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LM3100MHX датащи(PDF) 10 Page - National Semiconductor (TI) |
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LM3100MHX датащи(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Functional Description The LM3100 Step Down Switching Regulator features all functions needed to implement a cost effective, efficient buck power converter capable of supplying 1.5A to a load. This voltage regulator contains Dual 40V N-Channel buck syn- chronous switches and is available in a thermally enhanced eTSSOP-20 package. The Constant ON-Time (COT) regu- lation scheme requires no loop compensation, results in fast load transient response, and simplifies circuit implementa- tion. It will work correctly even with an all ceramic output capacitor network and does not rely on the output capacitor’s ESR for stability. The operating frequency remains constant with line and load variations due to the inverse relationship between the input voltage and the on-time. The valley cur- rent limit detection circuit, internally set at 1.9A, inhibits the high-side switch until the inductor current level subsides. Please refer to the functional block diagram with a typical application circuit. The LM3100 can be applied in numerous applications and can operate efficiently from inputs as high as 36V. Protection features include: Thermal shutdown, V CC under-voltage lockout, gate drive under-voltage lockout. Hysteretic Control Circuit Overview The LM3100 buck DC-DC regulator employs a control scheme in which the high-side switch on-time varies in- versely with the line voltage (V IN). Control is based on a comparator and the one-shot on-timer, with the output volt- age feedback (FB) compared with an internal reference of 0.8V. If the FB level is below the reference the buck switch is turned on for a fixed time determined by the input voltage and a programming resistor (R ON). Following the on-time, the switch remains off for a minimum of 260ns. If FB is below the reference at that time the switch turns on again for another on-time period. The switching will continue until regulation is achieved. The regulator will operate in discontinuous conduction mode at light load currents, and continuous conduction mode with heavy load current. In discontinuous conduction mode (DCM), current through the output inductor starts at zero and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the internal refer- ence. Until then the inductor current remains zero and the load is supplied entirely by the output capacitor. In this mode the operating frequency is lower than in continuous conduc- tion mode, and varies with load current. Conversion effi- ciency is maintained since the switching losses are reduced with the reduction in load and switching frequency. The discontinuous operating frequency can be calculated ap- proximately as follows: (1) where R L = the load resistance 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 rela- tively constant with load and line variations. The CCM oper- ating frequency can be calculated approximately as follows: (2) The output voltage is set by two external resistors (R FB1, R FB2). The regulated output voltage is calculated as follows: V OUT =0.8Vx(RFB1 +RFB2)/RFB2 (3) Start-up Regulator (V CC) 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 to 40V. The V CC output regulates at 6V, and is current limited to 65 mA. Upon power up, the regulator sources current into the external capacitor at V CC (C VCC). 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 (C SS) to charge. The minimum input voltage is determined by the dropout voltage of V CC, and the VCC UVLO falling threshold ( )3.7 V). If VIN is less than )4.0V, the V CC UVLO activates to shut off the output. Regulation Comparator The feedback voltage at FB 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. Bias current at the FB pin is nomi- nally 100 nA. Over-Voltage Comparator The voltage at FB is compared to an internal 0.92V refer- ence. If the voltage at FB 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 and until the inductor current decays to zero. The low side switch will be turned off. ON-Time Timer, Shutdown The LM3100 main switch is determined by the R ON resistor and the input voltage (V IN), and is calculated from: (4) The inverse relationship with VIN results in a nearly constant frequency as V IN is varied. RON should be selected for a minimum on-time (at maximum V IN) greater than 200 ns for proper current limit operation. This requirement limits the maximum frequency for each application, depending on V IN and V OUT, calculated from the following: www.national.com 10 |
Аналогичный номер детали - LM3100MHX |
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Аналогичное описание - LM3100MHX |
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