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IC-WDADFN10 Datasheet(PDF) 6 Page - IC-Haus GmbH |
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IC-WDADFN10 Datasheet(HTML) 6 Page - IC-Haus GmbH |
6 / 12 page iC-WD A/B/C SWITCHED-MODE DUAL VOLTAGE REGULATOR Rev D1, Page 6/12 DESCRIPTION OF FUNCTIONS Fig. 1 illustrates the operating principle of the switching converter in simplified form. When the switch S closes in steady-state condition, a linearly increasing charg- ing current for the capacitor CVH flows through the coil LVH in addition to the load current in RL. The energy from the supply VB is stored in the coil’s magnetic field. When the switch opens, the current flows via the diode through the coil; its energy content is supplied to ca- pacitor and load. VH VB Vsat V(LVH) VD S LVH CVH RL Figure 1: Principle of operation The block diagram on page 1 shows the iC-WD with typical wiring. The internally generated clock pulse closes the switch between VBR and VHL and the cur- rent in the coil rises (charging phase). A control vari- able, ∆VR in accordance with the regulating charac- teristic in Fig. 2, is obtained from the voltage VH and the internal reference voltage and is compared to the voltage at shunt RVB. When the cut-off current Ioff = ∆ VR/RVB is reached, the switch opens and the coil current runs free via the integrated power diode (dis- charge phase). When the next clock signal occurs, this charging and discharging process is repeated. Fig. 6 shows the resulting current and voltage characteristics. The current rise (tr ) and fall times (tf ) depend on the voltage VH at the inductor. The following approxima- tion applies: tr = LVH Ioff VB − Vsat − VH tf = LVH Ioff VH + VD (1) Vsat = VB − VHL: Saturation voltage of the switching transistor plus voltage drop at RVB VD: Forward voltage of the free-wheeling diode The current dependencies of the saturation and diode forward voltage (Fig. 3 and 4) are ignored here, as are the losses due to the internal resistance of the coil. The regulator operates at a constant frequency under load. To prevent VH from rising without load, the os- cillator frequency is reduced as the level of voltage VH rises (Fig. 5). 5.5V 6.0V 6.5V 7.0V 7.5V VH 500mV 400mV 300mV 200mV 100mV 0V typ. Figure 2: Regulating characteristic ∆VR = f (VH) 0A 100mA 200mA 300mA 400mA 500mA I(VHL) 1.5V 1.0V 0.5V 0V typ. max. Figure 3: Saturation voltage of switching transistor 0A 100mA 200mA 300mA 400mA 500mA I(VHL) 1.5V 1.0V 0.5V 0V max. typ. Figure 4: Forward voltage of free-wheeling diode |
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