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LM2432 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No. de pieza LM2432
Descripción Electrónicos  220V Monolithic Single Channel 37 MHz HDTV CRT Driver
Download  13 Pages
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Fabricante Electrónico  NSC [National Semiconductor (TI)]
Página de inicio  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2432 Datasheet(HTML) 6 Page - National Semiconductor (TI)

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Theory of Operation
The LM2432 is a high voltage monolithic single channel CRT
driver suitable for HDTV applications. The LM2432 typically
operates with 220V and 12V power supplies. The part is
housed in a staggered 7-lead TO-220 molded plastic power
package.
The circuit diagram of the LM2432 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.
The LM2432 has an IK current output (pin 5) that produces a
replica of the actual cathode current into V
OUT (pin 2). The IK
output pin is internally connected to the collector of Q4. If IK
feedback is not used in the application, the IK pin should be
connected to the same ground as pin 3 (GND). Otherwise,
the IK output can interface with a RGB video processor with
IK feedback for automatic cathode calibration. Note: During
the non-blanking period, video current levels can be as high
as several mA, which is much higher than the reference
currents (in µA range) produced during the IK measurement
interval. These high currents have the potential to produce
large voltages at the IK output pin. To avoid damage to Q4,
the IK output voltage, V
IK, must not exceed +16V (VIKMAX).
Please see the section “Cathode Current Output for IK Feed-
back Systems” for more information on the usage and pro-
tection of the IK output.
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 LM2432 performance is targeted for the HDTV market.
The application circuits shown in this document to optimize
performance and to protect against damage from CRT ar-
cover are designed specifically for the LM2432. If another
member of the NSC DTV CRT Driver family is used, please
refer to its data sheet.
POWER SUPPLY BYPASS
Since the LM2432 is a wide bandwidth amplifier, proper
power supply bypassing is critical for optimum performance
and for robustness against arcover. Improper power supply
bypassing can result in large overshoot, ringing or oscilla-
tion, and even arcover failure. 0.1 µF capacitors should be
connected from the supply pins, V
CC and VBB, to ground
using very short traces. Additionally, a 10 µF or larger elec-
trolytic capacitor should be connected from both supply pins
to ground reasonably close to the LM2432.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. This fast, high voltage, high energy pulse can
damage the LM2432 output stage since it is DC coupled to
the cathode. In a DC coupled application, an external spark
gap with an arcover voltage rating of 200 to 300V
DC on the
cathode is NOT recommended. The internal CRT socket
spark gap (1 to 2 kV
DC rating) can sufficiently reduce the
initial arcover voltage seen at the cathode. The output circuit
shown in Figure 13 is designed to help clamp the voltage at
the output of the LM2432 to a safe level during an arcover.
External arc protection clamp diodes, D1 and D2, should
have a fast transient response, high peak current rating, low
series impedance and low shunt capacitance. 1SS83 or
equivalent diodes like BAV21 are recommended. D1 and D2
should have short, low impedance connections to V
CC and
ground respectively. The cathode of D1 should have a very
short connection to a separate V
CC bypass capacitor, C3.
The ground connection of D2 and the C3 should have a
short, direct path to ground. This will significantly reduce the
high frequency voltage transients that the LM2432 would be
subjected to during an arcover.
Resistor R2, which limits the arcover current that is seen by
the diodes, should be a 12W solid carbon type resistor. R1
limits the current into the LM2432 as well as the voltage
stress at the outputs of the device and can be a 14W metal or
carbon film type resistor. Having large value resistors for R1
and R2 would be desirable, but this has the effect of increas-
ing rise and fall times. Inductor L1 is critical to reduce the
initial high frequency voltage levels that the LM2432 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.
www.national.com
6


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