LM2599SX-12/NOPB National Semiconductor, LM2599SX-12/NOPB Datasheet - Page 27

• Manufacturer Part Number: LM2599SX-12/NOPB
• Description: IC REG SW 12V 3A STP DN TO-263-7
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM2599SX-12/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 12V
• Current - Output: 3A
• Frequency - Switching: 150kHz
• Voltage - Input: 4.5 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: D²Pak, TO-263 (7 leads + tab)
• For Use With: 551011367-021 - BOARD WEBENCH BUILD IT LM2599
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM2599SX-12; *LM2599SX-12/NOPB; LM2599SX-12

Application Information
lNVERTING REGULATOR
The circuit in Figure 27 converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulator’s ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
This example uses the LM2599-5 to generate a −5V output,
but other output voltages are possible by selecting other
output voltage versions, including the adjustable version.
Since this regulator topology can produce an output voltage
that is either greater than or less than the input voltage, the
maximum output current greatly depends on both the input
and output voltage. The curve shown in Figure 28 provides a
guide as to the amount of output load current possible for the
different input and output voltage conditions.
The maximum voltage appearing across the regulator is the
absolute sum of the input and output voltage, and this must
be limited to a maximum of 40V. In this example, when
converting +20V to −5V, the regulator would see 25V be-
tween the input pin and ground pin. The LM2599 has a
maximum input voltage rating of 40V.
FIGURE 28. Maximum Load Current for
Inverting Regulator Circuit
Inverting Regulator
FIGURE 27. Inverting −5V Regulator With Shutdown and Soft-start
DS012582-44
(Continued)
27
An additional diode is required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the C
or no load conditions. Also, this diode isolation changes the
topology to closely resemble a buck configuration thus pro-
viding good closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a IN5400 diode could be
used.
Because of differences in the operation of the inverting
regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 33 µH,
3.5A inductor is the best choice. Capacitor selection can also
be narrowed down to just a few values. Using the values
shown in Figure 27 will provide good results in the majority of
inverting designs.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2599 current limit
(approximately 4.5A) are needed for 2 ms or more, until the
output reaches its nominal output voltage. The actual time
depends on the output voltage and the size of the output
capacitor. Input power sources that are current limited or
sources that can not deliver these currents without getting
loaded down, may not work correctly. Because of the rela-
tively high startup currents required by the inverting topology,
the Soft-start feature shown in Figure 27 is recommended.
Also shown in Figure 27 are several shutdown methods for
the inverting configuration. With the inverting configuration,
some level shifting is required, because the ground pin of the
regulator is no longer at ground, but is now at the negative
output voltage. The shutdown methods shown accept
ground referenced shutdown signals.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. Figure
29 contains a undervoltage lockout circuit for a buck configu-
ration, while Figure 30 and 30 are for the inverting types
(only the circuitry pertaining to the undervoltage lockout is
IN
capacitor to the output, under light
DS012582-43
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