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LM2734 Datasheet(PDF) 11 Page - Texas Instruments |
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LM2734 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 37 page  LM2734 BOOST SW GND L D1 D2 COUT CBOOST VOUT CIN VIN VIN VBOOST LM2734 www.ti.com SNVS288J – SEPTEMBER 2004 – REVISED DECEMBER 2014 8 Application and Implementation NOTE Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality. 8.1 Application Information 8.1.1 Boost Function Capacitor CBOOST and diode D2 in Figure 13 are used to generate a voltage VBOOST. VBOOST - VSW is the gate drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time, VBOOST needs to be at least 1.6 V greater than VSW. Although the LM2734 device will operate with this minimum voltage, it may not have sufficient gate drive to supply large values of output current. Therefore, it is recommended that VBOOST be greater than 2.5 V above VSW for best efficiency. VBOOST – VSW should not exceed the maximum operating limit of 5.5 V. 5.5 V > VBOOST – VSW > 2.5 V for best performance. Figure 13. VOUT Charges CBOOST When the LM2734 device starts up, internal circuitry from the BOOST pin supplies a maximum of 20 mA to CBOOST. This current charges CBOOST to a voltage sufficient to turn the switch on. The BOOST pin will continue to source current to CBOOST until the voltage at the feedback pin is greater than 0.76 V. There are various methods to derive VBOOST: 1. From the input voltage (VIN) 2. From the output voltage (VOUT) 3. From an external distributed voltage rail (VEXT) 4. From a shunt or series Zener diode In the simplified block diagram of Functional Block Diagram, capacitor CBOOST and diode D2 supply the gate- drive current for the NMOS switch. Capacitor CBOOST is charged via diode D2 by VIN. During a normal switching cycle, when the internal NMOS control switch is off (TOFF) (refer to Figure 12), VBOOST equals VIN minus the forward voltage of D2 (VFD2), during which the current in the inductor (L) forward biases the Schottky diode D1 (VFD1). Therefore the voltage stored across CBOOST is: VBOOST - VSW = VIN - VFD2 + VFD1 (1) When the NMOS switch turns on (TON), the switch pin rises to: VSW = VIN – (RDSON × IL), (2) forcing VBOOST to rise thus reverse biasing D2. The voltage at VBOOST is then: VBOOST = 2VIN – (RDSON × IL) – VFD2 + VFD1 (3) which is approximately: 2VIN - 0.4 V (4) Copyright © 2004–2014, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2734 |
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