lm5000sdx-6 National Semiconductor Corporation, lm5000sdx-6 Datasheet - Page 9

• Manufacturer Part Number: lm5000sdx-6
• Description: High Voltage Switch Mode Regulator
• Manufacturer: National Semiconductor Corporation

Block Diagram
Boost Regulator Operation
The LM5000 utilizes a PWM control scheme to regulate the
output voltage over all load conditions. The operation can best
be understood referring to the block diagram and Figure 3. At
the start of each cycle, the oscillator sets the driver logic and
turns on the NMOS power device conducting current through
the inductor, cycle 1 of Figure 3 (a). During this cycle, the
voltage at the COMP pin controls the peak inductor current.
The COMP voltage will increase with larger loads and de-
crease with smaller. This voltage is compared with the sum-
mation of the SW volatge and the ramp compensation.The
ramp compensation is used in PWM architectures to eliminate
the sub-harmonic oscillations that occur during duty cycles
greater than 50%. Once the summation of the ramp compen-
sation and switch voltage equals the COMP voltage, the PWM
comparator resets the driver logic turning off the NMOS power
device. The inductor current then flows through the output
diode to the load and output capacitor, cycle 2 of Figure 3 (b).
The NMOS power device is then set by the oscillator at the
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end of the period and current flows through the inductor once
again.
The LM5000 has dedicated protection circuitry running during
the normal operation to protect the IC. The Thermal Shutdown
circuitry turns off the NMOS power device when the die tem-
perature reaches excessive levels. The UVP comparator pro-
tects the NMOS power device during supply power startup
and shutdown to prevent operation at voltages less than the
minimum input voltage. The OVP comparator is used to pre-
vent the output voltage from rising at no loads allowing full
PWM operation over all load conditions. The LM5000 also
features a shutdown mode. An external capacitor sets the
softstart time by limiting the error amp output range, as the
capacitor charges up via an internal 10µA current source.
The LM5000 is available in two operating frequency ranges.
The LM5000-3 is pin selectable for either 300kHz (FS
Grounded) or 700kHz (FS Open). The LM5000-6 is pin se-
lectable for either 600kHz (FS Grounded) or 1.3MHz (FS
Open)
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