AN1464 datasheet(9/27 Pages) STMICROELECTRONICS

9 / 27 page

9/27

LOW-COST DOUBLE LI-ION BATTERY CHARGER USING ST6255C/ST6265C MCU

ST6 pins are open-drain outputs, so

Here, unlike previous equations, (VBE)SAT and (VCE)SAT refer to the NPN transistor values.

RPWM guarantees that (IS + IEN)/IPWM ≤ 10 even if (IS + IEN) is maximum.

Voh is the output high voltage of the ST6 pin emitting the PWM signal.

Example with the evaluation board hardware:

At maximum current (IS = 65 mA), typical values are (VBE)SAT = 0.9 V and (VCE)SAT = 0.1 V.

For IPWM = 7 mA, VOH = 4 V typically (cf. ST6 datasheet).

REN = 620 Ω and RPWM = 430 meet the requirements.

2.2.1 Measurement Circuitry

A shunt (RS) is connected to the battery in order to measure the charging current. The MCU

reads the VS voltage with its on-chip A/D converter. The converter has a voltage range be-

tween ground and VDD. When VS is too low, like in this case, an amplification circuit is needed,

e.g. with an OpAmp.

In our case, Vsupply = 6 V and the microcontroller is supplied with VDD = 5 V. Therefore, it is

safer not to read Vbat directly, but to attenuate this voltage by using a resistor bridge (Ra, Rb).

However, this attenuation must not be too strong to take maximum profit of the whole ADC

input range (0 to VDD). This must be taken into account when choosing a proper Ra/Rb ratio.

Note that the ST6 does not measure VBAT but VB, which is proportional to (VBAT +RS*IBAT).

Some calculation must be performed on the conversion results to access the real battery

voltage.

2.2.2 Battery Discharge Protection

If the charger is not powered on or if the battery is already fully charged, the PNP transistor is

kept permanently off. The diode prevents the battery from discharging into the capacitor.

Therefore, the battery discharges into RA, RB and RS, which requires less current.

2.2.3 Power Supply Restrictions

The battery characteristics have a direct influence on the choice of the DC power supply:

– The supply must be able to drive enough current to charge the battery, even in fast charge

mode.

()

sat

−

()

PWM

sat

PWM

−

AN1464 Datasheet(PDF) 9 Page - STMicroelectronics