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LP3905 Datasheet(PDF) 11 Page - Texas Instruments |
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LP3905 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 24 page IPFM = 112 mA + VIN 27: VOUT TIME (4 Ps/DIV) 200 mA/DIV IL VSW 2V/DIV 20 mV/DIV AC Coupled VIN = 3.6V VOUT = 1.5V IOUT = 20 mA Typically IMODE < 30 mA + VIN 42: LP3905 www.ti.com SNVS374D – JUNE 2006 – REVISED MAY 2013 PFM OPERATION At very light loads, the converters enters PFM mode and operate with reduced switching frequency and supply current to maintain high efficiency. The Bucks will 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 IMODE level, (1) Figure 8. Typical PFM Operation During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output FETs such that the output voltage ramps between ~0.6% and ~1.7% 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 reaches the ‘high’ PFM threshold or the peak current exceeds the IPFM level set for PFM mode. The typical peak current in PFM mode is: (2) 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 ), the PMOS switch is again turned on and the cycle is repeated 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 16µA (typ), 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 (average voltage in pfm mode) to ∼1.15% above the nominal PWM output voltage. If the load current should increase during PFM mode causing the output voltage to fall below the ‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode. When VIN =2.8V the part transitions from PWM to PFM mode at ~35mA output current and from PFM to PWM mode at ~85mA , when VIN=3.6V, PWM to PFM transition happens at ~50mA and PFM to PWM transition happens at ~100mA, when VIN =4.5V, PWM to PFM transition happens at ~65mA and PFM to PWM transition happens at ~115mA. Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LP3905 |
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