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LM2734ZQMKE bảng dữ liệu(PDF) 10 Page - Texas Instruments |
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10 / 36 page LM2734 VIN BOOST SW GND L D1 D2 D3 R3 C4 VBOOST CBOOST VZ V IN C IN V OUT C OUT LM2734Z, LM2734Z-Q1 SNVS334F – JANUARY 2005 – REVISED JANUARY 2016 www.ti.com Feature Description (continued) Resistor R3 must be chosen to provide enough RMS current to the Zener diode (D3) and to the BOOST pin. A recommended choice for the Zener current (IZENER) is 1 mA. The current IBOOST into the BOOST pin supplies the gate current of the NMOS control switch and varies typically according to Equation 8. IBOOST = (D + 0.5) × (VZENER – VD2) mA where • D is the duty cycle • VZENER and VD2 are in volts • IBOOST is in milliamps • VZENER is the voltage applied to the anode of the boost diode (D2) • VD2 is the average forward voltage across D2 (8) NOTE Equation 8 for IBOOST gives typical current. For the worst case IBOOST, increase the current by 25%. In that case, the worse-case boost current is: IBOOST-MAX = 1.25 × IBOOST (9) R3 is then given by Equation 10. R3 = (VIN - VZENER) / (1.25 × IBOOST + IZENER) (10) For example, let VIN = 10 V, VZENER = 5 V, VD2 = 0.7 V, IZENER = 1 mA, and duty cycle D = 50%. Then: IBOOST = (0.5 + 0.5) × (5 - 0.7) mA = 4.3 mA (11) R3 = (10 V - 5 V) / (1.25 × 4.3 mA + 1 mA) = 787 Ω (12) Figure 10. Boost Voltage Supplied from the Shunt Zener on VIN 7.3.3 Soft-Start This function forces VOUT to increase at a controlled rate during start-up. During soft-start, the reference voltage of the error amplifier ramps from 0 V to its nominal value of 0.8 V in approximately 200 µs. This forces the regulator output to ramp up in a more linear and controlled fashion, which helps reduce inrush current. 7.3.4 Output Overvoltage Protection The overvoltage comparator compares the FB pin voltage to a voltage that is 10% higher than the internal reference Vref. Once the FB pin voltage goes 10% above the internal reference, the internal NMOS control switch is turned off, which allows the output voltage to decrease toward regulation. 10 Submit Documentation Feedback Copyright © 2005–2016, Texas Instruments Incorporated Product Folder Links: LM2734Z LM2734Z-Q1 |
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