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CS51412EMNR2G Datasheet(PDF) 11 Page - ON Semiconductor |
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CS51412EMNR2G Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 20 page CS51411, CS51412, CS51413, CS51414 http://onsemi.com 11 The Soft−Start duration can be calculated by TSS + VC CCOMP ISOURCE where: VC = VC pin steady−state voltage, which is approximately equal to error amplifier’s reference voltage. CCOMP = Compensation capacitor connected to the VC pin ISOURCE = Output Source Current of the error amplifier. Using a 0.1 mF CCOMP, the calculation shows a TSS over 5.0 ms which is adequate to avoid any current stresses. Figure 15 shows the gradual rise of the VC, VO and envelope of the VSW during power up. There is no voltage overshoot after the output voltage reaches the regulation. If the supply voltage rises slower than the VC pin, output voltage may overshoot. Figure 15. The Power Up Transition of CS5141X Regulator Short Circuit When the VFB pin voltage drops below Foldback Threshold, the regulator reduces the peak current limit by 40% and switching frequency to 1/4 of the nominal frequency. These features are designed to protect the IC and external components during overload or short circuit conditions. In those conditions, peak switching current is clamped to the current limit threshold. The reduced switching frequency significantly increases the ripple current, and thus lowers the DC current. The short circuit can cause the minimum duty cycle to be limited by Minimum Output Pulse Width. The foldback frequency reduces the minimum duty cycle by extending the switching cycle. This protects the IC from overheating, and also limits the power that can be transferred to the output. The current limit foldback effectively reduces the current stress on the inductor and diode. When the output is shorted, the DC current of the inductor and diode can approach the current limit threshold. Therefore, reducing the current limit by 40% can result in an equal percentage drop of the inductor and diode current. The short circuit waveforms are captured in Figure 16, and the benefit of the foldback frequency and current limit is self−evident. Figure 16. In Short Circuit, the Foldback Current and Foldback Frequency Limit the Switching Current to Protect the IC, Inductor and Catch Diode Thermal Considerations A calculation of the power dissipation of the IC is always necessary prior to the adoption of the regulator. The current drawn by the IC includes quiescent current, predriver current, and power switch base current. The quiescent current drives the low power circuits in the IC, which include comparators, error amplifier and other logic blocks. Therefore, this current is independent of the switching current and generates power equal to WQ + VIN IQ where: IQ = quiescent current. The predriver current is used to turn on/off the power switch and is approximately equal to 12 mA in worst case. During steady state operation, the IC draws this current from the Boost pin when the power switch is on and then receives it from the VIN pin when the switch is off. The predriver current always returns to the VSW pin. Since the predriver current goes out to the regulator’s output even when the power switch is turned off, a minimum load is required to prevent overvoltage in light load conditions. If the Boost pin voltage is equal to VIN + VO when the switch is on, the power dissipation due to predriver current can be calculated by WDRV + 12 mA (VIN * VO ) VO2 VIN ) The base current of a bipolar transistor is equal to collector current divided by beta of the device. Beta of 60 is used here to estimate the base current. The Boost pin provides the base current when the transistor needs to be on. |
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