National Semiconductor LM1575 Series, Manual

Page: 21 / 27

Additional Applications
(Continued)
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off
until the input voltage reaches a certain threshold. An und-
ervoltage lockout circuit which accomplishes this task is
shown in
Figure 12 , while Figure 13 shows the same circuit
applied to a buck-boost configuration. These circuits keep
the regulator off until the input voltage reaches a predeter-
mined level.
V
TH
≈
V
Z1
+ 2V
BE
(Q1)
DELAYED STARTUP
The ON /OFF pin can be used to provide a delayed startup
feature as shown in
Figure 14 . With an input voltage of 20V
and for the part values shown, the circuit provides approxi-
mately 10 ms of delay time before the circuit begins switch-
ing. Increasing the RC time constant can provide longer
delay times. But excessively large RC time constants can
cause problems with input voltages that are high in 60 Hz or
120 Hz ripple, by coupling the ripple into the ON /OFF pin.
ADJUSTABLE OUTPUT, LOW-RIPPLE
POWER SUPPLY
A 1A power supply that features an adjustable output voltage
is shown in
Figure 15 . An additional L-C filter that reduces
the output ripple by a factor of 10 or more is included in this
circuit.
01147516
Typical Load Current
200 mA for V
IN
= −5.2V
500 mA for V
IN
= −7V
Note: Pin numbers are for TO-220 package.
FIGURE 11. Negative Boost
01147517
Note: Complete circuit not shown.
Note: Pin numbers are for the TO-220 package.
FIGURE 12. Undervoltage Lockout for Buck Circuit
01147518
Note: Complete circuit not shown (see Figure 10 ).
Note: Pin numbers are for the TO-220 package.
FIGURE 13. Undervoltage Lockout
for Buck-Boost Circuit
01147519
Note: Complete circuit not shown.
Note: Pin numbers are for the TO-220 package.
FIGURE 14. Delayed Startup
LM1575/LM2575/LM2575HV
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