Test Circuit
*
(Continued)
DS008711-3
Op amp supplies are
±
15V
DVM input resistance
>
100 MΩ
*
LM1578 max duty cycle is 90%
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 kΩ 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 kΩ 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
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