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MBR0540 Datasheet(PDF) 8 Page - ON Semiconductor |
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MBR0540 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 13 page NCP1546 http://onsemi.com 8 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 mF 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: VF = diode forward voltage. The anode of the diode can be connected to any DC voltage as well as the regulated output voltage (Figure 1). However, the maximum voltage on the BOOST pin shall not exceed 40 V. As shown in Figure 7, the BOOST pin current includes a constant 7.0 mA pre−driver current and base current proportional to switch conducting current. A detailed discussion of this current is conducted in Thermal Consideration section. A 0.1 mF capacitor is usually adequate for maintaining the Boost pin voltage during the on time. Figure 7. The Boost Pin Current Includes 7.0 mA Pre−Driver Current and Base Current when the Switch is Turned On. The Beta Decline of the Power Switch Further Increases the Base Current at High Switching Current 0 0.5 1.0 1.5 SWITCHING CURRENT (A) 0 5 10 15 20 25 30 Shutdown The internal power switch will not turn on until the VIN pin rises above the Startup Voltage. This ensures no switching will occur until adequate supply voltage is provided to the IC. The IC enters a sleep mode when the SHDNB pin is pulled below the Shutdown Threshold Voltage. In sleep mode, the power switch is kept open and the supply current reduces to Shutdown Quiescent Current ( 1 mA typically). This pin has an internal pull−down current. When not in use, pull this pin up to VCC with a resistor (See Figure 1). Startup During power up, the regulator tends to quickly charge up the output capacitors to reach voltage regulation. This gives rise to an excessive in−rush current which can be detrimental to the inductor, IC and catch diode. In V2 control , the compensation capacitor provides Soft−Start with no need for extra pin or circuitry. During the power up, the Output Source Current of the error amplifier charges the compensation capacitor which forces VC pin and thus output voltage ramp up gradually. 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 8 shows the gradual rise of the VC, VO and envelope of the VSW during power up. There is no voltage over−shoot after the output voltage reaches the regulation. If the supply voltage rises slower than the VC pin, output voltage may over−shoot. Figure 8. The Power Up Transition of NCP1546 Regulator |
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