LM2574M-5.0 National Semiconductor, LM2574M-5.0 Datasheet - Page 20

• Manufacturer Part Number: LM2574M-5.0
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2574M-5.0

• Input Voltage: 40V
• Output Voltage: 5V
• No. Of Pins: 14
• Mounting Type: Surface Mount
• Output Current Max: 0.5A
• Peak Reflow Compatible (260 C): No
• Supply Voltage Max: 40V
• Leaded Process Compatible: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Additional Applications
Note: Pin numbers are for 8-pin DIP package.
Because of the boosting function of this type of regulator, the
switch current is relatively high, especially at low input volt-
ages. Output load current limitations are a result of the maxi-
mum current rating of the switch. Also, boost regulators can
not provide current limiting load protection in the event of a
shorted load, so some other means (such as a fuse) may be
necessary.
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 13 while Figure 14 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.
V
TH
V
Z1
+ 2V
FIGURE 12. Negative Boost
BE
(Q1)
(Continued)
DS011394-20
20
Note: Complete circuit not shown.
Note: Pin numbers are for 8-pin DIP package.
Note: Complete circuit not shown (see Figure 11 ).
Note: Pin numbers are for 8-pin DIP package.
DELAYED STARTUP
The ON /OFF pin can be used to provide a delayed startup
feature as shown in Figure 15 . 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 500 mA power supply that features an adjustable output
voltage is shown in Figure 16 . An additional L-C filter that re-
duces the output ripple by a factor of 10 or more is included
in this circuit.
Note: Complete circuit not shown.
Note: Pin numbers are for 8-pin DIP package.
FIGURE 13. Undervoltage Lockout for Buck Circuit
FIGURE 14. Undervoltage Lockout
FIGURE 15. Delayed Startup
for Buck-Boost Circuit
DS011394-23
DS011394-21
DS011394-22