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.
Applications Information (Continued)
DS008711-18
FIGURE 5. Current Limit Transient Suppressor,
Vin Referred
C.L. SENSE VOLTAGE MULTIPLICATION
DS008711-22
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 Vf + 110 mV).
FIGURE 8. Under-Voltage Lockout
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.
DS008711-19
FIGURE 6. Current Limit Sense Voltage Multiplication,
Ground Referred
DS008711-21
FIGURE 9. Maximum Duty Cycle Limiting
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%.
DS008711-20
FIGURE 7. Current Limit Sense Voltage Multiplication,
Vin Referred
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.
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
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