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LM2738 数据表(PDF) 11 Page - Texas Instruments |
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LM2738 数据表(HTML) 11 Page - Texas Instruments |
11 / 40 page L R1 R2 D1 D2 BOOST Output Control Logic Current Limit Thermal Shutdown Under Voltage Lockout Corrective Ramp Reset Pulse PWM Comparator Current-Sense Amplifier RSENSE + + Internal Regulator and Enable Circuit Oscillator Driver 0.25: Switch Internal Compensation SW EN FB GND Error Amplifier - + VREF 0.8V COUT ON OFF VBOOST IL VSW + - CBOOST VOUT CIN VIN VIN ISENSE + - + - 0.93V - + OVP Comparator Error Signal 11 LM2738 www.ti.com SNVS556C – APRIL 2008 – REVISED JANUARY 2016 Product Folder Links: LM2738 Submit Documentation Feedback Copyright © 2008–2016, Texas Instruments Incorporated 7.2 Functional Block Diagram 7.3 Feature Description 7.3.1 Boost Function Capacitor CBOOST and diode D2 in Figure 26 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 must be at least 2.5 V greater than VSW. TI recommends that VBOOST be greater than 2.5 V above VSW for best efficiency. VBOOST – VSW must not exceed the maximum operating limit of 5.5 V. For best performance, see Equation 1. 5.5 V > VBOOST – VSW > 2.5 V (1) When the LM2738 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 continues 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 (3 V < VIN < 5.5 V) 2. From the output voltage (2.5 V < VOUT < 5.5 V) 3. From an external distributed voltage rail (2.5 V < VEXT < 5.5 V) 4. From a shunt or series Zener diode As seen on the Functional Block Diagram, 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 25), 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 Equation 2: VBOOST – VSW = VIN – VFD2 + VFD1 (2) When the NMOS switch turns on (TON), the switch pin rises to Equation 3: VSW = VIN – (RDSON × IL), (3) forcing VBOOST to rise, thus reverse biasing D2. The voltage at VBOOST is then Equation 4: VBOOST = 2 VIN – (RDSON × IL) – VFD2 + VFD1 (4) which is approximately 2 VIN – 0.4 V for many applications. Thus the gate-drive voltage of the NMOS switch is approximately VIN – 0.2 V. |
类似零件编号 - LM2738_16 |
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类似说明 - LM2738_16 |
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