LM2904MXNOPB National Semiconductor, LM2904MXNOPB Datasheet

LM2904MXNOPB

Manufacturer Part Number

LM2904MXNOPB

Description

SOP8

Manufacturer

National Semiconductor

Datasheet

1.LM2904MXNOPB.pdf (9 pages)

Specifications of LM2904MXNOPB

Date_code

08+

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RSDS

Design Guidelines Part 2

Overview

This is the second half of the flat panel display design

guidelines. Part 1 focused on the Timing Controller (TCON)

and Column Driver RSDS

voltage references and power supply generation. In a typical

flat panel display, there can be 12 or more different supply

voltages that need to be generated and stabilized to maxi-

mize the panel performance.

DC-DC Conversion

Power supply design is very different from the high-speed

digital design that dominates the rest of the flat panel display

system. National Semiconductor has an industry best sup-

port structure for analog and power supply design including

the award winning WebBench

application notes (AN-556: Introduction to Power Supplie-

sand AN-1149: Layout Guidelines for Switching Power Sup-

plies are particularly helpful). All of National’s documentation

is available on the corporate website (www.national.com) or

through your local National Semiconductor representative.

Panels will normally have only a low voltage digital supply

(typically +3.3V), and will use that supply to generate the

higher voltages needed by the column drivers and the row

drivers. The most common method for developing these

voltages is to use a step-up converter. There are many

variations on the market, and National Semiconductor has a

variety of options designed specifically to meet the needs of

flat panel displays.

In this application note, examples are given that will illustrate

how to use National’s LM2622 and a +3.3V supply to gen-

erate the following voltages:

• +9.0V for Column Driver Analog Supply

• +18.0V for Row Driver "on" Voltage

• −9.0V for Row Driver "off" Voltage

The basic operation of the LM2622 is outlined in Figure 1

and Figure 2. All of the components shown are external to

the device except the switch, which is attached to the circuit

by the SW pin on the IC. In the first cycle, the switch is

closed (shown in Figure 1) and the diode is reverse biased

(the diode acts like an open circuit and has been removed

from the figure). Energy is stored in the inductor and the

current for the load is supplied by C

(shown in Figure 2), the switch is open and the energy stored

in the inductor is transferred to the load and C

details on the operation of the LM2622 and switching power

supplies in general, please see the LM2622 datasheet and

Linear and Switching Voltage Regulator Fundamentals,

which is available on National’s corporate website or through

your National Semiconductor representative.

™

Flat Panel Display

™

interface. Part 2 focuses on the

™

software and numerous

OUT

. In the second cycle

AN200471

OUT

. For more

National Semiconductor

Application Note 1237

April 2003

+9.0V for Column Driver Analog Supply

Normally, the analog supply for the column drivers is be-

tween +8.0V and +10.0V. The LM2622 is capable of gener-

ating a controlled, stable voltage of up to +12.0V from a

+3.3V supply. Column drivers will consume 8-10mA of ana-

log current under worst-case conditions. For XGA and SXGA

applications, the DC-DC converter must be able to supply

100-150mA of current. National’s LM2622 can provide 100-

200mA of current at over 90% efficiency, making it perfectly

suited to supply the column driver analog voltage

Figure 3 shows an example of how to generate +9.0V using

the LM2622. In this configuration, the LM2622 is set to

operate at 600kHz (it can also operate at 1.3MHz if the

FSLCT pin is tied high). The circuit comprising of L1, D1, C3,

and the SW pin of the IC operate as described above in

Figure 1 and Figure 2. The analog output voltage is con-

trolled by the resistor-dividor of R1 and R2. The voltage on

the FB pin is internally fixed at +1.26V, so in order to have a

+9.0V output, resistor values of 51kΩ for R1 and 8.3kΩ for

R2 can be used. Voltages other than +9.0V can be gener-

ated by changing the ratio of R1 and R2.

FIGURE 1. Basic Operation of the LM2622 (switch

FIGURE 2. Basic Operation of the LM2622 (switch

closed)

open)

www.national.com

20047101

20047102

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