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LM2736 Datasheet(PDF) 9 Page - Texas Instruments |
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LM2736 Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 27 page  BOOST SW GND L D1 D2 COUT CBOOST VOUT CIN VIN VIN VBOOST LM2736 www.ti.com SNVS316E – SEPTEMBER 2004 – REVISED SEPTEMBER 2011 Figure 5. VOUT Charges CBOOST When the LM2736 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 1.18V. 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 Simplifed Block Diagram of Figure 3, 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 4), 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 5. 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 6. 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) > 1.6V Copyright © 2004–2011, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2736 |
Similar Part No. - LM2736_14 |
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Similar Description - LM2736_14 |
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