LM3676SD-ADJEV National Semiconductor, LM3676SD-ADJEV Datasheet - Page 12

• Manufacturer Part Number: LM3676SD-ADJEV
• Description: BOARD EVALUATION LM3676SD-ADJ
• Manufacturer: National Semiconductor

Specifications of LM3676SD-ADJEV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1 ~ 3.3V
• Current - Output: 600mA
• Voltage - Input: 2.9 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3676
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

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During PFM operation, the converter positions the output volt-
age 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 switch
FIGURE 6. Typical PFM Operation
FIGURE 7. Operation in PFM Mode and Transfer to PWM Mode
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is turned on. It remains on until the output voltage reaches the
‘high’ PFM threshold or the peak current exceeds the I
level set for PFM mode. The typical peak 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 be-
low the ‘high’ PFM comparator threshold (see Figure 8), 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 ex-
tremely low power mode. Quiescent supply current during this
‘sleep’ mode is 16µA (typ), which allows the part to achieve
high efficiency under extremely light load conditions.
If the load current should increase during PFM mode (see
Figure 8) causing the output voltage to fall below the ‘low2’
PFM threshold, the part will automatically transition into fixed-
frequency PWM mode. When V
from PWM to PFM mode at ~35mA output current and from
PFM to PWM mode at ~85mA , when V
transition happens at ~50mA and PFM to PWM transition
happens at ~100mA, when V
happens at ~65mA and PFM to PWM transition happens at
~115mA.
PFM
= 112mA + V
IN
/27Ω .
IN
=4.5V, PWM to PFM transition
IN
=2.9V the part transitions
IN
=3.6V, PWM to PFM
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PFM