High PFM Threshold

~1.017*Vout

Low1 PFM Threshold

~1.006*Vout

PFM Mode at Light Load

PWM Mode at

Moderate to Heavy

Loads

Pfet on

until

Ipfm limit

reached

Nfet on

drains

conductor

current

until

I inductor=0

High PFM

Voltage

Threshold

reached,

go into

sleep mode

Low PFM

Threshold,

turn on

PFET

Current load

increases,

draws Vout

towards

Low2 PFM

Threshold

Low2 PFM Threshold,

switch back to PWMmode

Load current

increases

Low2 PFM Threshold

Vout

TIME (4 Ps/DIV)

200 mA/DIV

2V/DIV

20 mV/DIV

AC Coupled

LM3676

www.ti.com

SNVS426B – NOVEMBER 2006 – REVISED MARCH 2007

Figure 6. 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

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 8), 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 efficiency under extremely light load conditions.

If the load current should increase during PFM mode (see Figure 8) causing the output voltage to fall below the

part transitions from PWM to PFM mode at ~35mA output current and from PFM to PWM mode at ~85mA , when

=4.5V, PWM to PFM transition happens at ~65mA and PFM to PWM transition happens at ~115mA.

Figure 7. Operation in PFM Mode and Transfer to PWM Mode

Copyright © 2006–2007, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM3676