lm2597hvn-5.0 National Semiconductor Corporation, lm2597hvn-5.0 Datasheet - Page 30

• Manufacturer Part Number: lm2597hvn-5.0
• Description: Simple Switcher Power Converter 150 Khz 0.5a Step-down Voltage Regulator, With Features
• Manufacturer: National Semiconductor Corporation

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 1N4001 diode could be
used.
Because of differences in the operation of the inverting
regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 100
µH, 1 Amp inductor is the best choice. Capacitor selection
can also be narrowed down to just a few values. Using the
values shown in Figure 29 will provide good results in the
majority of inverting 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 LM2597 current limit
(approximately 0.8A) are needed for 1 ms or more, until the
output reaches its nominal output voltage. The actual time
depends 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 29 is recommended.
Also shown in Figure 29 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
31 contains a undervoltage lockout circuit for a buck configu-
ration, while Figure 32 and Figure 33 are for the inverting
types (only the circuitry pertaining to the undervoltage lock-
out is shown). Figure 31 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 30. Maximum Load Current for Inverting
Regulator Circuit
IN
capacitor to the output, under light
DS012440-49
(Continued)
30
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 32 and Figure 33 apply the same feature to an
inverting circuit. Figure 32 features a constant threshold
voltage for turn on and turn off (zener voltage plus approxi-
mately one volt). If hysteresis is needed, the circuit in Figure
33 has a turn ON voltage which is different than the turn OFF
voltage. The amount of hysteresis is approximately equal to
the value of the output voltage. Since the SD /SS pin has an
internal 7V zener clamp, R2 is needed to limit the current into
this pin to approximately 1 mA when Q1 is on.
NEGATIVE VOLTAGE CHARGE PUMP
Occasionally a low current negative voltage is needed for
biasing 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
Figure 34 . This unregulated negative voltage is approxi-
mately equal to the positive input voltage (minus a few volts),
and can supply up to a 100 mA of output current. There is a
requirement however, that there be a minimum load of sev-
FIGURE 31. Undervoltage Lockout for a Buck
FIGURE 32. Undervoltage Lockout Without
FIGURE 33. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
Hysteresis for an Inverting Regulator
Regulator
DS012440-50
DS012440-53
DS012440-52