LM2698 datasheet(8/17 Pages) NSC

8 / 17 page

Operation

Continuous Conduction Mode

The LM2698 is a current-mode, PWM boost regulator. A

boost regulator steps the input voltage up to a higher output

voltage. In continuous conduction mode (when the inductor

current never reaches zero at steady state), the boost regu-

lator operates in two cycles.

In the first cycle of operation, shown in

Figure 1 (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 1 (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 as:

where D is the duty cycle of the switch.

Inductor

The inductor is one of the two energy storage elements in a

boost converter.

Figure 2 shows how the inductor current

varies during a switching cycle. The current through an

inductor is quantified as:

20012602

FIGURE 1. Simplified Boost Converter Diagram

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

20012605

FIGURE 2. (a) Inductor Current (b) Diode Current

LM2698 Datasheet(PDF) 8 Page - National Semiconductor (TI)