LM3671MFX-1.375 National Semiconductor, LM3671MFX-1.375 Datasheet - Page 11

• Manufacturer Part Number: LM3671MFX-1.375
• Description: 2MHz / 600mA Step-Down DC-DC Converter in SOT23-5
• Manufacturer: National Semiconductor

Operation Description
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
SHUTDOWN MODE
Setting the EN input pin low (
shutdown mode. During shutdown the PFET switch, NFET
switch, reference, control and bias circuitry of the LM3671
are turned off. Setting EN high (
FIGURE 6. Typical PFM Operation
FIGURE 7. Operation in PFM Mode and Transfer to PWM Mode
<
0.4V) places the LM3671 in
>
1.0V) enables normal
(Continued)
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11
switch is turned on. It remains on until the output voltage
reaches the ‘high’ PFM threshold or the peak current ex-
ceeds the I
current in PFM mode is: I
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 7),
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 16µA (typ), which allows the part
to achieve high efficiencies under extremely light load con-
ditions. When the output drops below the ‘low’ PFM thresh-
old, the cycle repeats to restore the output voltage (average
voltage in pfm mode) to ∼1.15% above the nominal PWM
output voltage.
If the load current should increase during PFM mode (see
Figure 7) causing the output voltage to fall below the ‘low2’
PFM threshold, the part will automatically transition into
fixed-frequency PWM mode. When V
sitions from PWM to PFM mode at ~ 35mA output current
and from PFM to PWM mode at ~ 85mA , when V
PWM to PFM transition happens at ~ 50mA and PFM to
PWM transition happens at ~ 100mA, when V
to PFM transition happens at ~ 65mA and PFM to PWM
transition happens at ~ 115mA.
operation. While turning on the device with EN, soft start is
activated. It is recommended to set EN pin low to turn off the
LM3671 during system power up and undervoltage condi-
PFM
level set for PFM mode. The typical peak
PFM
= 112mA + V
IN
=2.8V the part tran-
IN
20108403
IN
/27Ω .
=4.5V, PWM
www.national.com
IN
=3.6V,