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LM2734Z Datasheet(PDF) 8 Page - Texas Instruments |
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LM2734Z Datasheet(HTML) 8 Page - Texas Instruments |
8 / 29 page LM2734 VIN BOOST SW GND CBOOST L D1 D2 D3 CIN VIN COUT VOUT VBOOST LM2734Z SNVS334D – JANUARY 2005 – REVISED JUNE 2008 www.ti.com VSW = VIN – (RDSON x IL), (2) forcing VBOOST to rise thus reverse biasing D2. The voltage at VBOOST is then VBOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1 (3) which is approximately 2VIN - 0.4V (4) for many applications. Thus the gate-drive voltage of the NMOS switch is approximately VIN - 0.2V (5) An alternate method for charging CBOOST is to connect D2 to the output as shown in Figure 4. The output voltage should be between 2.5V and 5.5V, so that proper gate voltage will be applied to the internal switch. In this circuit, CBOOST provides a gate drive voltage that is slightly less than VOUT. In applications where both VIN and VOUT are greater than 5.5V, or less than 3V, CBOOST cannot be charged directly from these voltages. If VIN and VOUT are greater than 5.5V, CBOOST can be charged from VIN or VOUT minus a zener voltage by placing a zener diode D3 in series with D2, as shown in Figure 5. When using a series zener diode from the input, ensure that the regulation of the input supply doesn’t create a voltage that falls outside the recommended VBOOST voltage. (VINMAX – VD3) < 5.5V (6) (VINMIN – VD3) > 1.6V (7) Figure 5. Zener Reduces Boost Voltage from VIN An alternative method is to place the zener diode D3 in a shunt configuration as shown in Figure 6. A small 350mW to 500mW 5.1V zener in a SOT-23 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: IBOOST = (D + 0.5) x (VZENER – VD2) mA (8) 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 25%. In that case, the worst case boost current will be IBOOST-MAX = 1.25 x IBOOST (9) R3 will then be given by R3 = (VIN - VZENER) / (1.25 x IBOOST + IZENER) (10) For example, let VIN = 10V, VZENER = 5V, VD2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%. Then IBOOST = (0.5 + 0.5) x (5 - 0.7) mA = 4.3mA (11) R3 = (10V - 5V) / (1.25 x 4.3mA + 1mA) = 787 Ω (12) 8 Submit Documentation Feedback Copyright © 2005–2008, Texas Instruments Incorporated Product Folder Links: LM2734Z |
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