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LM2717MT Scheda tecnica(PDF) 9 Page - National Semiconductor (TI) |
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LM2717MT Scheda tecnica(HTML) 9 Page - National Semiconductor (TI) |
9 / 12 page Buck Operation PROTECTION (BOTH REGULATORS) The LM2717 has dedicated protection circuitry running dur- ing normal operation to protect the IC. The Thermal Shut- down circuitry turns off the power devices when the die temperature reaches excessive levels. The UVP comparator protects the power devices during supply power startup and shutdown to prevent operation at voltages less than the minimum input voltage. The OVP comparator is used to prevent the output voltage from rising at no loads allowing full PWM operation over all load conditions. The LM2717 also features a shutdown mode for each converter decreas- ing the supply current to approximately 10µA (both in shut- down mode). CONTINUOUS CONDUCTION MODE The LM2717 contains current-mode, PWM buck regulators. A buck regulator steps the input voltage down to a lower output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady state), the buck regulator operates in two cycles. The power switch is connected between V IN and SW1 and SW2. In the first cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and the load current is supplied by C OUT and the rising current through the inductor. During the second cycle the transistor is open and the diode is forward biased due to the fact that the inductor current cannot instantaneously change direction. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined approximately as: where D is the duty cycle of the switch, D and D' will be required for design calculations. DESIGN PROCEDURE This section presents guidelines for selecting external com- ponents. SETTING THE OUTPUT VOLTAGE (ADJUSTABLE REGULATOR) The output voltage is set using the feedback pin and a resistor divider connected to the output as shown in Figure 1. The feedback pin voltage is 1.26V, so the ratio of the feed- back resistors sets the output voltage according to the fol- lowing equation: INPUT CAPACITOR A low ESR aluminum, tantalum, or ceramic capacitor is needed betwen the input pin and power ground. This capaci- tor prevents large voltage transients from appearing at the input. The capacitor is selected based on the RMS current and voltage requirements. The RMS current is given by: The RMS current reaches its maximum (I OUT/2) when V IN equals 2VOUT. This value should be calculated for both regulators and added to give a total RMS current rating. For an aluminum or ceramic capacitor, the voltage rating should be at least 25% higher than the maximum input voltage. If a tantalum capacitor is used, the voltage rating required is about twice the maximum input voltage. The tantalum ca- pacitor should be surge current tested by the manufacturer to prevent being shorted by the inrush current. The minimum capacitor value should be 47µF for lower output load current applications and less dynamic (quickly changing) load con- ditions. For higher output current applications or dynamic load conditions a 68µF to 100µF low ESR capacitor is rec- ommended. It is also recommended to put a small ceramic capacitor (0.1µF to 4.7µF) between the input pins and ground to reduce high frequency spikes. INDUCTOR SELECTION The most critical parameters for the inductor are the induc- tance, peak current and the DC resistance. The inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages (for 300kHz operation): A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, and current stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple current increases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resis- tance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss equal 2% of the output power. OUTPUT CAPACITOR The selection of C OUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant fre- quency, PWM mode is approximated by: The ESR term usually plays the dominant role in determining the voltage ripple. Low ESR ceramic, aluminum electrolytic, or tantalum capacitors (such as Taiyo Yuden MLCC, Nichi- con PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX TPS, and CDE polymer aluminum) is recommended. An electrolytic capacitor is not recommended for temperatures below −25˚C since its ESR rises dramatically at cold tem- perature. Ceramic or tantalum capacitors have much better ESR specifications at cold temperature and is preferred for low temperature applications. www.national.com 9 |
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