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595D685X9016A2T Datasheet(PDF) 4 Page - ON Semiconductor |
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595D685X9016A2T Datasheet(HTML) 4 Page - ON Semiconductor |
4 / 10 page AND8098/D http://onsemi.com 4 In non-isolated topologies such as buck or buck-boost, the circuits are mainly designed for CCM. The CCM burst-mode waveform is different to the PWM waveform in Figure 5. Because of this characteristic, burst mode requires a higher peak value of the inductor current in order to have the same level of averaged inductor current (or output current). Figure 5. CCM Inductor Currents in Burst Mode and traditional PWM Control Burst mode PWM As shown in Figure 4 and 5 burst-mode control produces low-frequency waveform comparing to the switching frequency. Part of the power loss in this low frequency becomes audible noise. Therefore, burst-mode control is not suitable for high power applications such as more than 20 W. VCC Capacitor The VCC capacitor C2 is the key component to make the circuit operate in normal mode or fault mode. The device recognizes a fault condition when there is no feedback current in the FB pin during the time from VCC = 8.5 V to 7.5 V. The VCC capacitor directly affects this time duration. In normal mode, the VCC follows a 8.5 V-7.5 V-8.5 V hysteresis loop. When the circuit is in fault mode, the VCC follows a 8.5 V-7.5 V-4.5 V-8.5 V hysteresis loop. The device keeps its MOSFET opened except for the time from VCC = 8.5 V to 7.5 V and delivers a little amount of power to the output in fault mode. A common and extreme case to enter fault condition is the startup. The MOSFET begins switching at the VCC is firstly charged to 8.5 V and hence output voltage rises. The output voltage needs some time to build up the output voltage from 0 V to a desired value. When the desired level is reached, a feedback current flows into the device to stop its switching. If the feedback current is determined before VCC reaches 7.5V, the circuit will remain in normal mode. Otherwise, the circuit will enter the fault mode and cannot provide the output voltage at its desired level. Therefore, the VCC capacitor is needed to be big enough to ensure sufficient time for VCC going from 8.5 V to 7.5 V to sample feedback current in startup. Figure 6. Startup Scenarios of the Circuits with Big Enough or Too Small VCC Capacitor Output waveforms with big enough VCC capacitor time FB current VCC Vout Output waveforms with too small VCC capacitor time VCC Vout Desired level of Vout Practically, the NCP1052 consumes approximately 0.5 mA in normal operation. The concerned fault sampling time for feedback signal is from 8.5 V to 7.5V. Hence, C + I dt dV + 0.5 10-3 1 · sampling time (eq. 4) + 0.5 10-3 · sampling time For example, if sampling time or startup transient is designed to be 20 ms, 10 µF VCC capacitor is needed. Inductor The 300 mA current limit in the NCP1052 is measured with a condition that the di/dt reaches 300 mA in 4 µs. When the buck or buck-boost circuit is designed for universal ac input voltage (85 to 265 Vac), the rectified input voltage will be possibly as high as 375 Vdc. In order to keep the 4 µs condition, the inductance value will be 5 mH by (5) and (6). For buck, di dt + Vin * Vout L [ Vin L (eq. 5) For buck-boost, di dt + Vin L (eq. 6) The 5 mH is practically too high and hence not very practical. Therefore, the inductor is basically selected by market available inductor models which is with a normally smaller inductance (but not too small). It must have enough saturation current level (>300 mA). If inductance is too |
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