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TSM1011 Datasheet(PDF) 5 Page  STMicroelectronics 

TSM1011 Datasheet(HTML) 5 Page  STMicroelectronics 
5 / 9 page Voltage and Current Control TSM1011 5/9 5 VOLTAGE AND CURRENT CONTROL 5.1 Voltage Control The voltage loop is controlled via a first transconductance operational amplifier, the resistor bridge R1, R2, and the optocoupler which is directly connected to the output. The relative values of R1 and R2 should be chosen in accordance with Equation 1: Equation 1 where Vout is the desired output voltage. To avoid discharge of the load, the resistor bridge R1, R2 should have high impedance. For this type of application, a total value of 100k Ω (or more) would be appropriate for the resistors R1 and R2. For example, if R2 = 100kΩ, Vout = 4.10V, Vref=2.5V, then R1 = 41.9KΩ. Note: If the low drop diode is to be inserted between the load and the voltage regulation resistor bridge to avoid current flowing from the load through the resistor bridge, this drop should be taken into account in the above calculations by replacing Vout by (Vout + Vdrop). 5.2 Current control The current loop is controlled via the second transconductance operational amplifier, the sense resistor Rsense, and the optocoupler. Vsense threshold is achieved externally by a resistor bridge tied to the Vref voltage reference. Its midpoint is tied to the positive input of the current control operational amplifier, and its foot is to be connected to lower potential point of the sense resistor, as shown in Figure 3. The resistors of this bridge are matched to provide the best precision possible. The control equation verifies that: Equation 2 Equation 2’ where Ilim is the desired limited current, and Vsense is the threshold voltage for the current control loop. Note that the Rsense resistor should be chosen taking into account the maximum dissipation (Plim) through it during full load operation. Equation 3 Therefore, for most adapter and battery charger applications, a quarterwatt, or halfwatt resistor to make the current sensing function is sufficient. The current sinking outputs of the two transconductance operational amplifiers are common (to the output of the IC). This makes an ORing function which ensures that whenever the current or the voltage reaches too high values, the optocoupler is activated. The relation between the controlled current and the controlled output voltage can be described with a square characteristic as shown in the following V/I outputpower graph. Fig. 3: Output voltage versus output current R 1 R 2 V ref V out V ref –  ⋅ = R sense I lim ⋅ V sense = V sense R 5 V ref R 4 R 5 +  ⋅ = I lim R 5 V ref ⋅ R 4 R 5 + () R sense ⋅  = P lim V se nse Ilim ⋅ = Vout Iout Voltage regulation TSM1011 Vcc : independent power supply 0 Secondary current regulation TSM1011 Vcc : On power output Primary current regulation 
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