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LM3100MHX bảng dữ liệu(PDF) 9 Page - Texas Instruments |
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LM3100MHX bảng dữ liệu(HTML) 9 Page - Texas Instruments |
9 / 21 page VOUT VIN(MAX) x 200 ns FSW(MAX) = VIN 1.3 x 10 -10 x R ON tON = VOUT 1.3 x 10 -10 x R ON FSW = LM3100 www.ti.com SNVS421F – JANUARY 2006 – REVISED DECEMBER 2009 In continuous conduction mode (CCM), current always flows through the inductor and never reaches zero during the off-time. In this mode, the operating frequency remains relatively constant with load and line variations. The CCM operating frequency can be calculated approximately as follows: (2) The output voltage is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated as follows: VOUT = 0.8V x (RFB1 + RFB2)/RFB2 (3) Start-up Regulator (VCC) The start-up regulator is integrated within LM3100. The input pin (VIN) can be connected directly to line voltage up to 36V, with transient capability of 40V. The VCC output regulates at 6V, and is current limited to 65 mA. Upon power up, the regulator sources current into the external capacitor at VCC (CVCC). CVCC must be at least 680nF for stability. When the voltage on the VCC pin reaches the under-voltage lockout threshold of 3.75V, the buck switch is enabled and the Soft-start pin is released to allow the soft-start capacitor (CSS) to charge. The minimum input voltage is determined by the dropout voltage of VCC regulator, and the VCC UVLO falling threshold ( ≊3.7 V). If VIN is less than ≊4.0V, the VCC UVLO activates to shut off the output. Regulation Comparator The feedback voltage at FB pin is compared to the internal reference voltage of 0.8V. In normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at FB falls below 0.8V. The buck switch stays on for the on-time, causing the FB voltage to rise above 0.8V. After the on-time period, the buck switch stays off until the FB voltage falls below 0.8V again. Bias current at the FB pin is nominally 100 nA. Over-Voltage Comparator The voltage at FB pin is compared to an internal 0.92V reference. If the feedback voltage rises above 0.92V the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load, changes suddenly. Once the OVP is activated, the buck switch remains off until the voltage at FB pin falls below 0.92V. The low side switch will stay on to discharge the inductor energy until the inductor current decays to zero. The low side switch will be turned off. ON-Time Timer, Shutdown The ON-Time of LM3100 main switch is determined by the RON resistor and the input voltage (VIN), and is calculated from: (4) The inverse relationship of tON and VIN results in a nearly constant switching frequency as VIN is varied. RON should be selected for a minimum on-time (at maximum VIN) greater than 200 ns for proper current limit operation. This requirement limits the maximum frequency for each application, depending on VIN and VOUT, calculated from equation 5: (5) The LM3100 can be remotely shut down by taking the EN pin below 1.1V. Refer to Figure 2. In this mode the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the EN pin allows normal operation to resume. For normal operation, the voltage at the EN pin is set between 1.5V and 3.0V, depending on VIN and the external pull-up resistor. For all cases, this voltage must be limited not to exceed 7V. Copyright © 2006–2009, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3100 |
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