lm2593hvsx-adj National Semiconductor Corporation, lm2593hvsx-adj Datasheet - Page 17

• Manufacturer Part Number: lm2593hvsx-adj
• Description: Simple Switcher Power Converter 150 Khz 2a Step-down Voltage Regulator, With Features
• Manufacturer: National Semiconductor Corporation

Application Information
suming 100% efficiency, which is never so. Therefore expect
I
the above equation.
The reader is also referred to Application Note AN-1157 for
examples based on positive to negative configuration.
The maximum voltage appearing across the regulator is the
absolute sum of the input and output voltage, and this must
be limited to a maximum of 60V. In this example, when
converting +20V to −5V, the regulator would see 25V be-
tween the input pin and ground pin. The LM2593HV has a
maximum input voltage rating of 60V.
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 IN5400 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 33 µH,
4A inductor is the best choice. Capacitor selection can also
be narrowed down to just a few values.
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 LM2593HV current limit
(approximately 4.0A) are needed for 2 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 10 is recommended.
Also shown in Figure 10 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 11
contains a undervoltage lockout circuit for a buck configura-
tion, while Figure 12 and Figure 13 are for the inverting types
(only the circuitry pertaining to the undervoltage lockout is
shown). Figure 11 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
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.
PEAK
to be an additional 10-20% higher than calculated from
IN
capacitor to the output, under light
(Continued)
17
Figure 12 and Figure 13 apply the same feature to an
inverting circuit. Figure 12 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
13 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 is shown in
Figure 14 . This unregulated negative voltage is approxi-
mately equal to the positive input voltage (minus a few volts),
and can supply up to a 600 mA of output current. There is a
requirement however, that there be a minimum load of 1.2A
FIGURE 11. Undervoltage Lockout for a Buck
FIGURE 12. Undervoltage Lockout Without
FIGURE 13. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
Hysteresis for an Inverting Regulator
Regulator
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