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LM2734ZMKX bảng dữ liệu(PDF) 9 Page - Texas Instruments |
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LM2734ZMKX bảng dữ liệu(HTML) 9 Page - Texas Instruments |
9 / 36 page LM2734 VIN BOOST SW GND CBOOST L D1 D2 D3 CIN VIN COUT VOUT VBOOST LM2734Z, LM2734Z-Q1 www.ti.com SNVS334F – JANUARY 2005 – REVISED JANUARY 2016 Feature Description (continued) 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 Functional Block Diagram, capacitor CBOOST and diode D2 supply the gate-drive current for the NMOS switch. Capacitor CBOOST is charged through diode D2 by VIN. During a normal switching cycle, when the internal NMOS control switch is off (TOFF) (refer to Figure 7), 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 calculated using Equation 1. VBOOST –VSW = VIN – VFD2 + VFD1 (1) When the NMOS switch turns on (TON), the switch pin rises to: VSW = VIN – (RDSON x IL), (2) forcing VBOOST to rise thus reverse biasing D2. The voltage at VBOOST is then: VBOOST = 2 VIN – (RDSON x IL) – VFD2 + VFD1 (3) which is approximately: 2 VIN – 0.4 V (4) for many applications. Thus the gate-drive voltage of the NMOS switch is approximately: VIN –0.2 V (5) An alternate method for charging CBOOST is to connect D2 to the output as shown in Figure 8. The output voltage must be between 2.5 V and 5.5 V, so that proper gate voltage is 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 5.5 V, or less than 3 V, CBOOST cannot be charged directly from these voltages. If VIN and VOUT are greater than 5.5 V, 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 9. When using a series Zener diode from the input, ensure that the regulation of the input supply does not create a voltage that falls outside the recommended VBOOST voltage. (VINMAX – VD3) < 5.5V (6) (VINMIN – VD3) > 1.6V (7) Figure 9. Zener Reduces Boost Voltage from VIN An alternative method is to place the Zener diode D3 in a shunt configuration as shown in Figure 10. A small 350-mW to 500-mW, 5.1-V Zener in a SOT or SOD package can be used for this purpose. A small ceramic capacitor such as a 6.3-V, 0.1-µF capacitor (C4) must 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. Copyright © 2005–2016, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2734Z LM2734Z-Q1 |
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