LM2578A, LM3578A

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SNVS767E – AUGUST 2000 – REVISED APRIL 2013

Figure 19. Current Limit Sense Voltage Multiplication, Ground Referred

UNDER-VOLTAGE LOCKOUT

breakdown voltage, the output transistor is turned off. This occurs because diode D1 will then become forward

biased, allowing resistor R3 to sink a greater current from the non-inverting input than is sunk by the parallel

combination of R1 and R2 at the inverting terminal. R3 should be one-fifth of the value of R1 and R2 in parallel.

Figure 21. Under-Voltage Lockout

MAXIMUM DUTY CYCLE LIMITING

The maximum duty cycle can be externally limited by adjusting the charge to discharge ratio of the oscillator

capacitor with a single external resistor. Typical values are 50

μA for the charge current, 450 μA for the

discharge current, and a voltage swing from 200 mV to 750 mV. Therefore, R1 is selected for the desired

charging and discharging slopes and C1 is readjusted to set the oscillator frequency.

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