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CS8321YDPR3G 데이터시트(PDF) 3 Page - ON Semiconductor |
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CS8321YDPR3G 데이터시트(HTML) 3 Page - ON Semiconductor |
3 / 6 page CS8321 http://onsemi.com 3 CIRCUIT DESCRIPTION AND APPLICATION NOTES VOLTAGE REFERENCE AND OUTPUT CIRCUITRY The CS8321 is a series pass voltage regulator. It consists of an error amplifier, bandgap voltage reference, PNP pass transistor with antisaturation control, and current limit. As the voltage at the input, VIN, is increased (Figure 1), QN is forward biased via R. QN provides base drive for QP. As QP becomes forward biased, the output voltage, VOUT, begins to rise as QP’s output current charges the output capacitor. Once VOUT rises to a certain level, the error amplifier becomes biased and provides the appropriate amount of base current to QP. The error amplifier monitors the scaled output voltage via an internal voltage divider, R1 and R2, and compares it to the bandgap voltage reference. The error amplifier’s output is a current which is equal to the error amplifier’s differential input voltage times its transconductance. Therefore, the error amplifier varies the base drive current to QN, which provides bias to QP, based on the difference between the reference voltage and the scaled output voltage, VOUT. Antisaturation Protection An antisaturation control circuit has also been added to prevent the pass transistor from going into deep saturation, which would cause excessive power dissipation due to large bias currents lost to the substrate via a parasitic PNP transistor, as shown in Figure 2. Figure 2. The Parasitic PNP Transistor Which Is Part of the Pass Transistor (QP) Structure VOUT VIN QP QParasitic Substrate Current Limit Limit The output stage is protected against short circuit conditions. As shown in Figure 3, the output current will fold back when the faulted load is continually increased. This technique has been incorporated to limit the total power dissipation across the device during a short circuit condition, since the device does not contain overtemperature shutdown. STABILITY CONSIDERATIONS The output or compensation capacitor helps determine three main characteristics of a linear regulator: start−up delay, load transient response and loop stability. Figure 3. Typical Current Limit and Fold Back Waveform 0.0 0.0 Output Voltage 0.34257 0.30831 0.27405 0.23980 0.20554 0.17128 0.13703 0.10277 0.06851 0.03426 0.51 1.02 1.52 2.03 2.54 3.05 3.56 4.06 4.57 5.08 Curve will vary with temperature and process variation. The capacitor value and type should be based on cost, availability, size and temperature constraints. A tantalum or aluminum electrolytic capacitor is best, since a film or ceramic capacitor with almost zero ESR can cause instability. The aluminum electrolytic capacitor is the least expensive solution, but, if the circuit operates at low temperatures (−25 °C to −40°C), both the value and ESR of the capacitor will vary considerably. The capacitor manufacturers data sheet usually provides this information. The value for the output capacitor COUT shown in Figure 4 should work for most applications, however it is not necessarily the best solution. Figure 4. Test and Application Circuit Showing Output Compensation CS8321 VIN CIN* 0.1 mF VOUT VOUTSense† COUT** 0.1 mF *CIN required if regulator is located far from the power supply filter. **COUT required for stability. Capacitor must operate at minimum temperature expected. †Pin internally shorted to VOUT in 3−pin applications. To determine an acceptable value for COUT for a particular application, start with a tantalum capacitor of the |
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