C.L. SENSE VOLTAGE MULTIPLICATION

When a larger sense resistor value is desired, the voltage di-

vider 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

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.

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 se-

lected for the desired charging and discharging slopes and

C1 is readjusted to set the oscillator frequency.

DUTY CYCLE ADJUSTMENT

When manual or mechanical selection of the output transis-

tor’s duty cycle is needed, the cirucit shown below may be

used. The output will turn on with the beginning of each os-

cillator 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.

DS008711-18

FIGURE 5. Current Limit Transient Suppressor,

V

DS008711-19

FIGURE 6. Current Limit Sense Voltage Multiplication,

Ground Referred

DS008711-20

FIGURE 7. Current Limit Sense Voltage Multiplication,

V

DS008711-22

FIGURE 8. Under-Voltage Lockout

DS008711-21

FIGURE 9. Maximum Duty Cycle Limiting

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