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LM3670MF-1.5 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM3670MF-1.5 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 17 page Operation Description (Continued) The part automatically transition into PFM mode when either of two conditions occurs for a duration of 32 or more clock cycles: A. The inductor current becomes discontinuous B. The peak PMOS switch current drops below the I MODE level: During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage in PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy load. The PFM comparator senses the output voltage via the feedback pin and control the switching of the output FETs such that the output voltage ramps between 0.8% and 1.6% (typ) above the nominal PWM output voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on. It remains on until the output voltage exceeds the ‘high’ PFM threshold or the peak current exceeds the I PFM level set for PFM mode. The peak current in PFM mode is: Once the PMOS power switch is turned off, the NMOS power switch is turned on until the inductor current ramps to zero. When the NMOS zero-current condition is detected, the NMOS power switch is turned off. If the output voltage is below the ‘high’ PFM comparator threshold (see Figure 5), the PMOS switch is again turned on and the cycle is re- peated until the output reaches the desired level. Once the output reaches the ‘high’ PFM threshold, the NMOS switch is turned on briefly to ramp the inductor current to zero and then both output switches are turned off and the part enters an extremely low power mode. Quiescent supply current during this ‘sleep’ mode is less than 30 µA, which allows the part to achieve high efficiencies under extremely light load conditions. When the output drops below the ‘low’ PFM threshold, the cycle repeats to restore the output voltage to ∼1.6% above the nominal PWM output voltage. If the load current should increase during PFM mode (see Figure 5) causing the output voltage to fall below the ‘low2’ PFM threshold, the part automatically transitions into fixed- frequency PWM mode. Soft-Start The LM3670 has a soft-start circuit that limits in-rush current during start-up. Typical start-up times with a 10µF output capacitor and 350mA load is 400µs: Inrush Current (mA) Duration (µSec) 032 70 224 140 256 Inrush Current (mA) Duration (µSec) 280 256 620 until soft start ends Note 6: The first 32µS are to allow the bias currents to stabilize 20075803 FIGURE 5. Operation in PFM Mode and Transition to PWM Mode www.national.com 11 |
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