LM2576H National Semiconductor, LM2576H Datasheet - Page 18

• Manufacturer Part Number: LM2576H
• Description: SIMPLE SWITCHER 3A Step-Down Voltage Regulator
• Manufacturer: National Semiconductor

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Additional Applications
Note: Complete circuit not shown (see Figure 10 ).
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-
Definition of Terms
BUCK REGULATOR
A switching regulator topology in which a higher voltage is
converted to a lower voltage. Also known as a step-down
switching regulator.
BUCK-BOOST REGULATOR
A switching regulator topology in which a positive voltage is
converted to a negative voltage without a transformer.
DUTY CYCLE (D)
Ratio of the output switch’s on-time to the oscillator period.
FIGURE 13. Undervoltage Lockout
for Buck-Boost Circuit
FIGURE 15. 1.2V to 55V Adjustable 3A Power Supply with Low Output Ripple
(Continued)
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18
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 3A 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.
Note: Complete circuit not shown.
CATCH DIODE OR CURRENT STEERING DIODE
The diode which provides a return path for the load current
when the LM2576 switch is OFF.
EFFICIENCY ( )
The proportion of input power actually delivered to the load.
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)
The purely resistive component of a real capacitor’s imped-
ance (see Figure 16 ). It causes power loss resulting in ca-
pacitor heating, which directly affects the capacitor’s operat-
ing lifetime. When used as a switching regulator output filter,
higher ESR values result in higher output ripple voltages.
FIGURE 16. Simple Model of a Real Capacitor
FIGURE 14. Delayed Startup
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