High PFM Threshold

~1.018*Vout

Low1 PFM Threshold

~1.002*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

LM3679

www.ti.com

SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

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 ~0.2% and ~1.8% 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

PFM mode. The typical peak current in PFM mode is:

(1)

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 25), 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.

If the load current should increase during PFM mode (Figure 25) 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 ~ 95mA ,

Figure 25. Operation in PFM Mode and Transfer to PWM Mode

SHUTDOWN MODE

Setting the EN input pin low (<0.4V) places the LM3679 in shutdown mode. During shutdown the PFET switch,

NFET switch, reference, control and bias circuitry of the LM3679 are turned off. Setting EN high (>1.0V) enables

normal operation. It is recommended to set EN pin low to turn off the LM3679 during system power up and

undervoltage conditions when the supply is less than 2.5V. Do not leave the EN pin floating.

Copyright © 2007–2008, Texas Instruments Incorporated

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