Functional Description
(Continued)
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.
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.
DS008711-15
FIGURE 2. Current Limit, Ground Referred
DS008711-16
DS008711-4
FIGURE 3. Current Limit, V
in
Referred
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 kΩ when referenced to ground, and less than 100Ω
when referenced to V
in
.
FIGURE 1. Value of Timing Capacitor vs
Oscillator Frequency
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.
DS008711-17
FIGURE 4. Current Limit Transient Suppressor,
Ground Referred
7
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