LM3670MFX-2.5 National Semiconductor, LM3670MFX-2.5 Datasheet - Page 11

• Manufacturer Part Number: LM3670MFX-2.5
• Description: Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
• Manufacturer: National Semiconductor

Operation Description
off, blocking current flow from the input, and then turns the
NFET synchronous rectifier on. The inductor draws current
from ground through the NFET to the output filter capacitor
and load, which ramps the inductor current down with a
slope of
The output filter stores charge when the inductor current is
high, and releases it when low, smoothing the voltage across
the load.
PWM OPERATION
During PWM operation the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve excellent load and line regulation.
The DC gain of the power stage is proportional to the input
voltage. To eliminate this dependence, feed forward in-
versely proportional to the input voltage is introduced.
Internal Synchronous Rectification
While in PWM mode, the LM3670 uses an internal NFET as
a synchronous rectifier to reduce rectifier forward voltage
drop and associated power loss. Synchronous rectification
provides a significant improvement in efficiency whenever
the output voltage is relatively low compared to the voltage
drop across an ordinary rectifier diode.
Current Limiting
A current limit feature allows the LM3670 to protect itself and
external components during overload conditions PWM mode
implements cycle-by-cycle current limiting using an internal
comparator that trips at 620 mA (typ).
PFM OPERATION
At very light loads, the converter enters PFM mode and
operates with reduced switching frequency and supply cur-
rent to maintain high efficiency.
The party will automatically transition into PFM mode when
either of two conditions occurs for a duration of 32 or more
clock cycles:
A. The inductor current becomes discontinuous
(Continued)
11
level:
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 0.8% and 1.6% (typ) 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 exceeds the ‘high’ PFM threshold or the peak current
exceeds the I
in PFM mode is:
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 5),
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 less than 30 µA, which allows the
part to achieve high efficiencies under extremely light load
conditions. When the output drops below the ‘low’ PFM
threshold, the cycle repeats to restore the output voltage to
∼1.6% above the nominal PWM output voltage.
If the load current should increase during PFM mode (see
Figure 5) causing the output voltage to fall below the ‘low2’
PFM threshold, the part will automatically transition into
fixed-frequency PWM mode.
B. The peak PMOS switch current drops below the I
PFM
level set for PFM mode. The peak current
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MODE