LM2597, LM2597HV
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SNVS119C –MARCH 1998–REVISED APRIL 2013
NOTE
The lower curve shown in Figure 32 shows the Soft-start region from 0% to 100%. This is
not the duty cycle percentage, but the output voltage percentage. Also, the Soft-start
voltage range has a negative temperature coefficient associated with it. See the Soft-start
curve in Electrical Characteristics.
4. Normal operation. Above 2.8V, the circuit operates as a standard Pulse Width Modulated switching
regulator. The capacitor will continue to charge up until it reaches the internal clamp voltage of approximately
7V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.
If the part is operated with an input voltage at or below the internal soft-start clamp voltage of approximately
7V, the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To
maintain proper function under these conditions, it is strongly recommended that the SD/SS pin be clamped
externally between the 3V maximum soft-start threshold and the 4.5V minimum input voltage. Figure 34 is an
example of an external 3.7V (approx.) clamp that prevents a line-step related glitch but does not interfere
with the soft-start behavior of the device.
Figure 32. Soft-start, Delay, Error, Output
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