LM2672
SNVS136K –SEPTEMBER 1998–REVISED APRIL 2013
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PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
6. Soft-Start Capacitor (CSS - optional)
6. Soft-Start Capacitor (CSS - optional)
This capacitor controls the rate at which the device starts up. The
formula for the soft-start capacitor CSS is:
For this application, selecting a start-up time of 10 ms and using the
formula for CSS results in a value of:
(1)
(2)
where:
ISS = Soft-Start Current: 4.5 μA typical.
tSS = Soft-Start Time: Selected.
VSSTH = Soft-Start Threshold Voltage :0.63V typical.
VOUT = Output Voltage: Selected.
VSCHOTTKY = Schottky Diode Voltage Drop: 0.4V typical.
VIN = Input Voltage: Selected.
If this feature is not desired, leave this pin open. With certain
softstart capacitor values and operating conditions, the LM2672 can
exhibit an overshoot on the output voltage during turn on. Especially
when starting up into no load or low load, the softstart function may
not be effective in preventing a larger voltage overshoot on the
output. With larger loads or lower input voltages during startup this
effect is minimized. In particular, avoid using softstart capacitors
between 0.033µF and 1µF.
7. Frequency Synchronization (optional)
7. Frequency Synchronization (optional)
The LM2672 (oscillator) can be synchronized to run with an external For all applications, use a 1 kΩ resistor and a 100 pF capacitor for
oscillator, using the sync pin (pin 3). By doing so, the LM2672 can
be operated at higher frequencies than the standard frequency of
260 kHz. This allows for a reduction in the size of the inductor and
output capacitor.
the RC filter.
As shown in the drawing below, a signal applied to a RC filter at the
sync pin causes the device to synchronize to the frequency of that
signal. For a signal with a peak-to-peak amplitude of 3V or greater, a
1 kΩ resistor and a 100 pF capacitor are suitable values.
Inductor Value Selection Guides
(For Continuous Mode Operation)
Figure 24. LM2672-3.3
Figure 25. LM2672-5.0
14
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