LM2710 datasheet(13/20 Pages) NSC

13 / 20 page

Operation

CONTINUOUS CONDUCTION MODE

The LM2710 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 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 the

typical operating circuit. The feedback pin voltage is 1.265V,

so the ratio of the feedback resistors sets the output voltage

according to the following equation:

SOFT-START CAPACITOR

The LM2710 has patented internal circuitry that is used to

limit the inductor inrush current on start-up. This inrush

current limiting circuitry serves as a soft-start. However,

many applications may require much more soft-start than

what is available with the internal circuitry. The external SS

pin is used to tailor the soft-start for a specific application. A

11µA current charges the external soft-start capacitor, Css.

The soft-start time can be estimated as:

Tss = Css*0.6V/11µA

The minimum soft-start time is set by the internal soft-start

circuitry, typically 7ms for 600kHz operation and approxi-

mately half that for 1.25MHz operation. Only longer soft-start

times may be implemented using the SS pin and a capacitor

SS. If a shorter time is designed for using the above equa-

tion, the internal soft-start circuitry will override it.

Due to the unique nature of the dual internal/external soft-

start, care was taken in the design to ensure temperature

stable operation. As you can see with the Iss data in the

Electrical Characterisitcs table and the graph "Soft-Start

Current vs. V

IN"inthe Typical Performance Characterisitcs

section, the soft start curent has a temperature coefficient

and would lead one to believe there would be significant

variation with temperature. Though the current has a tem-

perature coefficient the actual programmed external soft

start time does not show this extreme of a temperature

variation. As you can see in the following transient plots:

20043402

FIGURE 1. Simplified Boost Converter Diagram

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

LM2710 Datasheet(PDF) 13 Page - National Semiconductor (TI)

LM2710 Datasheet(HTML) 13 Page - National Semiconductor (TI)

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