Monolithic Triple 7.5nS CRT Driver With Integrated
Clamp and G1 Blanking
The LM2457 driver is an integrated high voltage triple CRT
driver circuit designed for use in color monitor applications.
The input signal interface to the IC is a multiplexed signal
containing both clamp and video signal information, relative
to an external 1.7 VDC reference.
The IC contains three high gain, differential input, high input
impedance, wide band amplifiers which directly drive the
RGB cathodes of a CRT. Each channel has its gain internally
set to -52 and can drive CRT capacitive loads as well as
resistive loads present in other applications, limited only by
the package’s power dissipation.
Integrated with the driver is a triple video clamp circuit for DC
recovery of each of the AC coupled video outputs. The DC
clamp circuit amplifies the clamp signal that is multiplexed on
the video signal input. The DC clamp amplifiers are high
gain, differential input, high input impedance amplifiers, set-
ting a low impedance DC level at the clamp output which can
be used to restore the DC level of the cathode drive. Each
channel has a gain that is internally set to +75.
Also integrated within the package is a 43Vp-p vertical blank-
ing driver that is designed to drive the vertical retrace blank-
ing signal to the G1. This is a current limited, low impedance
output capable of driving normal G1 decoupling capaci-
tances via an external resistor.
The output of the G1 driver can also be used to drive a
voltage boost capacitor (22uF). When connected between
the G1 drive output and the 120v supply input pin, a 120v
boost supply is achieved which can be used to drive the
internal DC clamp circuit, thereby eliminating the require-
ment for a 120V clamp supply.
The IC is packaged in an industry standard 15 lead TO-220
molded plastic power package.
n 7.5ns low power driver
n Three integrated active clamp circuits
n Integrated 120V supply and G1 vertical blank drive
n Multiplexed signal interface to LM125X OSD Pre-amp
minimizes pin count while increasing functionality
n External System voltage reference
n Convenient TO-220 staggered lead package
n 1024 x 768 Resolution displays up to 85Hz refresh
n Pixel clock frequencies up to 95MHz
n Monitor requiring horizontal video blanking
FIGURE 1. LM2457 Block Diagram
© 2000 National Semiconductor Corporation DS101379
FIGURE 2. LM2457 Package Pinout
Order Number LM2457TA