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LM2734YMK bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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LM2734YMK bảng dữ liệu(HTML) 11 Page - Texas Instruments |
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11 / 31 page  LM2734 VIN BOOST SW GND L D1 D2 D3 R3 C4 VBOOST CBOOST VZ V IN C IN V OUT C OUT LM2734 VIN BOOST SW GND CBOOST L D1 D2 D3 CIN VIN COUT VOUT VBOOST LM2734 www.ti.com SNVS288I – SEPTEMBER 2004 – REVISED APRIIL 2013 Figure 25. Zener Reduces Boost Voltage from VIN An alternative method is to place the zener diode D3 in a shunt configuration as shown in Figure 26. A small 350mW to 500mW 5.1V zener in a SOT or SOD package can be used for this purpose. A small ceramic capacitor such as a 6.3V, 0.1µF capacitor (C4) should be placed in parallel with the zener diode. When the internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The 0.1 µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time. Resistor R3 should 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 the following formula for the X version: IBOOST = 0.56 x (D + 0.54) x (VZENER – VD2) mA (6) IBOOST can be calculated for the Y version using the following: IBOOST = 0.22 x (D + 0.54) x (VZENER - VD2) µA (7) where D is the duty cycle, VZENER and VD2 are in volts, and IBOOST is in milliamps. VZENER is the voltage applied to the anode of the boost diode (D2), and VD2 is the average forward voltage across D2. Note that this formula for IBOOST gives typical current. For the worst case IBOOST, increase the current by 40%. In that case, the worst case boost current will be IBOOST-MAX = 1.4 x IBOOST (8) R3 will then be given by R3 = (VIN - VZENER) / (1.4 x IBOOST + IZENER) (9) For example, using the X-version let VIN = 10V, VZENER = 5V, VD2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%. Then IBOOST = 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA (10) R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11k Ω (11) Figure 26. Boost Voltage Supplied from the Shunt Zener on VIN Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2734 |
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