Test Circuit)

Unless otherwise noted: V

HF IMPEDANCE.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

I

G1-H

Typical G1 High Output Voltage

V

49

V

DC

V

G1-L

Typical G1 Low Output Voltage

V

6

V

DC

V

G1PP

Typical G1 Output Swing

43

V

P-P

V

G1TH

G1 Blanking Input Threshold

0.9

V

V

120BOOST

Typical 120V Boost Supply

123

V

I

BOOST

Typical 120V Boost Charge Current

50

mA

120V Boost/G1 Drive Test Circuit

Theory of Operation

The LM2453 is a high voltage monolithic three channel CRT

driver and three-channel DC clamp circuit suitable for high-

resolution display applications. The device also contains a

G1 blanking signal driver, which also can be used to gener-

ate a 120 VDC supply to power the DC clamp circuit. The

LM2453 operates using 80V and 8V power supplies. An ex-

ternal supply can also be used to power the clamp circuit if

the user desires to do so. The part is housed in the industry

standard 15-lead TO-220 molded plastic power package.

A simplified circuit block diagram of the LM2453 is shown in

Figure 1. A reference supply of approximately 1.7 VDC is

supplied to pin 9 of the device. This provides a reference

level for the minus input of the video and clamp buffer ampli-

fiers as well as for the vertical blank comparator. The output

of the buffer amplifier drives the video amplifier which pro-

vides a gain of -52 to the input signal and only amplifies the

portion of the input signal that is above the reference volt-

age. When the input signal is at or below V

puts are driven to the +80V supply rail. The output of the

driver is a low impedance complimentary emitter follower cir-

cuit that minimizes the effect of capacitive load variations on

transient response.

The clamp amplifier provides a gain of 73 and only amplifies

the portion of the input signal that is below the reference volt-

age. When the input signal is at or above V

outputs are driven to the V

blank comparator drives the boost circuit, which generates a

40 V

loads. When the first vertical blanking pulse is received, the

G1 drive output enters a current limited latched state until the

120V boost capacitor is fully charged. Thereafter, the G1

output pulses in response to the vertical blanking pulse re-

ceived on the multiplexed V

Figures 4, 11, 13 show the typical test circuits for evaluation

of the LM2453.

Application Hints

INTRODUCTION

National Semiconductor (NSC) is committed to provide ap-

plication information that assists our customers in obtaining

the best performance possible from our products. The follow-

ing information is provided in order to support this commit-

ment. The reader should be aware that the optimization of

performance was done using a specific printed circuit board

designed at NSC. Variations in performance can be realized

due to physical changes in the printed circuit board and the

application. Therefore, the designer should know that com-

ponent value changes might be required in order to optimize

performance in a given application. The values shown in this

document can be used as a starting point for evaluation pur-

poses. When working with high bandwidth circuits, good lay-

out practices are always critical to achieving maximum per-

formance.

DS101302-13

FIGURE 13. 120V Boost/G1 Drive Test Circuit

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