LM2427.PDF

April 1995

LM2427

Triple 80 MHz CRT Driver

General Description

The LM2427 is a high performance triple CRT driver for

simplifying color monitor designs. The device contains three

large signal transimpedance amplifiers, and provides direct

cathode drive capability. A plastic power package and pin-

to-pin compatibility make the LM2427 ideal for new designs

or as a low cost replacement for designs using the LH2426

or CR5527.

Features

Y Low-cost plastic power package

Y Typical rise/fall times of 3.5 ns

Y 80 MHz video bandwidth at 50 V PP with 8 pF load

Y Operation from 80V power supply

Applications

Y CRT driver for color monitors

Y Drives CRT cathode directly

Y Pin-to-pin compatible with the LH2426 and CR5527

CRT drivers

Schematic and Connection Diagrams

(One Section)

TL/H/11967±2

Top View

Order Number LM2427T

See NS Package Number MKT-TA12A

TL/H/11967±1

C 1995 National Semiconductor Corporation

TL/H/11967

RRD-B30M115/Printed in U. S. A.

Absolute Maximum Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Supply Voltage, V a

a 85V

Storage Temperature Range, T STG b 25 § Cto a 100 § C

Operating Temperature Range, T CASE b 20 § Cto a 90 § C

Lead Temperature (Soldering, 10 sec.)

300 § C

ESD Tolerance

TBD

Safe Operating Power Consumption

14W

Electrical Characteristics V ae 80V, R G e 430 X ,C1 e 47 pF, C L e 8 pF, 50 V PP output swing with 40V DC

offset. SeeFigure1.T CASE e 25 § C unless otherwise noted.

LM2427

Symbol

Parameter

Conditions

Units

Min Typical Max

I CC

Supply Current (per Amplifier) No Input or Output Load

30 mA

V INDC

Input Offset Voltage

1.4

1.6

1.8

V OUTDC Output Offset Voltage

t R

Rise Time

10% to 90% (Note 1)

3.5

t F

Fall Time

90% to 10% (Note 1)

3.5

A V

Voltage Gain

b 11 b 13 b 14 V/V

Linearity Error

V OUT from a 10V to a 70V (Note 2)

D A V

Gain Matching

(Note 3)

0.2

Note 1: Input signal: t r ,t f k 1.5 ns, f in e 1 MHz.

Note 2: Linearity error is defined as: The variation in small signal gain from a 20V to a 70V output with a 100 mVAC, 1 MHz, input signal.

Note 3: Calculated value from voltage gain test on each channel.

Typical Performance Characteristics

Typical Test Circuit (One Section)

TL/H/11967±3

Note: C L , total load capacltance, includes all parasitic capacitances.

FIGURE 1. Test Circuit (One Section)

This test circuit is used for both characteristic plots.

Typical Rise Time vs Capacitive Loading

Typical Fall Time vs Capacitive Loading

TL/H/11967±10

TL/H/11967±5

Test Circuit

Figure 1 shows a typical test circuit for evaluation of the

LM2427. This circuit is designed to allow testing of the

LM2427 in a 50 X environment, such as a pulse generator,

oscilloscope or network analyzer. The 4950 X resistor in se-

ries with the output of the LM2427 forms a 100:1 voltage

divider when connected to a 50 X -input oscilloscope or net-

work analyzer. To calibrate pulse generator, set to 2.4 V PP

into 50 X .

THEORY OF OPERATlON

The LM2427 is a triple channel transimpedance amplifier for

CRT's, suitable for SVGA, XGA, IBM and Macintosh display

resolution monitors. The LM2427 is pin-to-pin compatible

with the LH2426 and CR5527 CRT drivers. The device is

packaged in the industry standard 12-lead SIP TO-220

molded plastic power package. The heat sink is electrically

isolated and may be grounded for ease of manufacturing

and RFI/EMI shielding.

Applying an input current to the LM2427 will result in an

output voltage. An input current of about g 4.5 mA will pro-

vide a full output swing of g 25V. A resistor in series with the

input converts the device into a voltage amplifier; with a

resistor value of 430 X the voltage gain becomes b 13.

The LM2427 is a two stage amplifier configured in a push-

pull configuration (see schematic on front page). Q2 is bi-

ased by resistors R4 and R5, Q1 gets its bias through a

5700 X feedback resistor and the input biasing current. The

bases of Q1 and Q2 are capacitively coupled and, therefore,

Q2 will be actively driven.

The emitter resistors of Q1 and Q2 are bypassed with small

capacitors. This increases the gain of the stage for high

frequencies and increases the bandwidth of the amplifier.

Emitter followers Q3 and Q4 isolate the input stage from the

output capacitance load, and minimizes the circuit sensitivity

to load capacitance.

The power supply pin is intemally bypassed. If low frequen-

cies are present in the power supply line, an electrolytic

capacitor is recommended.

Bypassing the resistor with a capacitor of about 47 pF will

restore the rise and fall times but will result in some over-

shoot. (Figure2b)

Adding a resistor in series with the 47 pF capacitor will re-

duce the overshoot but also increases the rise and fall

times. (Figure2c)

The addition of a second capacitor offers a compromise

between the above networks by improving the rise and fall

times at the expense of some overshoot. (Figure2d)

Suggested values for the resistors and capacitors are

shown, however, optimum values may differ depending

upon the stray inductances and capacitances present in dif-

ferent board layouts.

Application Hints

The LM2427 is designed as a triple power amplifier for deliv-

ering red, green, and blue video signals to a cathode ray

tube (CRT). It can provide a 50V output swing and energize

a 12 ns pixel at a CRT cathode with 8 pF of capacitance.

As with any CRT driver, when designing a video amplifier

board with the LM2427, careful attention should be paid at

reducing stray capacitance along the entire video signal

path. This is especially important in the path between the

output of the CRT driver and the cathodes, because any

additional capacitance load will increase rise and fall times

and will result in reduced picture quality.

INPUT NETWORKS

The voltage gain and the response of the amplifiers can be

set by adding an R-C network to the input.

A 430 X resistor in series with the input will set the voltage

gain to b 13, but this will increase the rise and fall times of

the system (see Figure2a).

TL/H/11967±6

FIGURE 2. Influence of Input Networks

on Switching Performance

Application Hints (Continued)

TILT AND OVERSHOOT COMPENSATlON

When a low frequency square is displayed on a monitor

screen, some tilt may appear on the video signal due to the

large power and thermal dissipation changes in the input

transistors. This problem is illustrated in Figure3.

PROTECTlNG AMPLIFIER OUTPUT

FROM TUBE ARCING

During normal CRT operation, internal arcing may occasion-

ally occur. Spark gap protectors will limit the maximum volt-

age, but to a value that is much higher than allowable on the

LM2427. This fast, high voltage, high energy pulse can dam-

age the LM2427 output stage. The addition of two current

limiting resistors of 50 X to 100 X total, and clamping diodes

D1 and D2, will provide protection but will slow down the

response. The diodes should have a fast transient re-

sponse, high peak current rating, low series impedance and

low shunt capacitance. FDH400 or equivalent diodes are

recommended. Adding a series peaking inductor of 100 nH

to 150 nH will restore the bandwidth and provide additional

protection. (See Figure5)

The value of the inductor can be calculated from:

Lp e (R O a R 1 a R 2 ) 2

2.4 C L

where C L is the total load and R O is the intrinsic high fre-

quency output resistance of the amplifier, generally 160 X .

TL/H/11967±7

FIGURE 3. Tilt on a Low Frequency

Signal and Its Effects

The tilt can be compensated by adding an external RC feed-

back network as shown in Figure4. The RC feedback helps

by reducing the gain of the amplifier during the edge tran-

sition for a duration corresponding to u . The values of R and

C should be selected so that the gain is reduced ( D V e 0)

for the duration of the tilt ( u ).

TL/H/11967±9

FIGURE 5. One Section of the LM2427

with Tilt Compensation, Arc Protection

and Peaking Inductance L P in the Output

SHORT CIRCUIT PROTECTION

WARNING!

To provide maximum output speed, the LM2427 does not

have short circuit protection. Shorting the output can de-

stroy the device.

SUPPLY BYPASSlNG

Although the LM2427 has internal supply bypassing, some

values of supply line inductance can cause ringing in the

supply lines. If this occurs, an additional bypass capacitor or

a low-pass filter should be placed as close as possible to

the supply (V a ) pins of the LM2427.

CAPACITlVE LOADS

The LM2427 is designed to drive capacitive loads, however,

the very high output slew rate of about 13,700 V/ m s can

result in charging currents of over 200 mA into a 20 pF load.

These very high currents can damage the output transistors.

HEAT SlNKlNG

Power consumption by the LM2427 will depend on the sup-

ply voltage used, the output loading, the peak-to-peak out-

put swing and the operating frequency. Since the LM2427

will dissipate up to 14W, an external heatsink is always re-

quired. The maximum allowed case temperature is 90 § C. To

calculate maximum heatsink thermal resistance, use the fol-

lowing formula:

TL/H/11967±8

FIGURE 4. RC Feedback Network for Tilt Compensation

To find the value of resistor R, the following formula can be

used:

(100 b x%)

x% R F

where x% is the percentage value of D V to the peak-to-

peak output swing (V PP ). R F is internally fixed to 5700 X .

The value of capacitor C is determined by:

C e u /R

where u is the duration of the tilt.

For optimum results in a specific application, the values for

R and C may need to be tested and adjusted in the given

application board.

(90 § C b Max Ambient)

R th e

R e

PC BOARD LAYOUT CONSIDERATIONS

Input pins 2, 6 and 10 are amplifier summing junctions. All

connections to these points should be as short as possible

and should be separated from other signals. The compo-

nents connected to these pins should be located close to

the LM2427, and the total conductor length connected to

these points should be no more than one inch.

For optimum performance, an adequate ground plane, isola-

tion between channels, good supply bypassing and minimiz-

ing unwanted feedback are necessary. Also, the length of

the signal traces from the preamplifier to the LM2427 and

from the LM2427 to the CRT cathode should be as short as

possible. The following references are recommended:

Ott, Henry W., ``NoiseReductionTechniques inElectronic

Systems'', John Wiley & Sons, New York, 1976.

``GuidetoCRTVideoDesign'', National Semiconductor Ap-

plication Note 861.

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