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參數(shù)資料
| 型號(hào)： | LM2427T |
| 廠(chǎng)商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類(lèi)： | 音頻／視頻放大 |
| 英文描述： | Triple 80 MHz CRT Driver |
| 中文描述： | 3 CHANNEL, VIDEO AMPLIFIER, PSFM12 |
| 封裝： | POWER, PLASTIC, TO-220, 12 PIN |
| 文件頁(yè)數(shù)： | 3/6頁(yè) |
| 文件大?。?/td> | 117K |
| 代理商： | LM2427T |
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.
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 Figure 2a).
Bypassing the resistor with a capacitor of about 47 pF will
restore the rise and fall times but will result in some over-
shoot. (Figure 2b)
Adding a resistor in series with the 47 pF capacitor will re-
duce the overshoot but also increases the rise and fall
times. (Figure 2c)
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. (Figure 2d)
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.
TL/H/11967–6
FIGURE 2. Influence of Input Networks
on Switching Performance
3
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