Datasheet

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
5. Boost Capacitor (C
B
)
This capacitor develops the necessary voltage to turn the switch
gate on fully. All applications should use a 0.01 μF, 50V ceramic
capacitor.
5. Boost Capacitor (C
B
)
For this application, and all applications, use a 0.01 μF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (C
SS
- optional)
This capacitor controls the rate at which the device starts up. The
formula for the soft-start capacitor C
SS
is:
6. Soft-Start Capacitor (C
SS
- optional)
For this application, selecting a start-up time of 10 ms and using
the formula for C
SS
results in a value of:
where:
I
SS
= Soft-Start Current :4.5 μA typical.
t
SS
= Soft-Start Time :Selected.
V
SSTH
= Soft-Start Threshold Voltage :0.63V typical.
V
OUT
= Output Voltage :Selected.
V
SCHOTTKY
= Schottky Diode Voltage Drop :0.4V typical.
V
IN
= Input Voltage :Selected.
If this feature is not desired, leave this pin open. With certain
softstart capacitor values and operating conditions, the LM2671
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)
The LM2671 (oscillator) can be synchronized to run with an
external oscillator, using the sync pin (pin 3). By doing so, the
LM2671 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.
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.
7. Frequency Synchronization (optional)
For all applications, use a 1 kΩ resistor and a 100 pF capacitor for
the RC filter.
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LM2671