TSM102/A
7/9
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. Prop-
er hysteresis is given thanks to R20. An improve-
ment 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 cor-
responding LED is polarized for convenient visual-
ization 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 refer-
ence gives a fixed 2.500V voltage.
I = U / R = [V
ref
( 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 = [ V
ref
R8 / (R7 + R8) ] / Rs
= [ 2.5 x 390 / (10000 + 390 ) ] / 0.375
= 250mA
I (LoBatt) = 250mA
Voltage Control :
V
out
= V
ref
/ [ R2 / (R1 + R2 + R3) ]
= 2.5 / [ 56 / (131.5 + 56 + 0.68 ) ]
= 8.400V
V
out
= 8.400V
Low Battery signal :
If R5 = 0Ω and R6 = open :
V
out
(LoBatt) = Vref / [ R19 / ( R17 + R19 ) ]
= 2.5 / [ 10 / (12.4 + 10) ]
= 5.6V
V
out
(LoBatt) = 5.6V
End of Charge signal :
V
out
(EOC) = Vref / [ (R2 + R3 ) / (R1 + R2 + R3) ]
= 2.5 / [(56 + 0.68) / (131.5 + 56 + 0.68)]
= 8.300V
V
out
(EOC)= 8.300V
