Figure 2 : The Application Schematic - Battery Charger Secondary Side

The first comparator ensures the ”Low Battery”

signal generation thanks to the comparison of a

part of the charger’s output voltage (resistor bridge

R17, R19) and the reference voltage. Proper hys-

teresis is given thanks to R20. An improvement to

the chargers security and to the battery’s life time

optimization is achieved by lowering the current

control measurement thanks to Q1 that shunts the

resistor R9 when the battery’s voltage is below the

”Low Battery” level.

The second comparator ensures the ”End of

Charge” signal generation thanks to the compari-

son of a part of the charger’s output voltage (resis-

tor bridge R1, R2, R3) and the reference voltage.

When either of these two signals is active, the

corresponding LED is polarized for convenient

visualization of the battery status.

3 - CALCULATION OF THE ELEMENTS

All the components values have been chosen for a

two-Lithium-Ion batteries charge application :

.

Current Control : 720mA (Low Battery current

control : 250mA)

.

Voltage Control : 8.4V (= 2x 4.2V)

.

Low Battery : 5.6V (= 2x 2.5V + 0.6V)

.

End of Charge : 8.3V (= 2x 4.15V)

Current Control :

The voltage reference is polarized thanks to the R4

resistor (2.5mA), and the cathode of the reference

gives a fixed 2.500V voltage.

I = U / R = [ Vref ( R8 + R9 ) / (R7 + R8 + R9) ] / Rs

= [ 2.5 x (390 + 820) / (10000 + 390 + 820) ] / 0.375

= 720mA

I = 720mA

P = power dissipation through the sense resistor = R I2

= 0.375 x 0.7202 = 194mW

In case of ”Low Battery” conditions, the current

control is lowered thanks to the following

equation :

I = U / R = = [ Vref R8 / (R7 + R8) ] / Rs

= [ 2.5 x 390 / (10000 + 390 ) ] / 0.375

= 250mA

I (LoBatt) = 250mA

Voltage Control :

Vout

= Vref / [ R2 / (R1 + R2 + R3) ]

TSM102

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