TSM101/A

6/13

Figure 2 : SMPS Using a TL431 as Voltage Controller

The optocoupler current increases and tends to

reduce the output voltage by the way of the PWM

controller.

By enabling the TSM101 current source (pin 2) it

is possible to offset the current sensing by a volt-

age equal to :

This offset lowers the output charge current and

this function can be used to charge two types of

batteries having different capacities. The current

source is enabled by connecting pin 2 to ground

3 - CALCULATION OF THE ELEMENTS

The charge current is regulated at 700mA (if the

charge control input is left open) or 200mA (if the

charge control input is put to ground ), allowing the

charge of two different types of batteries.

3.1 - Voltage limitation

The end-of- charge voltage is limited at 1.45V/cell,

this is the recommended voltage for an ambient

temperature at 25oC.

A diode is generally inserted at the output of the

charger to avoid the discharge of the battery if the

charger is not powered. This diode is sometimes

directly integrated in the battery pack. The influ-

ence of this diode on the charge is negligible if the

voltage drop (0.7V) is taken into account during

the design of the charger.

The voltage at the output of the charger is :

• V

and regarding R6 and R7 :

• R6=

P1, which is a part of R6 and R7 is not considered

in this equation.

The following values are used on the application

board :

• R7 = 12kΩ

• R6 = 1kΩ

• P1 = 220Ω, adjust for V

battery replaced by a 1k

Ω resistor

• R10 = short circuit

• C3 = 100nF

3.2 - Current regulation

R5 is the sense resistor used for current measure-

ment.

The current regulation is effective when the volt-

age drop across R5 is equal to the voltage on pin

5 of the TSM101 (assuming that the internal cur-

rent source is disabled).

For medium currents (<1A), a voltage drop across

R5 of 200mV = Vr5 is a good value, R5 can be re-

alized with standard low cost 0.5W resistors in

parallel.

• R5 =

, R5 = 0.285

Ω (four 1.2Ω resistor in

parallel)

R2 and R3 can be chosen using the following for-

mula :

• R2 =

CHARGE CONTROL

If the pin 2 is left open, the charge current is nom-

inal at # 700mA.

R6

R 7

+

R6

----------------------

V

r

×

Vref

Vout

Vref

–

--------------------------------



 R7

×

Vr5

Ich

----------

R3

Vref

Vr5

–

Vr5

----------------------------





×