lm2425 National Semiconductor Corporation, lm2425 Datasheet - Page 6

• Manufacturer Part Number: lm2425
• Description: 220v Monolithic Triple Channel 10 Mhz Crt Dtv Driver
• Manufacturer: National Semiconductor Corporation

www.national.com
Theory of Operation
The LM2425 is a high voltage monolithic three channel CRT
driver suitable for DTV applications. The LM2425 operates
with 220V and 12V power supplies. The part is housed in the
industry standard 11-lead TO-220 molded plastic power
package with thin leads for improved metal-to-metal spacing.
The circuit diagram of the LM2425 is shown in Figure 2 . The
PNP emitter follower, Q5, provides input buffering. Q1 and
Q2 form a fixed gain cascode amplifier with resistors R1 and
R2 setting the gain at −53. Emitter followers Q3 and Q4
isolate the high output impedance of the cascode stage from
the capacitance of the CRT cathode, which decreases the
sensitivity of the device to load capacitance. Q6 provides
biasing to the output emitter follower stage to reduce cross-
over distortion at low signal levels.
Figure 3 shows a typical test circuit for evaluation of the
LM2425. This circuit is designed to allow testing of the
LM2425 in a 50Ω environment without the use of an expen-
sive FET probe. In this test circuit, the two 4.99 kΩ resistors
form a 400:1 wideband, low capacitance probe when con-
nected to a 50Ω coaxial cable and a 50Ω load (such as a
50Ω oscilloscope input). The input signal from the generator
is ac coupled to the base of Q5.
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 may 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 also critical to achieving maximum per-
formance.
IMPORTANT INFORMATION
The LM2425 performance is targeted for the DTV market.
The application circuits shown in this document to optimize
performance and to protect against damage from CRT arc
over are designed specifically for the LM2425. If another
member of the LM242X family is used, please refer to its
datasheet.
POWER SUPPLY BYPASS
Since the LM2425 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 or oscillation. 0.1 µF capacitors should be
connected from the supply pins, V
close to the LM2425 as is practical. Additionally, a 22 µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2425.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. This fast, high voltage, high-energy pulse can
damage the LM2425 output stage. The application circuit
CC
and V
BB
, to ground, as
6
shown in Figure 13 is designed to help clamp the voltage at
the output of the LM2425 to a safe level. The clamp diodes,
D1 and D2, should have a fast transient response, high peak
current rating, low series impedance and low shunt capaci-
tance. 1SS83 or equivalent diodes are recommended. D1
and D2 should have short, low impedance connections to
V
located very close to a separately decoupled bypass capaci-
tor (C3 in Figure 13 ). The ground connection of D2 and the
decoupling capacitor should be very close to the LM2425
ground. This will significantly reduce the high frequency
voltage transients that the LM2425 would be subjected to
during an arc over condition. Resistor R2 limits the arc over
current that is seen by the diodes while R1 limits the current
into the LM2425 as well as the voltage stress at the outputs
of the device. R2 should be a
R1 can be a
large value resistors for R1 and R2 would be desirable, but
this has the effect of increasing rise and fall times. Inductor
L1 is critical to reduce the initial high frequency voltage
levels that the LM2425 would be subjected to. The inductor
will not only help protect the device but it will also help
minimize rise and fall times as well as minimize EMI. For
proper arc protection, it is important to not omit any of the arc
protection components shown in Figure 13 .
EFFECT OF LOAD CAPACITANCE
Figure 7 shows the effect of increased load capacitance on
the speed of the device. This demonstrates the importance
of knowing the load capacitance in the application. Increas-
ing the load capacitance from 10 pF to 20 pF adds about
12 ns to both the rise and fall times. It is important to keep
the board capacitance as low as possible to maximize the
speed of the driver.
EFFECT OF OFFSET
Figure 8 shows the variation in rise and fall times when the
output offset of the device is varied from 120V to 130V
The rise and fall times both show a variation of about 6%
relative to the center data point (125V
increases less than 2 ns when the offset is changed from
120V to 130V. The fall time has about a
from the 125V center point, increasing in speed with more in
offset voltage.
THERMAL CONSIDERATIONS
Figure 9 shows the performance of the LM2425 in the test
circuit shown in Figure 3 as a function of case temperature.
The figure shows that the rise time of the LM2425 increase
by approximately 19% as the case temperature increases
from 30˚C to 110˚C. This corresponds to a speed degrada-
CC
FIGURE 13. One Channel of the LM2425 with the
and ground respectively. The cathode of D1 should be
Recommended Application Circuit
1
⁄
4
W metal or carbon film type resistor. Having
1
⁄
2
W solid carbon type resistor.
DC
±
). The rise time
2 ns . Variation
20124113
DC
.