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

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

Ω

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