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1、LM1578A LM2578A LM3578A Switching Regulator General Description The LM1578A is a switching regulator which can easily be set up for such DC to DC voltage conversion circuits as the buck boost and inverting configurations The LM1578A fea tures a unique comparator input stage which not only has separa
2、te pins for both the inverting and non inverting inputs but also provides an internal 1 0V reference to each input thereby simplifying circuit design and p c board layout The output can switch up to 750 mA and has output pins for its collector and emitter to promote design flexibility An external cu
3、rrent limit terminal may be referenced to either the ground or the Vinterminal depending upon the application In addi tion the LM1578A has an on board oscillator which sets the switching frequency with a single external capacitor from100 M LM1578 max duty cycle is 90 6 Functional Description Continu
4、ed and R2 Thus both inputs to the comparator will have the po tential of the 1 0V reference VA When one input for ex ample the non inverting input is pulled V away from VA 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
5、 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
6、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 na
7、l capacitor C1 as shown inFigure 1 and follows the equation fOSC 8x10 5 C1 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
8、 less than 0 9V seeCollector Saturation VoltageandEmitter Saturation Voltagecurves The emitter must not be pulled more than 1V below ground this limit is 0 6V for TJ 100 C Because of this limit an ex ternal transistor must be used to develop negative output voltages see the Inverting Regulator Typic
9、al 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 Vinpins and operates on a cycle by cycle ba sis The current
10、limit section consists of two comparators one with its non inverting input referenced to a voltage 110 mV below Vin 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 e
11、ither Vinor ground Applications Information CURRENT LIMIT As mentioned in the functional description the current limit terminal may be referenced to either the Vinor the ground terminal Resistor R3 converts the current to be sensed into a voltage for current limit detection CURRENT LIMIT TRANSIENT S
12、UPPRESSION 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
13、 should be less than 2 k when referenced to ground and less than 100 when referenced to Vin 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 VinReferred DS008711 17 FIGURE 4 Current Limit Trans
14、ient Suppressor Ground Referred 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 dio
15、de to increase current limit sense voltage to about 800 mV diode Vf 110 mV UNDER VOLTAGE LOCKOUT Under voltage lockout is accomplished with few external components When Vinbecomes lower than the zener breakdown voltage the output transistor is turned off This occurs because diode D1 will then become
16、 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 A for the charge current 450 A for the discharge current and a v
