lm5000sdx-6ep National Semiconductor Corporation, lm5000sdx-6ep Datasheet - Page 11

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

Operation
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FIGURE 3. Simplified Boost Converter Diagram
(a) First Cycle of Operation (b) Second Cycle Of Operation
CONTINUOUS CONDUCTION MODE
The LM5000EP is a current-mode, PWM regulator. When
used as a boost regulator the input voltage is stepped up to
a higher output voltage. In continuous conduction mode
(when the inductor current never reaches zero at steady
state), the boost regulator operates in two cycles.
INTRODUCTION TO COMPENSATION
In the first cycle of operation, shown in Figure 3 (a), the
transistor is closed and the diode is reverse biased. Energy
is collected in the inductor and the load current is supplied by
C
.
OUT
The second cycle is shown in Figure 3 (b). During this cycle,
the transistor is open and the diode is forward biased. The
energy stored in the inductor is transferred to the load and
output capacitor.
The ratio of these two cycles determines the output voltage.
The output voltage is defined approximately as:
where D is the duty cycle of the switch, D and D' will be
required for design calculations.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the feedback pin and a
resistor divider connected to the output as shown in Figure 1.
The feedback pin is always at 1.259V, so the ratio of the
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feedback resistors sets the output voltage.
FIGURE 4. (a) Inductor current. (b) Diode current.
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