LM3578AM datasheet(9/20 Pages) NSC

9 / 20 page

Applications Information (Continued)

C.L. SENSE VOLTAGE MULTIPLICATION

When a larger sense resistor value is desired, the voltage

divider network, consisting of R1 and R2, may be used. This

effectively multiplies the sense voltage by (1 + R1/R2). Also,

R1 can be replaced by a diode to increase current limit

sense voltage to about 800 mV (diode V

f + 110 mV).

UNDER-VOLTAGE LOCKOUT

Under-voltage lockout is accomplished with few external

components. When V

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.

MAXIMUM DUTY CYCLE LIMITING

The maximum duty cycle can be externally limited by adjust-

ing 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.

00871117

FIGURE 4. Current Limit Transient Suppressor,

Ground Referred

00871118

FIGURE 5. Current Limit Transient Suppressor,

in Referred

00871119

FIGURE 6. Current Limit Sense Voltage Multiplication,

Ground Referred

00871120

FIGURE 7. Current Limit Sense Voltage Multiplication,

in Referred

00871122

FIGURE 8. Under-Voltage Lockout

LM3578AM Scheda tecnica(PDF) 9 Page - National Semiconductor (TI)