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LM1578 Folha de dados(PDF) 8 Page - National Semiconductor (TI) |
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LM1578 Folha de dados(HTML) 8 Page - National Semiconductor (TI) |
8 / 18 page Applications Information (Continued) 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 f + 110 mV). 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 in Referred DS008711-19 FIGURE 6. Current Limit Sense Voltage Multiplication, Ground Referred DS008711-20 FIGURE 7. Current Limit Sense Voltage Multiplication, V in Referred DS008711-22 FIGURE 8. Under-Voltage Lockout DS008711-21 FIGURE 9. Maximum Duty Cycle Limiting www.national.com 8 |
Nº de peça semelhante - LM1578 |
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Descrição semelhante - LM1578 |
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