LM2599SX-12/NOPB National Semiconductor, LM2599SX-12/NOPB Datasheet - Page 28

• 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

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Application Information
shown). Figure 29 uses a zener diode to establish the
threshold voltage when the switcher begins operating. When
the input voltage is less than the zener voltage, resistors R1
and R2 hold the Shutdown /Soft-start pin low, keeping the
regulator in the shutdown mode. As the input voltage ex-
ceeds the zener voltage, the zener conducts, pulling the
Shutdown /Soft-start pin high, allowing the regulator to begin
switching. The threshold voltage for the undervoltage lockout
feature is approximately 1.5V greater than the zener voltage.
Figure 30 and 30 apply the same feature to an inverting
circuit. Figure 30 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 31 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. Since the SD /SS pin has an internal
7V zener clamp, R2 is needed to limit the current into this pin
to approximately 1 mA when Q1 is on.
FIGURE 29. Undervoltage Lockout for a Buck
Regulator
(Continued)
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28
NEGATIVE VOLTAGE CHARGE PUMP
Occasionally a low current negative voltage is needed for
biasing parts of a circuit. A simple method of generating a
negative voltage using a charge pump technique and the
switching waveform present at the OUT pin, is shown in
Figure 32 . This unregulated negative voltage is approxi-
mately equal to the positive input voltage (minus a few volts),
and can supply up to a 600 mA of output current. There is a
requirement however, that there be a minimum load of 1.2A
on the regulated positive output for the charge pump to work
correctly. Also, resistor R1 is required to limit the charging
current of C1 to some value less than the LM2599 current
limit (typically 4.5A).
This method of generating a negative output voltage without
an additional inductor can be used with other members of
the Simple Switcher Family, using either the buck or boost
topology.
FIGURE 32. Charge Pump for Generating a
FIGURE 30. Undervoltage Lockout Without
FIGURE 31. Undervoltage Lockout With
Low Current, Negative Output Voltage
Hysteresis for an Inverting Regulator
Hysteresis for an Inverting Regulator
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