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TSM108 Datasheet(PDF) 6 Page - STMicroelectronics |
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TSM108 Datasheet(HTML) 6 Page - STMicroelectronics |
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6 / 13 page  6/13 Description of a DC/DC step down battery charger 1. Voltage and Current Controller The device has been designed to drive an external P-Channel MOSFET in PWM mode and in Step Down topology. Its two integrated operational am- plifiers ensure accurate Voltage and Current Reg- ulation loops. The Voltage Control dedicated operational ampli- fier acts as an error amplifier and compares a part of the output voltage (external voltage divider) to an integrated highly precise voltage reference (V ref). The Current Control dedicated operational amplifi- er acts as an error amplifier and compares the drop voltage through the sense resistor to an inte- grated low value voltage reference (Vs). These two amplified errors are ORed through di- odes, and the resulting signal (“max of”) is a refer- ence for the PWM generator to set the switching duty cycle of the P-Channel MOSFET transistor. The PWM generator comprises an oscillator (saw tooth) and a comparator which gives a variable duty cycle from 0 to 95%. This PWM signal is the direct command of the output totem pole stage to drive the Gate of the P-Channel MOSFET. Thanks to this architecture, the TSM108 is ideal to be used from a DC power supply to control the charging Voltage and Current of a battery in appli- cations such as accessories for cellular phones chargers and post-regulation in power supplies. 2. Voltage Control The Voltage Control loop can be set by an exter- nal voltage divider connected between the output positive line and the Ground reference. The mid- dle point has to be connected to the V ctrl pin of TSM108, and, if R1 is the upper resistor, and R2, the lower resistor of the voltage divider, the values of R1 and R2 are: ❑ eq1: Vref = Vout x R2 / (R1 + R2) In Constant Voltage Control mode, the output volt- age is fixed by to the R1/R2 resistor. The total value of R1 + R2 voltage divider will de- termine the necessary bleeding current to keep the Voltage Control loop effective, even under “no load” conditions. The current compensation loop is directly accessi- ble from the pins Vcomp and Vref The compensa- tion network is highly dependant of the operating conditions (switching frequency, external compo- nents R, L, C, MOSFET, output capacitor, etc...). 3. Current Control The Current control loop can be set by the sense resistor that has to be placed in series on the out- put positive line. The output side of the Sense re- sistor has to be to Ictrl pin,and the common point between Rsense and the choke has to be con- nected to the Vs pin. If Ilim is the value of the charging current limit The value of Rsense should verify: ❑ eq2: V s = Rsense x Ilim In Constant Current Control mode, the output cur- rent is fixed by to the Rsense resistor (under out- put short circuit conditions, please refer to this cor- responding section). The wattage calibration (W) of the sense resistor should be chosen according to: ❑ eq2a: W > R sense x Ilim 2 The current compensation loop is directly accessi- ble from Icomp and Ictrl pin. The compensation network is highly dependant on the operating conditions. 4. PWM frequency The internal oscillator frequency is a saw tooth waveform that can be adjusted. In accessories like battery chargers, it is suggested to set the switch- ing frequency at a typical 100kHz in order to ob- tain a good compromise between the ripple cur- rent and the choke size. An external capacitor connected between Osc pin and ground set the switching frequency The maximum duty cycle of the PWM function is limited to 95% in order to ensure safe driving of the MOSFET. TSM108 PRINCIPLE OF OPERATION AND APPLICATION HINTS |
Similar Part No. - TSM108 |
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Similar Description - TSM108 |
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