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LM3475MF Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM3475MF Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page Operation Description (Continued) device enters soft-start. During soft-start, the reference volt- age is ramped up to the nominal value of 0.8V in approxi- mately 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 consid- ered as overcurrent protection or output short circuit protec- tion. PGATE During switching, the PGATE pin swings from V IN (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 V IN, 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 capaci- tance is recommended. MINIMUM ON/OFF TIME To ensure accurate comparator switching, the LM3475 im- poses 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 hys- teretic comparator from improperly responding to switching noise spikes (See Reducing Switching Noise) and ESL spikes (See Output Capacitor Selection) at the output. At very low or very high duty cycle operation, maximum frequency will be limited by the blanking time. The maximum operating frequency can be determined by the following equations: F MAX =D/tonmin F MAX = (1-D) / toffmin Where D is the duty cycle, defined as V OUT/VIN, and tonmin and toff min is the sum of the blanking time, the propagation delay time and the PFET delay time (see Figure 1). ENABLE PIN (EN) The LM3475 provides a shutdown function via the EN pin to disable the device. The device is active when the EN pin is pulled above 1.5V (typ) and in shutdown mode when EN is below 1.135V (typ). In shutdown mode, total quiescent cur- rent is less than 10µA. The EN pin can be directly connected to V IN for always-on operation. www.national.com 8 |
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