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LM3475 Fiches technique(PDF) 8 Page - Texas Instruments |
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LM3475 Fiches technique(HTML) 8 Page - Texas Instruments |
8 / 21 page td VOUT (DC) VHYST VOUT ripple VIN -VD tON Switch Voltage Iout 'IL Inductor Current Output Voltage tOFF td LM3475 SNVS239A – OCTOBER 2004 – REVISED JANUARY 2005 www.ti.com Figure 12. Hysteretic Waveforms The LM3475 operates in discontinuous conduction mode at light load current and continuous conduction mode at heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage. Until then, the inductor current remains zero. Operating frequency is low, as are switching losses. In continuous conduction mode, current always flows through the inductor and never ramps down to zero. SOFT-START The LM3475 includes an internal soft-start function to protect components from excessive inrush current and output voltage overshoot. As VIN rises above 2.7V (typical), the internal bias circuitry becomes active. When EN goes high, the device enters soft-start. During soft-start, the reference voltage is ramped up to the nominal value of 0.8V in approximately 4ms. Duty cycle and output voltage will increase as the reference voltage is ramped up. UNDER VOLTAGE DETECTION When the output voltage falls below 70% (typical) of the normal voltage, as measured at the FB pin, the device turns off PFET and restarts a new soft-start cycle. In short circuit, the PFET is always on, and the converter is effectively a resistor divider from input to output to ground. Whether the part restarts depends on the power path resistance and the short circuit resistance. This feature should not be considered as overcurrent protection or output short circuit protection. PGATE During switching, the PGATE pin swings from VIN (off) to ground (on). As input voltage increases, the time it takes to slew the gate of the PFET on and off also increases. Also, as the PFET gate voltage approaches VIN, the PGATE current driving capability decreases. This can cause a significant additional delay in turning the switch off when using a PFET with a low threshold voltage. These two effects will increase power dissipation and reduce efficiency. Therefore, a PFET with relatively high threshold voltage and low gate capacitance is recommended. MINIMUM ON/OFF TIME To ensure accurate comparator switching, the LM3475 imposes a blanking time after each comparator state change. This blanking time is 180ns typically. Immediately after the comparator goes high or low, it will be held in that state for the duration of the blanking time. This helps keep the hysteretic comparator from improperly responding to switching noise spikes (See REDUCING SWITCHING NOISE) and ESL spikes (See OUTPUT CAPACITOR SELECTION) at the output. 8 Submit Documentation Feedback Copyright © 2004–2005, Texas Instruments Incorporated Product Folder Links: LM3475 |
Numéro de pièce similaire - LM3475_14 |
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Description similaire - LM3475_14 |
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