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LM74670-Q1 bảng dữ liệu(PDF) 8 Page - Texas Instruments |
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LM74670-Q1 bảng dữ liệu(HTML) 8 Page - Texas Instruments |
8 / 26 page tT1t T0 Body Diode Voltage Drop VOUT FET is OFF FET is ON VGS 0 V LM74670-Q1 SNOSD08A – SEPTEMBER 2015 – REVISED OCTOBER 2015 www.ti.com Feature Description (continued) Figure 8. Output Voltage and VGSOperation at 1A Output Current 7.3.2 During T1 Once the voltage on the capacitor reaches a higher voltage level of 6.3V (typical), the charge pump is disabled and the MOSFET turns ON. The energy stored in the capacitor is used to provide the gate drive for the MOSFET (T1 in Figure 8). When the MOSFET is ON, it provides a low resistive path for the drain current to flow and minimizes the power dissipation associated with forward conduction. The power losses during the MOSFET ON state depend primarily on the RDSON of the selected MOSFET and load current. At time when the capacitor voltage reaches its lower threshold VcapL 5.15V (typical), the MOSFET gate turns OFF. The drain current ID will then begin to flow through the body diode of the MOSFET, causing the MOSFET body diode voltage drop to appear across Anode and Cathode pins. The charge pump circuitry is re-activated and begins charging the Vcap. The LM74670-Q1 operation keeps the MOSFET ON at approximately 98% duty cycle (typical) regardless of the external charge pump capacitor value. This is the key factor to minimizing the power losses. The forward voltage drop during this time is limited by the RDSON of the MOSFET. 7.3.3 Pin Operation 7.3.3.1 Anode and Cathode Pins The LM74670-Q1 Anode and Cathode pins are connected to the source and drain of the external MOSFET. The current into the Anode pin is 30 µA (typical). When power is initially applied, the load current flows through the body diode of the external MOSFET, the voltage across Anode and Cathode pins is equal to the forward diode drop . The minimum value of diode voltage drop required to enable the charge pump circuitry is 0.48V. Once the MOSFET is turned ON, the Anode and Cathode pins constantly sense the voltage difference across the MOSFET to determine the magnitude and polarity of the voltage across it. When the MOSFET is on, the voltage difference across Anode and Cathode pins depends on the RDSON and load current. If voltage difference across source and drain of the external MOSFET becomes negative, this is sensed as a fault condition by Anode and Cathode pins and gate is turned off by Gate Pull Down pin as shown in Figure 1. The reverse voltage threshold across Anode and Cathode to detect the fault condition is -20 mV. The consistent sensing of voltage polarity across the MOSFET enables the LM74670-Q1 to provide a fast response to the power source failure and limit the amount and duration of the reverse current flow. 7.3.3.2 VcapH and VcapL Pins VcapH and VcapL are high and low voltage thresholds respectively that the LM74670-Q1 uses to detect when to turn the charge pump circuitry ON and OFF. The capacitor charging and discharging time can be correlated to the duty cycle of the MOSFET gate. Figure 9 shows the voltage behavior across the Vcap. During the time period T0, the capacitor is storing energy from the charge pump. The MOSFET is turned off and current flow is only through the body diode during this time period. The conduction though body diode of the MOSFET is for a 8 Submit Documentation Feedback Copyright © 2015, Texas Instruments Incorporated Product Folder Links: LM74670-Q1 |
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