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CS51413G データシート(PDF) 7 Page - ON Semiconductor |
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CS51413G データシート(HTML) 7 Page - ON Semiconductor |
7 / 16 page CS51411, CS51412, CS51413, CS51414 http://onsemi.com 7 frequency foldback feature is disabled during the sync mode. Figure 4. A CS51411 Buck Regulator is Synced by an External 350 kHz Pulse Signal Power Switch and Current Limit The collector of the built–in NPN power switch is connected to the VIN pin, and the emitter to the VSW pin. When the switch turns on, the VSW voltage is equal to the VIN minus switch Saturation Voltage. In the buck regulator, the VSW voltage swings to one diode drop below ground when the power switch turns off, and the inductor current is commutated to the catch diode. Due to the presence of high pulsed current, the traces connecting the VSW pin, inductor and diode should be kept as short as possible to minimize the noise and radiation. For the same reason, the input capacitor should be placed close to the VIN pin and the anode of the diode. The saturation voltage of the power switch is dependent on the switching current, as shown in Figure 5. Figure 5. The Saturation Voltage of the Power Switch Increases with the Conducting Current 0 0.5 1.0 1.5 Switching Current (A) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Members of the CS5141X family contain pulse–by–pulse current limiting to protect the power switch and external components. When the peak of the switching current reaches the Current Limit, the power switch turns off after the Current Limit Delay. The switch will not turn on until the next switching cycle. The current limit threshold is independent of switching duty cycle. The maximum load current, given by the following formula under continuous conduction mode, is less than the Current Limit due to the ripple current. IO(MAX) + ILIM * VO(VIN * VO) 2(L)(VIN)(fs) where: fS = switching frequency, ILIM = current limit threshold, VO = output voltage, VIN = input voltage, L = inductor value. When the regulator runs under current limit, the subharmonic oscillation may cause low frequency oscillation, as shown in Figure 6. Similar to current mode control, this oscillation occurs at the duty cycle greater than 50% and can be alleviated by using a larger inductor value. The current limit threshold is reduced to Foldback Current when the FB pin falls below Foldback Threshold. This feature protects the IC and external components under the power up or over–load conditions. Figure 6. The Regulator in Current Limit BOOST Pin The BOOST pin provides base driving current for the power switch. A voltage higher than VIN provides required headroom to turn on the power switch. This in turn reduces IC power dissipation and improves overall system efficiency. The BOOST pin can be connected to an external boost–strapping circuit which typically uses a 0.1 µF capacitor and a 1N914 or 1N4148 diode, as shown in Figure 1. When the power switch is turned on, the voltage on the BOOST pin is equal to VBOOST + VIN ) VO * VF where: |
同様の部品番号 - CS51413G |
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同様の説明 - CS51413G |
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