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LM2738 Folha de dados(PDF) 11 Page - Texas Instruments |
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LM2738 Folha de dados(HTML) 11 Page - Texas Instruments |
11 / 38 page LM2738 BOOST SW GND L D1 D2 COUT CBOOST VOUT CIN VIN VIN VBOOST LM2738 www.ti.com SNVS556B – APRIL 2008 – REVISED APRIL 2013 BOOST FUNCTION Capacitor CBOOST and diode D2 in Figure 29 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 2.5V greater than VSW. It is recommended that VBOOST be greater than 2.5V above VSW for best efficiency. VBOOST – VSW should not exceed the maximum operating limit of 5.5V. 5.5V > VBOOST – VSW > 2.5V for best performance. When the LM2738 starts up, internal circuitry from the BOOST pin supplies a maximum of 20mA 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 0.76V. There are various methods to derive VBOOST: 1. From the input voltage (3.0V < VIN < 5.5V) 2. From the output voltage (2.5V < VOUT < 5.5V) 3. From an external distributed voltage rail (2.5V < VEXT < 5.5V) 4. From a shunt or series zener diode In the Simplifed Block Diagram of (Figure 27), capacitor CBOOST and diode D2 supply the gate-drive voltage 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 28), 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 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 29. 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. Figure 29. VOUT Charges CBOOST 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 30. 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 (VINMIN – VD3) > 2.5V Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2738 |
Nº de peça semelhante - LM2738_15 |
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Descrição semelhante - LM2738_15 |
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