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TC2575-3.3VAT Datasheet(PDF) 6 Page - TelCom Semiconductor, Inc |
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TC2575-3.3VAT Datasheet(HTML) 6 Page - TelCom Semiconductor, Inc |
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6 / 25 page  TC2575 1.0A Step-Down Switching Regulator 6 TC2575-1 3/13/00 PCB LAYOUT GUIDELINES As in any switching regulator, the layout of the printed circuit board is very important. Rapidly switching currents associated with wiring inductance, stray capacitance and parasitic inductance of the printed circuit board traces can generate voltage transients which can generate electro- magnetic interferences (EMI) and affect the desired opera- tion. As indicated in the Figure 2, to minimize inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible. For best results, single–point grounding (as indicated) or ground plane construction should be used. On the other hand, the PCB area connected to the Pin 2 (emitter of the internal switch) of the TC2575 should be kept to a minimum in order to minimize coupling to sensitive circuitry. Another sensitive part of the circuit is the feedback. It is important to keep the sensitive feedback wiring short. To assure this, physically locate the programming resistors near to the regulator, when using the adjustable version of the TC2575 regulator. DESIGN PROCEDURE Buck Converter Basics The TC2575 is a “Buck” or Step–Down Converter which is the most elementary forward–mode converter. Its basic schematic can be seen in Figure 3. The operation of this regulator topology has two distinct time periods. The first one occurs when the series switch is on, the input voltage is connected to the input of the inductor. The output of the inductor is the output voltage, and the rectifier (or catch diode) is reverse biased. During this period, since there is a constant voltage source connected across the inductor, the inductor current begins to linearly ramp upwards, as described by the following equation: IL (ON) = (VIN – VOUT ) tON L During this “on” period, energy is stored within the core material in the form of magnetic flux. If the inductor is properly designed, there is sufficient energy stored to carry the requirements of the load during the “off” period. The next period is the “off” period of the power switch. When the power switch turns off, the voltage across the inductor reverses its polarity and is clamped at one diode voltage drop below ground by the catch diode. Current now flows through the catch diode thus maintaining the load current loop. This removes the stored energy from the inductor. The inductor current during this time is: IL (OFF) = (VOUT – VD ) tOFF L This period ends when the power switch is once again turned on. Regulation of the converter is accomplished by varying the duty cycle of the power switch. It is possible to describe the duty cycle as follows: d = tON , where T is the period of switching. T For the buck converter with ideal components, the duty cycle can also be described as: d = VOUT VIN Figure 4 shows the buck converter idealized waveforms of the catch diode voltage and the inductor current. Figure 3. Basic Buck Converter COUT+ VOUT RLOAD + – VIN D1 Power Switch L Figure 4. Buck Converter Idealized Waveforms Time VD/(FWD) Power Switch Off Power Switch Diode Power Switch Power Switch Off VON (SW) Power Switch On Power Switch On Time IMIN Diode ILOAD (AV) IPK |
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