LM2578A, LM3578A
SNVS767E –AUGUST 2000–REVISED APRIL 2013
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Figure 22. Maximum Duty Cycle Limiting
DUTY CYCLE ADJUSTMENT
When manual or mechanical selection of the output transistor's duty cycle is needed, the cirucit shown below
may be used. The output will turn on with the beginning of each oscillator cycle and turn off when the current
sunk by R2 and R3 from the non-inverting terminal becomes greater than the current sunk from the inverting
terminal.
With the resistor values as shown, R3 can be used to adjust the duty cycle from 0% to 90%.
When the sum of R2 and R3 is twice the value of R1, the duty cycle will be about 50%. C1 may be a large
electrolytic capacitor to lower the oscillator frequency below 1 Hz.
Figure 23. Duty Cycle Adjustment
REMOTE SHUTDOWN
The LM2578A may be remotely shutdown by sinking a greater current from the non-inverting input than from the
inverting input. This may be accomplished by selecting resistor R3 to be approximately one-half the value of R1
and R2 in parallel.
Figure 24. Shutdown Occurs when VL is High
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