and R2. Thus, both inputs to the comparator will have the po-

tential of the 1.0V reference, V

ample the non-inverting input, is pulled

∆V away from V

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

equation

f

1

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 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

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

sis.

The current limit section consists of two comparators: one

with its non-inverting input referenced to a voltage 110 mV

below V

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

Applications Information

CURRENT LIMIT

As mentioned in the functional description, the current limit

terminal may be referenced to either the V

terminal. Resistor R3 converts the current to be sensed into

a voltage for current limit detection.

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 k

Ω when referenced to ground, and less than 100Ω

when referenced to V

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, V

DS008711-17

FIGURE 4. Current Limit Transient Suppressor,

Ground Referred

7

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