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參數(shù)資料
| 型號： | LM2457 |
| 文件頁數(shù)： | 6/14頁 |
| 文件大?。?/td> | 823K |
| 代理商： | LM2457 |
G1 DRIVER/BOOST Electrical Characteristic Targets and Limits
(See Figure 13 for
Test Circuit)
Unless otherwise noted: V
CC
= +80V, V
BB
= +8V, T
C
= 50C, V
REF
= 1.735V. SW1, SW2 open. HEATSINK MUST BE
GROUNDED.
Symbol
V
G1-H
V
G1-L
V
G1PP
V
120BOOST
I
BOOST
Note 7:
Input from signal generator: V
HIGH
>
4.3V
DC
, V
LOW
<
3.0 V
DC
;
Pulse:
t
HIGH
= 300 μs, t
LOW
= 10 ms. Rise/fall time
<
0.1 μs.
Parameter
Conditions
Min
Typ
79
36
43
122
40
Max
Units
V
DC
V
DC
V
P-P
V
mA
Typical G1 High Output Voltage
Typical G1 Low Output Voltage
Typical G1 Output P-P
Typical 120V Boost Supply
Typical 120V Boost Current
V
BLANK
Input High, See (Note 7).
V
BLANK
Input Low, See (Note 7).
(Note 7)
(Note 7)
(Note 7)
120V Boost/G1 Drive Test Circuit
Theory of Operation
The LM2457 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
LM2457 operates using 80V and 8V power supplies. An
external 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 LM2457 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
amplifiers as well as for the vertical blank comparator. The
output of the buffer amplifier drives the video amplifier which
provides a gain of -52 to the input signal and only amplifies
the portion of the input signal that is above the reference
voltage. When the input signal is at or below V
the video
outputs are driven to the +80V supply rail. The output of the
driver is a low impedance complimentary emitter follower
circuit that minimizes the effect of capacitive load variations
on transient response.
The clamp amplifier provides a gain of 75 and only amplifies
the portion of the input signal that is below the reference
voltage. When the input signal is at or above V
the clamp
outputs are driven to the V
/120V supply rail. The vertical
blank comparator drives the boost circuit, which generates a
43V
p-p
signal that can directly drive typical G1 capacitive
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
received on the multiplexed V
REF
signal line.
Figures 4, 11, 13 show the typical test circuits for evaluation
of the LM2457.
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 fol-
lowing information is provided in order to support this com-
mitment. 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
purposes. When working with high bandwidth circuits, good
layout practices are always critical to achieving maximum
performance.
DS101379-13
FIGURE 13. 120V Boost/G1 Drive Test Circuit
L
www.national.com
6
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