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NCV47721 Scheda tecnica(PDF) 11 Page - ON Semiconductor |
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NCV47721 Scheda tecnica(HTML) 11 Page - ON Semiconductor |
11 / 16 page NCV47721 www.onsemi.com 11 APPLICATIONS INFORMATION Circuit Description The NCV47721 is an integrated low dropout regulator that provides a regulated voltage at 200 mA to output. It is enabled with an input to the enable pin. The regulator voltage is provided by a PNP pass transistor controlled by an error amplifier with a bandgap reference, which gives it the lowest possible dropout voltage. The output current capability of the LDO is 200 mA and the base drive quiescent current is controlled to prevent oversaturation when the input voltage is low or when the output is overloaded. The integrated current sense feature provides diagnosis and system protection functionality. The current limit of the device is adjustable by resistor connected to CSO pin. Voltage on CSO pin is proportional to output current. The regulator is protected by both current limit and thermal shutdown. Thermal shutdown occurs above 150 °C to protect the IC during overloads and extreme ambient temperatures. Regulator The error amplifier compares the reference voltage to a sample of the output voltage (Vout) and drives the base of a PNP series pass transistor via a buffer. The reference is a bandgap design to give it a temperature stable output. Saturation control of the PNP is a function of the load current and input voltage. Oversaturation of the output power device is prevented, and quiescent current in the ground pin is minimized. Regulator Stability Considerations The input capacitor (Cin) is necessary to stabilize the input impedance to avoid voltage line influences. The output capacitor (Cout) helps determine three main characteristics of a linear regulator: startup delay, load transient response and loop stability. The capacitor value and type should be based on cost, availability, size and temperature constraints. 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 manufacturer’s data sheet usually provides this information. The value for the output capacitor Cout, shown in Figure 1 should work for most applications; see also Figure 16 for output stability at various load and Output Capacitor ESR conditions. Stable region of ESR in Figure 16 shows ESR values at which the LDO output voltage does not have any permanent oscillations at any dynamic changes of output load current. Marginal ESR is the value at which the output voltage waving is fully damped during four periods after the load change and no oscillation is further observable. ESR characteristics were measured with ceramic capacitors and additional series resistors to emulate ESR. Low duty cycle pulse load current technique has been used to maintain junction temperature close to ambient temperature. Calculating Bypass Capacitor If improved stability (reducing output voltage ringing during transients) is demanded, connect the bypass capacitor Cb between Adjustable Input pin and Vout pin according to Applications circuit at Figure 1. Parallel combination of bypass capacitor Cb with the feedback resistor R1 contributes in the device transfer function as an additional zero and affects the device loop stability, therefore its value must be optimized. Attention to the Output Capacitor value and its ESR must be paid. See also Stability in High Speed Linear LDO Regulators Application Note, AND8037/D for more information. Optimal value of bypass capacitor is given by following expression Cn + 1 2 p fz R 1 (F) (eq. 1) where R1 the upper feedback resistor fz the frequency of the zero added into the device transfer function by R1 and Cb external components. Set the R1 resistor according to output voltage requirement. Choose the fz with regard on the output capacitance Cout, refer to the table below. Cout (mF) 10 22 47 100 fZ range (kHz) max 24 max 37 N/A* N/A* NOTE: * For Cout = 47 mF and higher, Cb capacitors are not needed for stability improvement. Cb capacitors are useful for noise reduction. See electrical characteristic table. Ceramic capacitors and their part numbers listed below have been used as low ESR output capacitors Cout from the table above to define the frequency ranges of additional zero required for stability: GRM31CR71C106KAC7 (10 mF, 16 V, X7R, 1206) GRM32ER71C226KE18 (22 mF, 16 V, X7R, 1210) GRM32ER61C476ME15 (47 mF, 16 V, X5R, 1210) GRM32ER60J107ME20 (100 mF, 6.3 V, X5R, 1210) Enable Input The enable pin is used to turn the regulator on or off. By holding the pin down to a voltage less than 0.99 V, the output of the regulator will be turned off. When the voltage on the enable pin is greater than 2.31 V, the output of the regulator will be enabled to power its output to the regulated output voltage. The enable pin may be connected directly to the input pin to give constant enable to the output regulator. Setting the Output Voltage The output voltage range can be set between 3.3 V and 20 V. This is accomplished with an external resistor divider feeding back the voltage to the IC back to the error amplifier by the voltage adjust pin ADJ. The internal reference voltage |
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