Datasheet

LM2578A, LM3578A
www.ti.com SNVS767E –AUGUST 2000–REVISED APRIL 2013
Figure 19. Current Limit Sense Voltage Multiplication, Ground Referred
Figure 20. Current Limit Sense Voltage Multiplication, V
in
Referred
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout is accomplished with few external components. When V
in
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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