge cycle is terminated and a corresponding timeout

fault signal is asserted (“Pre-charge Timeout” in register

36h).

Constant Current Mode

When the battery voltage exceeds the pre-charge to

fast-charge voltage threshold, the device enters the

constant current (fast charge) mode. During this mode,

the fast charge current level is set by either the

USB500/100 input (see below) or the corresponding

47.5mA to 525mA (16 steps), assuming a sense

resistor of 100m

Ω (Register 00h).

Constant Voltage Mode

When the battery voltage reaches the pre-defined float

voltage, the fast-charge current starts diminishing. The

float voltage is programmable from +4.00V to +4.62V in

20mV steps and is

±1% accurate over the 0°C to +70°C

temperature range. The higher float voltage settings of

the SMB135 enable the charging of modern battery

packs with a required float voltage of 4.3V, 4.4V, and

4.5V. Furthermore, the ability to dynamically adjust the

float voltage allows the implementation of sophisticated

battery charging and control algorithms.

Charge Completion

The charge cycle is considered complete when the

charge current reaches the programmed termination

current

threshold.

The

termination

current

programmable from 25mA to 130mA in 15mA steps,

assuming a sense resistor of 100m

Ω (Register 01h). If

the termination current threshold is not met before the

fast-charge timer expires, the charge cycle is

terminated and a corresponding timeout fault signal is

asserted (“Fast-charge Timeout” in register 36h).

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