LM2702 datasheet(15/21 Pages) NSC

15 / 21 page

Operation

CONTINUOUS CONDUCTION MODE

The LM2702 is a TFT Panel Module containing 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 regulator 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 LM2702 has patented internal circuitry that is used to

limit the inductor inrush current on start-up of the boost

DC/DC switching regulator. This inrush current limiting cir-

cuitry 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 12µA current charges

the external soft-start capacitor, C

SS. The soft-start time can

be estimated as:

SS =CSS*0.6V/12µA

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

circuitry, typically 7ms. Only longer soft-start times may be

implemented using the SS pin and a capacitor C

SS.Ifa

shorter time is designed for using the above equation, 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

20051102

FIGURE 1. Simplified Boost Converter Diagram

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

LM2702 Datasheet(PDF) 15 Page - National Semiconductor (TI)

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