LM2598S-ADJ National Semiconductor, LM2598S-ADJ Datasheet - Page 28

• Manufacturer Part Number: LM2598S-ADJ
• Description: SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

www.national.com
Application Information
An additional diode is required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the C
or no load conditions. Also, this diode isolation changes the
topology to closely resemble a buck configuration thus pro-
viding good closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a 1N5400 diode could be
used.
Because of differences in the operation of the inverting regu-
lator, the standard design procedure is not used to select the
inductor value. In the majority of designs, a 68 µH, 1.5 Amp
inductor is the best choice. Capacitor selection can also be
narrowed down to just a few values. Using the values shown
in Figure 26 will provide good results in the majority of invert-
ing designs.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2598 current limit (ap-
proximately 1.5A) are needed for 2 ms or more, until the out-
put reaches its nominal output voltage. The actual time de-
pends on the output voltage and the size of the output
capacitor. Input power sources that are current limited or
sources that can not deliver these currents without getting
loaded down, may not work correctly. Because of the rela-
tively high startup currents required by the inverting topology,
the Soft-start feature shown in Figure 26 is recommended.
Also shown in Figure 26 are several shutdown methods for
the inverting configuration. With the inverting configuration,
some level shifting is required, because the ground pin of the
regulator is no longer at ground, but is now at the negative
output voltage. The shutdown methods shown accept
ground referenced shutdown signals.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. Figure
28 contains a undervoltage lockout circuit for a buck configu-
ration, while Figure 29 and Figure 30 are for the inverting
types (only the circuitry pertaining to the undervoltage lock-
out is shown). Figure 28 uses a zener diode to establish the
threshold voltage when the switcher begins operating. When
the input voltage is less than the zener voltage, resistors R1
and R2 hold the Shutdown /Soft-start pin low, keeping the
regulator in the shutdown mode. As the input voltage ex-
ceeds the zener voltage, the zener conducts, pulling the
FIGURE 27. Maximum Load Current for Inverting
Regulator Circuit
IN
capacitor to the output, under light
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(Continued)
28
Shutdown /Soft-start pin high, allowing the regulator to begin
switching. The threshold voltage for the undervoltage lockout
feature is approximately 1.5V greater than the zener voltage.
Figure 29 and Figure 30 apply the same feature to an invert-
ing circuit. Figure 29 features a constant threshold voltage
for turn on and turn off (zener voltage plus approximately
one volt). Since the SD /SS pin has an internal 7V zener
clamp, R2 is needed to limit the current into this pin to ap-
proximately 1 mA when Q1 is on. If hysteresis is needed, the
circuit in Figure 30 has a turn ON voltage which is different
than the turn OFF voltage. The amount of hysteresis is ap-
proximately equal to the value of the output voltage.
NEGATIVE VOLTAGE CHARGE PUMP
Occasionally a low current negative voltage is needed for bi-
asing parts of a circuit. A simple method of generating a
negative voltage using a charge pump technique and the
switching waveform present at the OUT pin, is shown in Fig-
ure 31 . This unregulated negative voltage is approximately
equal to the positive input voltage (minus a few volts), and
can supply up to a 200 mA of output current. There is a re-
quirement however, that there be a minimum load of several
FIGURE 28. Undervoltage Lockout for a Buck
FIGURE 29. Undervoltage Lockout Without
FIGURE 30. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
Hysteresis for an Inverting Regulator
Regulator
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