LM2578AN/NOPB National Semiconductor, LM2578AN/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM2578AN/NOPB
• Description: IC REG SIMPLE SWITCHER 8-DIP
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck), Step-Up (Boost), Inverting, Flybackr

Specifications of LM2578AN/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 750mA
• Frequency - Switching: 20kHz
• Voltage - Input: 2 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• No. Of Outputs: 1
• Output Voltage: 1V
• Output Current: 750mA
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Msl: MSL 1 - Unlimited
• Input Voltage: 40V
• Input Voltage Primary Max: 40V
• Rohs Compliant: Yes
• Primary Input Voltage: 40V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Power - Output: -
• Other names: *LM2578AN; *LM2578AN/NOPB; LM2578AN

www.national.com
Functional Description
and R2. Thus, both inputs to the comparator will have the
potential of the 1.0V reference, V
example the non-inverting input, is pulled ∆V away from V
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
inputs 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
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, C
equation
f
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
external 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 LM2578A’s current limit may be referenced to either the
ground or the V
basis.
The current limit section consists of two comparators: one
with its non-inverting input referenced to a voltage 110 mV
below V
OSC
= 8x10
FIGURE 1. Value of Timing Capacitor vs
in
, the other with its inverting input referenced
−5
/C
1
in
Oscillator Frequency
pins, and operates on a cycle-by-cycle
1
, as shown in Figure 1, and follows the
J
≥ 100˚C). Because of this limit, an
A
. When one input, for
(Continued)
00871104
A
,
8
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 kΩ when referenced to ground, and less than 100Ω
when referenced to V
FIGURE 2. Current Limit, Ground Referred
FIGURE 3. Current Limit, V
in
.
in
in
or ground.
Referred
00871116
00871115
in
or the ground