Datasheet
LM2598
SNVS125C –MARCH 1998–REVISED APRIL 2013
www.ti.com
Package thermal resistance and junction temperature rise numbers are all approximate, and there are many
factors that will affect these numbers. Some of these factors include board size, shape, thickness, position,
location, and even board temperature. Other factors are, trace width, total printed circuit copper area, copper
thickness, single- or double-sided, multilayer board and the amount of solder on the board. The effectiveness of
the PC board to dissipate heat also depends on the size, quantity and spacing of other components on the
board, as well as whether the surrounding air is still or moving. Furthermore, some of these components such as
the catch diode will add heat to the PC board and the heat can vary as the input voltage changes. For the
inductor, depending on the physical size, type of core material and the DC resistance, it could either act as a
heat sink taking heat away from the board, or it could add heat to the board.
SHUTDOWN /SOFT-START
The circuit shown in Figure 43 is a standard buck regulator with 24V in, 12V out, 280 mA load, and using a 0.068
μF Soft-start capacitor. The photo in Figure 41 and Figure 42 show the effects of Soft-start on the output voltage,
the input current, with, and without a Soft-start capacitor. Figure 41 also shows the error flag output going high
when the output voltage reaches 95% of the nominal output voltage. The reduced input current required at
startup is very evident when comparing the two photos. The Soft-start feature reduces the startup current from
1A down to 240 mA, and delays and slows down the output voltage rise time.
This reduction in start up current is useful in situations where the input power source is limited in the amount of
current it can deliver. In some applications Soft-start can be used to replace undervoltage lockout or delayed
startup functions.
If a very slow output voltage ramp is desired, the Soft-start capacitor can be made much larger. Many seconds or
even minutes are possible.
If only the shutdown feature is needed, the Soft-start capacitor can be eliminated.
Figure 41. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
Figure 42. Output Voltage, Input Current, at Start-Up,
WITHOUT Soft-start
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