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LM3578 Scheda tecnica(PDF) 7 Page - National Semiconductor (TI) |
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LM3578 Scheda tecnica(HTML) 7 Page - National Semiconductor (TI) |
7 / 18 page Functional Description (Continued) and R2. Thus, both inputs to the comparator will have the po- tential of the 1.0V reference, V A. When one input, for ex- ample the non-inverting input, is pulled ∆V away from V A,a current of ∆V/R1 will flow through R1. This same current flows through R2, and the comparator sees a total voltage of 2 ∆V between its inputs. The high gain of the system, through feedback, will correct for this imbalance and return both in- puts to the 1.0V level. This unusual comparator input stage increases circuit flex- ibility, while minimizing the total number of external compo- nents required for a voltage regulator system. The inverting switching regulator configuration, for example, can be set up without having to use an external op amp for feedback polar- ity reversal (see TYPICAL APPLICATIONS). OSCILLATOR The LM1578A provides an on-board oscillator which can be adjusted up to 100 kHz. Its frequency is set by a single exter- nal capacitor, C 1, as shown in Figure 1, and follows the equation f OSC = 8x10 −5/C 1 The oscillator provides a blanking pulse to limit maximum duty cycle to 90%, and a reset pulse to the internal circuitry. OUTPUT TRANSISTOR The output transistor is capable of delivering up to 750 mA with a saturation voltage of less than 0.9V. (see Collector Saturation Voltage and Emitter Saturation Voltage curves). The emitter must not be pulled more than 1V below ground (this limit is 0.6V for T J ≥ 100˚C). Because of this limit, an ex- ternal transistor must be used to develop negative output voltages (see the Inverting Regulator Typical Application). Other configurations may need protection against violation of this limit (see the Emitter Output section of the Applica- tions Information). CURRENT LIMIT The LM1578A’s current limit may be referenced to either the ground or the V in pins, and operates on a cycle-by-cycle ba- sis. The current limit section consists of two comparators: one with its non-inverting input referenced to a voltage 110 mV below V in, the other with its inverting input referenced 110 mV above ground (see FUNCTIONAL DIAGRAM). The current limit is activated whenever the current limit terminal is pulled 110 mV away from either V in or ground. Applications Information CURRENT LIMIT As mentioned in the functional description, the current limit terminal may be referenced to either the V in or the ground terminal. Resistor R3 converts the current to be sensed into a voltage for current limit detection. CURRENT LIMIT TRANSIENT SUPPRESSION When noise spikes and switching transients interfere with proper current limit operation, R1 and C1 act together as a low pass filter to control the current limit circuitry’s response time. Because the sense current of the current limit terminal varies according to where it is referenced, R1 should be less than 2 k Ω when referenced to ground, and less than 100Ω when referenced to V in. DS008711-4 FIGURE 1. Value of Timing Capacitor vs Oscillator Frequency DS008711-15 FIGURE 2. Current Limit, Ground Referred DS008711-16 FIGURE 3. Current Limit, V in Referred DS008711-17 FIGURE 4. Current Limit Transient Suppressor, Ground Referred 7 www.national.com |
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