lm2433 National Semiconductor Corporation, lm2433 Datasheet - Page 6

• Manufacturer Part Number: lm2433
• Description: 220v Monolithic Single Channel 16 Mhz Edtv Crt Driver
• Manufacturer: National Semiconductor Corporation

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Theory of Operation
The LM2433 is a high voltage monolithic single channel CRT
driver suitable for EDTV applications. The LM2433 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 LM2433 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 emitter of Q1 is accessible via pin 6 and enables a
higher voltage gain, if needed. The gain improvement can be
helpful in applications where a channel’s gain needs to be
increased to compensate for a limited preamplifier drive
register adjustment. An external resistor, R
nected between pin 6 and GND, so it’s in parallel with
internal emitter resistor, R2. A properly chosen resistor will
decrease the total emitter resistance to produce a higher
voltage gain. Figure 4 shows typical transfer curves for
various values of R
new voltage gain, A
R
application does not require higher gain, then pin 6 should
be left floating.
The LM2433 has an IK current output (pin 5) that produces a
replica of the actual cathode current into V
output pin is internally connected to the collector of Q4. The
IK output can be connected through an interface circuit to a
RGB video processor with IK feedback for automatic cath-
ode 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 pin should be
protected with a clamp diode to V
not exceed +16V (V
Current Output for IK Feedback Systems” for more informa-
tion on the usage and protection of the IK output. If the IK
output is not used in the application, it should be connected
to the same ground as pin 3 (GND).
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
EM
= 10.6 / ( |A
EM
V
IK MAX
’| – 53 ), where R
. To determine the value of R
V
’, use the following approximation:
). Please see the section “Cathode
BB
so it’s voltage, V
EM
OUT
EM
is in kΩ. If the
(pin 2). The IK
, can be con-
EM
IK
for the
, does
6
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 LM2433 performance is targeted for the EDTV 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 LM2433. If another
member of NSC’s DTV CRT Driver family is used, please
refer to its data sheet.
POWER SUPPLY BYPASS
Since the LM2433 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
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 LM2433.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. This fast, high voltage, high energy pulse can
damage the LM2433 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
cathode is NOT recommended. The internal CRT socket
spark gap (1 to 2 kV
initial arcover voltage seen at the cathode. The output circuit
shown in Figure 12 is designed to help clamp the voltage at
the output of the LM2433 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
ground respectively. The cathode of D1 should have a very
short connection to a separate V
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 LM2433 would be
subjected to during an arcover.
Resistor R2, which limits the arcover current that is seen by
the diodes, should be a
limits the current into the LM2433 as well as the voltage
stress at the outputs of the device and can be a
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 LM2433 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 12 .
DC
1
rating) can sufficiently reduce the
⁄
2
W solid carbon type resistor. R1
CC
CC
bypass capacitor, C3.
and V
BB
1
⁄
, to ground
4
W metal or
DC
CC
on the
and