Datasheet TSM1012 (STMicroelectronics) - 7

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TSM1012 Principle of operation and application hints 6 Principle of operation and application hints 6.1 Voltage and current control 6.1.1 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 relation between the values of the R1 and R2 should be chosen as written in Equation 1.
Equation 1
R1 = R2 x Vref / (Vout - Vref) 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 100 K (or more) would be appropriate for the resistors R1 and R2. As an example, with R2 = 100 K, Vout = 4.10 V, Vref = 1.210 V, then R1 = 41.9 K. Note: If the low drop diode should 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 Equation 1 by replacing Vout by (Vout + Vdrop).
6.1.2 Current control
The current loop is controlled via the second transconductance operational amplifier, the sense resistor Rsense, and the optocoupler. The Vsense threshold is achieved externally by a resistor bridge tied to the Vref voltage reference. Its middle point is tied to the positive input of the current control operational amplifier, and its foot is to be connected to the lower potential point of the sense resistor as shown in Figure 4. The resistors of this bridge are matched to provide the best precision possible. The control equation verifies:
Equation 2
Rsense x Ilim = Vsense Vsense = R5 x Vref / (R4 + R5)
Equation 3
Ilim = R5 x Vref / (R4 + R5) x Rsense 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 the full load operation. DocID10124 Rev 2 7/14 14 Document Outline Figure 1. Pin connections (top view) Table 1. Order codes 1 Pin descriptions Table 2. SO-8 pinout 2 Absolute maximum ratings Table 3. Absolute maximum ratings 3 Operating conditions Table 4. Operating conditions 4 Electrical characteristics Table 5. Electrical characteristics 5 Internal schematics Figure 2. Internal schematic Figure 3. Typical adapter or battery charger application using TSM1012 6 Principle of operation and application hints 6.1 Voltage and current control 6.1.1 Voltage control 6.1.2 Current control Figure 4. Output voltage versus output current 6.2 Compensation 6.3 Start-up and short-circuit conditions 6.4 Voltage clamp Figure 5. Clamp voltage Figure 6. Typical application schematic 7 Package information 7.1 SO-8 package information Figure 7. SO-8 package outline Table 6. SO-8 package mechanical data 8 Revision history Table 7. Document revision history