LM3687 Datasheet, PDF (20/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
SNVS473A – DECEMBER 2007 – REVISED JULY 2008
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V_BATT = 3.6V
V_SW
I_L
V_OUT = 1.8V
I_OUT = 20 mA
V_OUT
TIME (1 µs/DIV)
Figure 7. Typical PFM Operation
During PFM operation, the DC-DC 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 I_PFM level
set for PFM mode. The typical peak current in PFM mode is: I_PFM = 134mA + V_BATT / 23Ω.
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 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 27µA (typ), which allows the part to achieve high efficiency under extremely light load conditions.
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 will automatically transition into fixed-frequency PWM mode.
When V_BATT =2.7V the part transitions from PWM to PFM mode at ~30mA output current and from PFM to PWM
mode at ~80mA , when V_BATT=3.6V, PWM to PFM transition happens at ~60mA and PFM to PWM transition
happens at ~90mA, when V_BATT =5.5V, PWM to PFM transition happens at ~100mA and PFM to PWM transition
happens at ~125mA.
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