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LM3578AN Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM3578AN Datasheet(HTML) 6 Page - National Semiconductor (TI) |
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6 / 18 page  Test Circuit* (Continued) Definition of Terms Input Reference Voltage: The voltage (referred to ground) that must be applied to either the inverting or non-inverting input to cause the regulator switch to change state (ON or OFF). Input Reference Current: The current that must be drawn from either the inverting or non-inverting input to cause the regulator switch to change state (ON or OFF). Input Level Shift Accuracy: This specification determines the output voltage tolerance of a regulator whose output con- trol depends on drawing equal currents from the inverting and non-inverting inputs (see the Inverting Regulator of Fig- ure 21, and the RS-232 Line Driver Power Supply of Figure 23). Level Shift Accuracy is tested by using two equal-value re- sistors to draw current from the inverting and non-inverting input terminals, then measuring the percentage difference in the voltages across the resistors that produces a controlled duty cycle at the switch output. Collector Saturation Voltage: With the inverting input ter- minal grounded thru a 10 k Ω resistor and the output transis- tor’s emitter connected to ground, the Collector Saturation- Voltage is the collector-to-emitter voltage for a given collector current. Emitter Saturation Voltage: With the inverting input termi- nal grounded thru a 10 k Ω resistor and the output transistor’s collector connected to V in, the Emitter Saturation Voltage is the collector-to-emitter voltage for a given emitter current. Collector Emitter Sustaining Voltage: The collector-emitter breakdown voltage of the output transistor, measured at a specified current. Current Limit Sense Voltage: The voltage at the Current Limit pin, referred to either the supply or the ground terminal, which (via logic circuitry) will cause the output transistor to turn OFF and resets cycle-by-cycle at the oscillator fre- quency. Current Limit Sense Current: The bias current for the Cur- rent Limit terminal with the applied voltage equal to the Cur- rent Limit Sense Voltage. Supply Current: The IC power supply current, excluding the current drawn through the output transistor, with the oscilla- tor operating. Functional Description The LM1578A is a pulse-width modulator designed for use as a switching regulator controller. It may also be used in other applications which require controlled pulse-width volt- age drive. A control signal, usually representing output voltage, fed into the LM1578A’s comparator is compared with an internally-generated reference. The resulting error signal and the oscillator’s output are fed to a logic network which determines when the output transistor will be turned ON or OFF. The following is a brief description of the subsections of the LM1578A. COMPARATOR INPUT STAGE The LM1578A’s comparator input stage is unique in that both the inverting and non-inverting inputs are available to the user, and both contain a 1.0V reference. This is accom- plished as follows: A 1.0V reference is fed into a modified voltage follower circuit (see FUNCTIONAL DIAGRAM). When both input pins are open, no current flows through R1 DS008711-3 Op amp supplies are ±15V DVM input resistance >100 M Ω *LM1578 max duty cycle is 90% www.national.com 6 |
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Similar Description - LM3578AN |
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