PROTECTION/SAFETY FEATURES :

The

µController is checking the charging process in several ways so as to make sure that the battery is

kept into safe operating conditions.

.

Temperature is measured via resistor bridge R21 and R22 (where R22 is an NTC) and PB6 config-

ured in A/D input. According to Li-Ion manufacturers information the cells should be charged only

when the temperature is between 0 and 45 °C. Thus when the temperature reaches outside this safe

operating range, the charging process is kept in standby (charge timings held, Power MOSFETs off

thus stopping the charge current, and user is warned of this condition by corresponding LED turning

orange). When the temperature is back to normal, charging process is resumed.

.

The 0 and 45 degrees threshold level have a programmable software hysteresis to prevent the

charger from oscillating form Temperature protection to Normal mode.

.

Open battery condition is detected using current measurement through PB7. If no current is flowing

for more than a programmable current establishment delay after the charge has started the battery

will be rejected, the charge terminated and the user warned of this condition by the corresponding

LED blinking orange.

.

Bad voltage battery condition : the

µController is continuously monitoring the voltage of the batteries.

According to the battery manufacturers specifications, and depending on the voltage, the actions of

the charger are summarized in Table 4.

Table 4 : CHARGER STATE AS A FUNCTION OF BATTERY VOLTAGE

Voltage

State

Action

Condition

0 - 1.5V

Pack protection circuitry active

Trickle charge

V > 1.5V within 1 second

1.5 - 2.7V

Very low

Trickle charge

V > 2.7V within 30 minutes

2.7 - 4.1/4.2V

Normal

Standard charge

V > 3.9V when battery is full

4.4V and over

Overvoltage

Stop charge

-

The Condition Column shows what the result of the

action must be for the charger to proceed with the

charging. In case this condition is not fulfilled, the

battery will be rejected, the charge terminated and

the user warned of this condition by the corre-

sponding LED blinking orange.

FULL BATTERY DETECTION :

This is made either by detecting a low current

flowing through the battery or by an overtime con-

dition (maximum time after standard charge has

been started). A minimum voltage of 3.9 volt must

be reached at that time or the battery will be re-

jected, and the user warned of this condition by the

corresponding LED blinking orange.

3 - CALCULATION OF THE ELEMENTS

All the components values have been chosen for a

two-Lithium-Ion batteries charge application:

PWM Frequency and Maximum swithching ra-

tio of the VIPER20 :

R02 > 1.2k

Ω

Fsw = 2.3 Dmax / (R02 x C06)

Dmax = 1 - 550 / (R02 - 150)

recommended Dmax values are :

100kHz : Dmax > 80%

200kHz : Dmax > 70%

R02 = 4.7k

Ω

C06 = 4.7nF

Dmax = 87.9%

Fsw = 91.5kHz

5V line power supply for TSM102A &

ST62T25C :

Vout = Vref ( 1 + R2 / R3 )

R2 = R3 = 10k

Ω

C1 = C2 = 10

µF

R1 = 1.5k

Ω

Voltage Control :

.

Vbatt = 4.100V

Vref = Vbatt x R6 / ( R6 + R4 )

With R6 = 100k

Ω, R4 = 64kΩ

AN1051 - APPLICATION NOTE

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