LM2406.pdf

June 1995

LM2406

Monolithic Triple 9 ns CRT Driver

General Description

The LM2406 is an integrated high voltage CRT driver circuit

designed for use in color monitor applications. The IC con-

tains three high input impedance, wide band amplifiers

which directly drive the RGB cathodes of a CRT. The gain of

each channel is internally set at b 14.5 and can drive CRT

capacitive loads as well as resistive loads presented by oth-

er applications, limited only by the package's power dissipa-

tion.

The IC is packaged in an industry standard 11 lead TO-220

molded plastic power package. See thermal considerations

on page 4.

Features

Y Output swing capability: 50 V PP for V CC e 80

40 V PP for V CC e 70

30 V PP for V CC e 60

Y Pinout designed for easy PCB layout

Y 1V to 7V input range

Y Stable with 0 pF±20 pF capacitive loads

Y Convenient TO-220 staggered lead style package

Applications

Y CRT driver for 1024 x 768 (Non-interlaced) and SVGA

display resolution color monitors

Y Pixel clock frequency up to 80 MHz

Schematic and Connection Diagrams

TL/H/12327±2

Note: Tab is at GND

Top View

TL/H/12327±1

FIGURE 1. Simplified Schematic Diagram (One Channel)

C 1995 National Semiconductor Corporation

TL/H/12327

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

Absolute Maximum Ratings (Notes 1, 3)

Supply Voltage, V CC a 95V

Bias Voltage, V BIAS a 16V

Input Voltage, V IN b 0.5V to V BIAS a 0.5V

Storage Temperature Range, T STG b 65 § Cto a 150 § C

Lead Temperature (Soldering, k 10 sec.)

300 § C

Operating Range (Note 2)

V CC a 60V to a 85V

V BIAS a 8V to a 15V

V IN a 1V to a 7V

Case Temperature, T CASE b 20 § Cto a 100 § C

Do not operate the part without a heat sink

ESD Tolerance

2 kV

Electrical Characteristics

Unless otherwise noted: V CC ea 80V, V BIAS ea 12V, V IN ea 3.3V, C L e 8 pF, Output e 40 V PP at 1 MHz, T A e 25 § C.

LM2406

Symbol

Parameter

Conditions

Units

Min

Typical Max

I CC

Supply Current (Per Channel) No Output Load

V OUT

Output Voltage

No Input Signal

V DC

t R

Rise Time

10%±90%, f e 1 MHz

t F

Fall Time

90%±10%, f e 1 MHz

A V

Voltage Gain

b 13 b 14.5 b 16

V/V

Linearity Error

(Note 4)

D A V

Gain Matching

(Note 5)

1.0

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.

Note 2: Operating ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and

test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may

change when the device is not operated under the listed test conditions.

Note 3: All voltages are measured with respect to GND, unless otherwise specified.

Note 4: Linearity Error is defined as the variation in small signal gain from a 30V to a 70V output with a 100 mV AC, 10 kHz input signal.

Note 5: Calculated value from Voltage Gain test on each channel.

Note 6: Input signal, V IN :t r ,t f k 2 nS.

AC Test Circuit

TL/H/12327±3

Note: 8 pF is total load including parasitic capacitance.

FIGURE 2. Test Circuit (One Channel)

Figure2shows a typical test circuit for evaluation of the LM2406. This circuit is designed to allow testing of the LM2406 in a 50 X

environment, such as a pulse generator, oscilloscope or network analyzer. The two series resistors at the output form a 100:1

voltage divider when connected to a 50 X load.

TL/H/12327±4

TL/H/12327±5

FIGURE 4. Power Dissipation vs V CC

FIGURE 3. V OUT vs V IN

TL/H/12327±7

TL/H/12327±6

FIGURE 5. Large Signal Frequency Response

FIGURE 6. Pulse Response

Theory of Operation

The LM2406 is a high voltage monolithic triple CRT driver

suitable for VGA and SVGA display applications. The

LM2406 features a 80V operation and low power dissipa-

tion. The part is housed in the industry standard 11-lead

TO-220 molded plastic power package.

The circuit diagram of the LM2406 is shown in Figure1. Q1

and R2 provide a conversion of input voltage to current,

while Q2 acts as a common base or cascode amplifier stage

to drive the load resistor R1. Emitter followers Q3 and Q4

isolate the impedance of R1 from the capacitance of the

CRT cathode, and make the circuit relative insensitive to

load capacitance. The gain of this circuit is b R1/(R2 ll R3)

and is fixed at b 14.5. The bandwidth of the circuit is set by

the collector time constant formed by the resistor R1 and

associated capacitance of Q2, Q3, Q4 and stray layout ca-

pacitance. Transistor Q6 and resistors R7 and R8 provide

biasing to the output emitter-follower stage to reduce cross-

over distortion at low signal levels, while R3 provides a DC

bias offset to match the output level characteristics of the

preamplifier stage.

Figure 2 shows a typical test circuit for evaluation of the

LM2406. This circuit is designed to allow testing of the

LM2406 in a 50 X environment, such as a pulse generator

and a scope, or a network analyzer. In this test circuit, two

resistors in series totaling 4.95 k X form a 100:1 wideband

low capacitance probe when connected to a 50 X cable and

load. The input signal from the generator is AC coupled to

the base of Q5, while a DC bias of a 12V is applied to the

base of Q2 (See Figure2).

(/4 × from the supply pin). Additionally,

a10 m F±100 m F electrolytic capacitor should be connected

from the supply pin to ground. The electrolytic capacitor

should also be placed reasonably close to the LM2406's

supply pin. A 0.1 m F capacitor should be connected from

the bias pin to ground, as close as is practical to the

LM2406.

The LM2406 is short circuit proof to momentary shorts to

ground ( k 1 sec.).

ARC PROTECTION

During normal CRT operation, internal arcing may occasion-

ally occur. Spark gaps of 200V±300V at the cathodes will

limit the maximum voltage, but to a value that is much high-

er than allowable on the LM2406. This fast, high voltage,

high energy pulse can damage the LM2406 output stage.

The addition of clamp diodes D1 and D2 (as shown in Fig-

ure 7) will help clamp the voltage at the output of the

LM2406 to a safe level. The clamp diodes should have a

fast transient response, high peak current rating, low series

impedance and low shunt capacitance. FDH400 or equiva-

lent diodes are recommended. Resistor R2 in Figure7 limits

the arcover current while R1 limits the current into the

LM2406 and reduces the power dissipation of the output

transistors when the output is stressed beyond the supply

voltage. Having large value resistors for R1 and R2 would

be desirable, but this has the effect of increasing rise and

fall times.

IMPROVING RISE AND FALL TIMES

Because of an emitter follower output stage, the rise and fall

times of the LM2406 are relatively insensitive to capacitive

loading. However, the series resistors R1 and R2 (see Fig-

ure7) will increase the rise and fall times when driving the

CRT's cathode which appears as a capacitive load. The ca-

pacitance at the cathode typically ranges from 8 pF±12 pF.

To improve the rise and fall times at the cathode, a small

inductor is often used in series with the output of the amplifi-

er. The inductor L P in Figure7peaks the amplifiers frequen-

cy response at the cathode, thus improving rise and fall

times The inductor value is empirically determined and is

dependent on the load. An inductor value of 0.1 m Hisa

good starting value. Note that peaking the amplifier's fre-

quency response will increase the overshoot.

THERMAL CONSIDERATIONS

Power supply current increases as the input signal increas-

es and consequently power dissipation also increases.

The LM2406 cannot be used without heat sinking. Typical

``average'' power dissipation with the device output voltage

at one half the supply voltage is 2.4W per channel for a total

dissipation of 7.2W package dissipation. Under white screen

conditions, i.e., 25V output, dissipation increases to 3.5W

per channel or 10.5W total. The LM2406 case temperature

must be maintained below 100 § C. If the maximum expected

ambient temperature is 50 § C, then a maximum heat sink

thermal resistance can be calculated:

R th e 100 § C b 50 § C

10.5W

e 4.76 § C/W.

This example assumes a typical CRT capacitive load and is

without a resistive load.

TYPICAL APPLICATION

A typical application of the LM2406 is shown in Figure 8.

Used in conjunction with an LM1207, a complete video

channel from monitor input to CRT cathode can be

achieved. Performance is satisfactory for all applications up

to 1024 x 768 non-interlaced.

TL/H/12327±8

FIGURE 7. One Section of the LM2406 with Arc Protection and Peaking Inductor L P

Application Hints

POWER SUPPLY BYPASS

Since the LM2406 is a wide bandwidth amplifier, proper

power supply bypassing is critical for optimum performance.

Improper power supply bypassing can result in large over-

shoot, ringing and oscillation. A 0.01 m F capacitor should be

connected as close to the supply pin, V a , as is practical

(preferably less than

TL/H/12327±9

Note: Unmarked capacitors 0.1 m F

FIGURE 8. Typical Application LM1207±LM2406

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