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

LM3673

SNVS434L – JULY 2006 – REVISED OCTOBER 2015

www.ti.com

Device Functional Modes (continued)

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 FB pin and control the switching of the output FETs

such that the output voltage ramps from approximately 0.6% to approximately 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

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 16), 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

16 µA (typical), which allows the device to achieve high efficiency under extremely light load conditions.

If the load current increases during PFM mode (see Figure 16) causing the output voltage to fall below the Low 2

device transitions from PWM mode to PFM mode at approximately 35-mA output current and from PFM mode to

happens at approximately 65 mA and PFM-to-PWM transition happens at approximately 115 mA.

Figure 16. Operation in PFM Mode and Transfer to PWM Mode

8.4.2 Shutdown Mode

Setting the EN input pin low (< 0.4 V) places the LM3673 in shutdown mode. During shutdown the PFET switch,

NFET switch, reference, control, and bias circuitry of the LM3673 are turned off. Setting EN high (> 1 V) enables

normal operation. Setting the EN pin low is recommended to turn off the LM3673 during system power up and

undervoltage conditions when the supply is less than 2.7 V. Do not leave the EN pin floating.

12

Submit Documentation Feedback

Copyright © 2006–2015, Texas Instruments Incorporated

Product Folder Links: LM3673