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LM27342-Q1 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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LM27342-Q1 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
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11 / 44 page  0 0 VIN -VD1 tON t t D = tON/TSW VSW tOFF TSW iL 'iL IOUT ILPK LM27341, LM27342, LM27341-Q1, LM27342-Q1 www.ti.com SNVS497E – NOVEMBER 2008 – REVISED APRIL 2013 Figure 32. LM27341/LM27342 Waveforms of SW Pin Voltage and Inductor Current BOOST FUNCTION Capacitor C2 in Figure 31, commonly referred to as CBOOST, is used to store a voltage VBOOST. When the LM27341/LM27342 starts up, an internal LDO charges CBOOST ,via an internal diode, to a voltage sufficient to turn the internal NMOS switch on. The gate drive voltage supplied to the internal NMOS switch is VBOOST - VSW. During a normal switching cycle, when the internal NMOS control switch is off (tOFF) (refer to Figure 32), VBOOST equals VLDO minus the forward voltage of the internal diode (VD2). At the same time the inductor current (iL) forward biases the catch diode D1 forcing the SW pin to swing below ground by the forward voltage drop of the catch diode (VD1). Therefore, the voltage stored across CBOOST is VBOOST - VSW = VLDO - VD2 + VD1 (1) Thus, VBOOST = VSW + VLDO - VD2 + VD1 (2) When the NMOS switch turns on (tON), the switch pin rises to VSW = VIN – (RDSON x IL), (3) reverse biasing D1, and forcing VBOOST to rise. The voltage at VBOOST is then VBOOST = VIN – (RDSON x IL) + VLDO – VD2 + VD1 (4) which is approximately VIN + VLDO- 0.4V (5) VBOOST has pulled itself up by its "bootstraps", or boosted to a higher voltage. LOW INPUT VOLTAGE CONSIDERATIONS When the input voltage is below 5V and the duty cycle is greater than 75 percent, the gate drive voltage developed across CBOOST might not be sufficient for proper operation of the NMOS switch. In this case, CBOOST should be charged via an external Schottky diode attached to a 5V voltage rail, see Figure 33. This ensures that the gate drive voltage is high enough for proper operation of the NMOS switch in the triode region. Maintain VBOOST - VSW less than the 6V absolute maximum rating. Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM27341 LM27342 LM27341-Q1 LM27342-Q1 |
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