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TSM105L Datasheet(PDF) 5 Page - STMicroelectronics |
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TSM105L Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 9 page 5/9 1. Voltage and Current Control 1.1. Voltage Control The voltage loop is controlled via a first transcon- ductance operational amplifier, the resistor bridge R1, R2, and the optocoupler which is directly con- nected to the output. The relation between the values of R1 and R2 should be chosen as writen in Equation 1. R1 = R2 x Vref / (Vout - Vref) Eq1 where Vout is the desired output voltage. To avoid the discharge of the load, the resistor bridge R1, R2 should be highly resistive. For this type of application, a total value of 100K Ω (or more) would be appropriate for the resistors R1 and R2. As an example, with R2 = 100K Ω, Vout = 4.10V, Vref = 1.210V, then R1 = 41.9K Ω. Note that if the low drop diode should be inserted between the load and the voltage regulation resis- tor bridge to avoid current flowing from the load through the resistor bridge, this drop should be taken into account in the above calculations by re- placing Vout by (Vout + Vdrop). 1.2. Current Control The current loop is controlled via the second trans-conductance operational amplifier, the sense resistor Rsense, and the optocoupler. The control equation verifies: Rsense x Ilim = Vsense eq2 Rsense = Vsense / Ilim eq2’ where Ilim is the desired limited current, and Vsense is the threshold voltage for the current control loop. As an example, with Ilim = 1A, Vsense = -200mV, then Rsense = 200m Ω. Note that the Rsense resistor should be chosen taking into account the maximum dissipation (Plim) through it during full load operation. Plim = Vsense x Ilim. eq3 As an example, with Ilim = 1A, and Vsense = 200mV, Plim = 200mW. Therefore, for most adapter and battery charger applications, a quarter-watt, or half-watt resistor to make the current sensing function is sufficient. Vsense threshold is achieved internally by a re- sistor bridge tied to the Vref voltage reference. Its middle point is tied to the positive input of the cur- rent control operational amplifier, and its foot is to be connected to lower potential point of the sense resistor as shown on the following figure. The re- sistors of this bridge are matched to provide the best precision possible The current sinking outputs of the two trans-con- nuctance operational amplifiers are common (to the output of the IC). This makes an ORing func- tion which ensures that whenever the current or the voltage reaches too high values, the optocou- pler is activated. The relation between the controlled current and the controlled output voltage can be described with a square characteristic as shown in the fol- lowing V/I output-power graph. Figure 3 : Output voltage versus output current 2. Compensation The voltage-control trans-conductance operation- al amplifier can be fully compensated. Both its out- put and the negative input are directly accessible for external compensation components. Vout Iout Voltage regulation TSM105 Vcc : independent power supply 0 Secondary current regulation TSM105 Vcc : On power output Primary current regulation TSM105 PRINCIPLE OF OPERATION AND APPLICATION HINTS |
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