LM3670MF-3.3/NOPB National Semiconductor, LM3670MF-3.3/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM3670MF-3.3/NOPB
• Description: IC CONV DC/DC STEP-DOWN SOT23-5
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM3670MF-3.3/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 3.3V
• Current - Output: 350mA
• Frequency - Switching: 1MHz
• Voltage - Input: 2.5 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM3670MF-3.3; LM3670MF-3.3; LM3670MF-3.3NOPB; LM3670MF-3.3NOPBTR; LM3670MF-3.3NOPBTR; LM3670MF-3.3TR

Operation Description
The part automatically transition into PFM mode when either
of two conditions occurs for a duration of 32 or more clock
cycles:
level:
During PFM operation, the converter positions the output
voltage slightly higher than the nominal output voltage in
PWM operation, allowing additional headroom for voltage
drop during a load transient from light to heavy load. The
PFM comparator senses 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:
Soft-Start
The LM3670 has a soft-start circuit that limits in-rush current
during start-up. Typical start-up times with a 10µF output
capacitor and 350mA load is 400µs:
Inrush Current (mA)
A. The inductor current becomes discontinuous
B. The peak PMOS switch current drops below the I
140
70
0
PFM
level set for PFM mode. The peak current
FIGURE 5. Operation in PFM Mode and Transition to PWM Mode
Duration (µSec)
(Continued)
224
256
32
MODE
11
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 automatically transitions into fixed-
frequency PWM mode.
Note 6: The first 32µS are to allow the bias currents to stabilize
Inrush Current (mA)
280
620
until soft start ends
Duration (µSec)
256
20075803
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