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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LM2696MXAX Fiches technique(PDF) 8 Page - National Semiconductor (TI) |
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LM2696MXAX Fiches technique(HTML) 8 Page - National Semiconductor (TI) |
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8 / 19 page  Application Information (Continued) limit. Following the termination of the on-time, a non- resetable extended off timer is initiated. The length of the off-time is proportional to the feedback voltage. When FB = 0V the off-time is preset to 20 µs. This condition is often a result of in short circuit operation when a maximum amount of off-time is required. This amount of time ensures safe short circuit operation up to the maximum input voltage of 24V. In cases of overload (not complete short circuit, FB > 0V) the current limit off-time is reduced. Reduction of the off-time during smaller overloads reduces the amount of fold back. This also reduces the initial startup time. N-CHANNEL HIGH SIDE SWITCH AND DRIVER The LM2696 utilizes an integrated N-Channel high side switch and associated floating high voltage gate driver. This gate driver circuit works in conjunction with an external bootstrap capacitor and an internal diode. The minimum off-time (165 ns) is set to ensure that the bootstrap capacitor has sufficient time to charge. THERMAL SHUTDOWN An internal thermal sensor is incorporated to monitor the die temperature. If the die temp exceeds 165oC then the sensor will trip causing the part to stop switching. Soft-start will restart after the temperature falls below 155oC. COMPONENT SELECTION As with any DC-DC converter, numerous trade-offs are present that allow the designer to optimize a design for efficiency, size and performance. These trade-offs are taken into consideration throughout this section. The first calculation for any buck converter is duty cycle. Ignoring voltage drops associated with parasitic resistances and non-ideal components, the duty cycle may be expressed as: A duty cycle relationship that considers the voltage drop across the internal FET and voltage drop across the external catch diode may be expressed as: Where V D is the forward voltage of the external catch diode (D CATCH) and VSW is the voltage drop across the internal FET. FREQUENCY SELECTION Switching frequency affects the selection of the output induc- tor, capacitor, and overall efficiency. The trade-offs in fre- quency selection may be summarized as; higher switching frequencies permit use of smaller inductors possibly saving board space at the trade-off of lower efficiency. It is recom- mended that a nominal frequency of 300 kHz should be used in the initial stages of design and iterated if necessary. The switching frequency of the LM2696 is set by the resistor connected to the RON pin. This resistor controls the current flowing into the RON pin and is directly related to the on-time pulse. Connecting a resistor from this pin to PVIN allows the switching frequency to remain constant as the input voltage changes. In normal operation this pin is approximately 0.65V above GND. In shutdown, this pin becomes a high imped- ance node to prevent current flow. The on time may be exoressed as: Where V IN is the voltage at the high side of the RON resistor (typically PV IN), VD is the diode voltage present at the RON pin (0.65V typical), R ON is in k Ω, and k ON is a constant value set internally (66 µA •µs nominal). This equation can be re-arranged such that R ON is a function of switching fre- quency: Where f SW is in kHz. In CCM the frequency may be determined using the relation- ship: (T ON is in µs) Which is typically used to set the switching frequency. Under no condition should a bypass capacitor be connected to the R ON pin. Doing so couples any AC perturbations into the pin and prevents proper operation. INDUCTOR SELECTION Selecting an inductor is a process that may require several iterations. The reason for this is that the size of the inductor influences the amount of ripple present at the output that is critical to the stability of an adaptive on-time circuit. Typically, an inductor is selected such that the maximum peak-to-peak ripple current is equal to 30% of the maximum load current. The inductor current ripple ( ∆I L) may be expressed as: Therefore, L can be initially set to the following by applying the 30% guideline: The other features of the inductor that should be taken into account are saturation current and core material. A shielded inductor or low profile unshielded inductor is recommended to reduce EMI. www.national.com 8 |
Numéro de pièce similaire - LM2696MXAX |
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Description similaire - LM2696MXAX |
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