LM2594M-5.0 National Semiconductor, LM2594M-5.0 Datasheet - Page 23

IC, STEP-DOWN REGULATOR, 8-SOIC

LM2594M-5.0

Manufacturer Part Number
LM2594M-5.0
Description
IC, STEP-DOWN REGULATOR, 8-SOIC
Manufacturer
National Semiconductor
Datasheet

Specifications of LM2594M-5.0

Primary Input Voltage
12V
No. Of Outputs
1
Output Voltage
5V
Output Current
500mA
No. Of Pins
8
Operating Temperature Range
-40°C To +125°C
Supply Voltage Range
4.5V To 40V
Termination Type
SMD
Dc To Dc Converter Type
Inverting/Step Down
Number Of Outputs
1
Pin Count
8
Input Voltage
40V
Switching Freq
173kHz
Efficiency
82%
Package Type
SOIC N
Output Type
Fixed
Switching Regulator
Yes
Mounting
Surface Mount
Input Voltage (min)
4.5V
Operating Temp Range
-40C to 125C
Operating Temperature Classification
Automotive
Filter Terminals
SMD
Rohs Compliant
No
Input Voltage Primary Max
40V
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant

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Application Information
The curves shown in Figure 19 and Figure 20 show the
LM2594 junction temperature rise above ambient tempera-
ture with a 500 mA load for various input and output volt-
ages. This data was taken with the circuit operating as a
buck switcher with all components mounted on a PC board
to simulate the junction temperature under actual operating
conditions. This curve is typical, and can be used for a quick
check on the maximum junction temperature for various con-
ditions, but keep in mind that there are many factors that can
affect the junction temperature.
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 22. Undervoltage Lockout
FIGURE 21. Delayed Startup
for Buck Regulator
FIGURE 23. Undervoltage Lockout for Inverting Regulator
(Continued)
DS012439-36
DS012439-37
23
DELAYED STARTUP
The circuit in Figure 21 uses the the ON /OFF pin to provide
a time delay between the time the input voltage is applied
and the time the output voltage comes up (only the circuitry
pertaining to the delayed start up is shown). As the input volt-
age rises, the charging of capacitor C1 pulls the ON /OFF pin
high, keeping the regulator off. Once the input voltage
reaches its final value and the capacitor stops charging, and
resistor R
cuit to start switching. Resistor R
maximum voltage applied to the ON /OFF pin (maximum of
25V), reduces power supply noise sensitivity, and also limits
the capacitor, C1, discharge current. When high input ripple
voltage exists, avoid long delay time, because this ripple can
be coupled into the ON /OFF pin and cause problems.
This delayed startup feature is useful in situations where the
input power source is limited in the amount of current it can
deliver. It allows the input voltage to rise to a higher voltage
before the regulator starts operating. Buck regulators require
less input current at higher input voltages.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. An und-
ervoltage lockout feature applied to a buck regulator is
shown in Figure 22 , while Figure 23 and Figure 24 applies
the same feature to an inverting circuit. The circuit in Figure
23 features a constant threshold voltage for turn on and turn
off (zener voltage plus approximately one volt). If hysteresis
is needed, the circuit in Figure 24 has a turn ON voltage
which is different than the turn OFF voltage. The amount of
hysteresis is approximately equal to the value of the output
voltage. If zener voltages greater than 25V are used, an ad-
ditional 47 k resistor is needed from the ON /OFF pin to the
ground pin to stay within the 25V maximum limit of the ON
/OFF pin.
INVERTING REGULATOR
The circuit in Figure 25 converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulators ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
2
pulls the ON /OFF pin low, thus allowing the cir-
DS012439-38
1
is included to limit the
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