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CS51413G Datasheet(PDF) 9 Page - ON Semiconductor |
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CS51413G Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 16 page CS51411, CS51412, CS51413, CS51414 http://onsemi.com 9 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 9, and the benefit of the foldback frequency and current limit is self–evident. Figure 9. 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, pre–driver 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 pre–driver 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 pre–driver current always returns to the VSW pin. Since the pre–driver 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 pre–driver 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. The power dissipated by the IC due to this current is WBASE + VO2 VIN IS 60 where: IS = DC switching current. When the power switch turns on, the saturation voltage and conduction current contribute to the power loss of a non–ideal switch. The power loss can be quantified as WSAT + VO VIN IS VSAT where: VSAT = saturation voltage of the power switch which is shown in Figure 5. The switching loss occurs when the switch experiences both high current and voltage during each switch transition. This regulator has a 30 ns turn–off time and associated power loss is equal to WS + IS VIN 2 20 ns fS The turn–on time is much shorter and thus turn–on loss is not considered here. The total power dissipated by the IC is sum of all the above WIC + WQ ) WDRV ) WBASE ) WSAT ) WS The IC junction temperature can be calculated from the ambient temperature, IC power dissipation and thermal resistance of the package. The equation is shown as follows, TJ + WIC R qJA ) TA The maximum IC junction temperature shall not exceed 125 °C to guarantee proper operation and avoid any damages to the IC. Minimum Load Requirement As pointed out in the previous section, a minimum load is required for this regulator due to the pre–driver current feeding the output. Placing a resistor equal to VO divided by 12 mA should prevent any voltage overshoot at light load conditions. Alternatively, the feedback resistors can be valued properly to consume 12 mA current. COMPONENT SELECTION Input Capacitor In a buck converter, the input capacitor witnesses pulsed current with an amplitude equal to the load current. This pulsed current and the ESR of the input capacitors determine the VIN ripple voltage, which is shown in Figure 10. For VIN ripple, low ESR is a critical requirement for the input capacitor selection. The pulsed input current possesses a significant AC component, which is absorbed by the input capacitors. The RMS current of the input capacitor can be calculated using: |
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