LM5000 datasheet(11/18 Pages) NSC

11 / 18 page

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.

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

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

feedback resistors sets the output voltage.

INTRODUCTION TO COMPENSATION

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FIGURE 3. Simplified Boost Converter Diagram

(a) First Cycle of Operation (b) Second Cycle Of Operation

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FIGURE 4. (a) Inductor current. (b) Diode current.

LM5000 Datasheet(PDF) 11 Page - National Semiconductor (TI)