LM1575 National Semiconductor, LM1575 Datasheet - Page 19

• Manufacturer Part Number: LM1575
• Description: SIMPLE SWITCHER 1A Step-Down Voltage Regulator
• Manufacturer: National Semiconductor

Additional Applications
Typical Load Current
200 mA for V
500 mA for V
Note: Pin numbers are for TO-220 package.
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off
until the input voltage reaches a certain threshold. An under-
voltage 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 regu-
lator off until the input voltage reaches a predetermined
level.
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 de-
lay 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.
IN
IN
= −5.2V
= −7V
FIGURE 11. Negative Boost
V
TH
V
Z1
+ 2V
BE
(Q1)
(Continued)
DS011475-16
19
Note: Complete circuit not shown.
Note: Pin numbers are for the TO-220 package.
Note: Complete circuit not shown (see Figure 10 ).
Note: Pin numbers are for the TO-220 package.
Note: Complete circuit not shown.
Note: Pin numbers are for the TO-220 package.
FIGURE 12. Undervoltage Lockout for Buck Circuit
FIGURE 13. Undervoltage Lockout
FIGURE 14. Delayed Startup
for Buck-Boost Circuit
DS011475-19
DS011475-17
DS011475-18
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