LM3679_15 Datasheet, PDF (11/24 Pages) Texas Instruments – 3MHz, 350mA Miniature Step-Down DC-DC Converter for Ultra Low Profile Applications (Height < 0.55mm)

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
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 = 112mA + V_IN/20Ω
(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
‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode. When V_IN =2.5V the
part transitions from PWM to PFM mode at ~ 35mA output current and from PFM to PWM mode at ~ 95mA ,
when V_IN=3.6V, PWM to PFM transition occurs at ~ 42mA and PFM to PWM transition occurs at ~ 115mA, when
V_IN =4.5V, PWM to PFM transition occurs at ~ 60mA and PFM to PWM transition occurs at ~ 135mA.
PFM Mode at Light Load
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
Low2 PFM Threshold,
switch back to PWMmode
Load current
increases
High PFM Threshold
~1.018*Vout
Low1 PFM Threshold
~1.002*Vout
Current load
increases,
draws Vout
towards
Low2 PFM
Threshold
Low2 PFM Threshold
Vout
PWM Mode at
Moderate to Heavy
Loads
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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