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

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

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