LM2578A, LM3578A
SNVS767E –AUGUST 2000–REVISED APRIL 2013
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Input Level Shift Accuracy: This specification determines the output voltage tolerance of a regulator whose
output control depends on drawing equal currents from the inverting and non-inverting inputs (see the Inverting
Regulator of Figure 34, and the RS-232 Line Driver Power Supply of Figure 36).
Level Shift Accuracy is tested by using two equal-value resistors 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 terminal grounded thru a 10 kΩ resistor and the output
transistor's emitter connected to ground, the Collector SaturationVoltage is the collector-to-emitter voltage for a
given collector current.
Emitter Saturation Voltage: With the inverting input terminal grounded thru a 10 kΩ resistor and the output
transistor's collector connected to Vin, 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 frequency.
Current Limit Sense Current: The bias current for the Current Limit terminal with the applied voltage equal to
the Current Limit Sense Voltage.
Supply Current: The IC power supply current, excluding the current drawn through the output transistor, with the
oscillator operating.
Functional Description
The LM2578A 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 voltage drive.
A control signal, usually representing output voltage, fed into the LM2578A'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 LM2578A.
COMPARATOR INPUT STAGE
The LM2578A'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 accomplished 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 and R2. Thus, both inputs to the comparator will have the potential of the 1.0V reference, VA. When
one input, for example the non-inverting input, is pulled ΔV away from VA, a current of ΔV/R1 will flow through
R1. This same current flows through R2, and the comparator sees a total voltage of 2ΔV between its inputs. The
high gain of the system, through feedback, will correct for this imbalance and return both inputs to the 1.0V level.
This unusual comparator input stage increases circuit flexibility, while minimizing the total number of external
components 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 polarity reversal (see TYPICAL
APPLICATIONS).
OSCILLATOR
The LM2578A provides an on-board oscillator which can be adjusted up to 100 kHz. Its frequency is set by a
single external capacitor, C1, as shown in Figure 14, and follows the equation
fOSC = 8×10−5/C1
The oscillator provides a blanking pulse to limit maximum duty cycle to 90%, and a reset pulse to the internal
circuitry.
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