AN896

AN896 APPLICATION NOTE A BATTERY CHARGER USING THE TSM101 by S. LAFFONT This technical note shows how to use the TSM101 integrated circuit with a switching mode power supply (SMPS) to realize a battery charger. An example of realization of a 12V Nickel-cadmium battery charger is given. 1 - TSM101 PRESENTATION The TSM101 integrated circuit incorporates a high stability series band gap voltage reference, two ORed operational amplifiers and a current source (Figure 1) Figure 1 : TSM101 Schematic Diagram 1 2 3 4 Vref 8 7 6 5 A great majority of low or medium end power supplies is voltage regulated by using shunt programmable voltage references like the TL431 (Figure 2). The galvanic insulation of the control information is done by using an opto-coupler in linear mode with a variable photo current depending on the difference between the actual output voltage and the desired one. A current limitation is used to protect the power supply against short circuits, but lacks precision. This limitation is generally realized by sensing the current of the power transistor, in the primary side of the SMPS. The role of the TSM101 is to make a fine regulation of the output current of the SMPS and a precise voltage limitation. The primary current limitation is conserved and acts as a security for a fail-safe operation if a short-circuit occurs at the output of the charger. 2 - PRINCIPLE OF OPERATION The current regulation loop and the voltage limitation loop use an internal 1.24V band-gap ...