LM2578A, LM3578A

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SNVS767E – AUGUST 2000 – REVISED APRIL 2013

Figure 19. Current Limit Sense Voltage Multiplication, Ground Referred

Figure 20. Current Limit Sense Voltage Multiplication, Vin Referred

UNDER-VOLTAGE LOCKOUT

Under-voltage lockout is accomplished with few external components. When Vin becomes lower than the zener

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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