LM2412T/NOPB National Semiconductor, LM2412T/NOPB Datasheet - Page 4

IC DRIVER MONOLITHIC TO-220-11

LM2412T/NOPB

Manufacturer Part Number
LM2412T/NOPB
Description
IC DRIVER MONOLITHIC TO-220-11
Manufacturer
National Semiconductor
Datasheet

Specifications of LM2412T/NOPB

Display Type
CRT
Current - Supply
21mA
Voltage - Supply
60 V ~ 85 V
Operating Temperature
-20°C ~ 100°C
Mounting Type
Through Hole
Package / Case
TO-220-11 (Bent and Staggered Leads)
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Interface
-
Configuration
-
Digits Or Characters
-
Other names
*LM2412T
*LM2412T/NOPB
LM2412T
www.national.com
Theory of Operation
The LM2412 is a high voltage monolithic three channel CRT
driver suitable for very high resolution display applications,
up to 1600 x 1200 at 85 Hz refresh rate. The LM2412
operates using 80V and 12V power supplies. The part is
housed in the industry standard 11-lead TO-220 molded
plastic power package.
The simplified circuit diagram of one channel of the LM2412
is shown in Figure 1 . A PNP emitter follower, Q5, provides
input buffering. This minimizes the current loading of the
video pre-amp. R9 is used to turn on Q5 when there is no
input. With Q5 turned on, Q1 will be almost completely off,
minimizing the current flow through Q1 and Q2. This will
drive the output stage near the V
power dissipation with no inputs. R6 is a pull-up resistor for
Q5 and also limits the current flow through Q5. R3 and R2
are used to set the current flow through Q1 and Q2. The ratio
of R1 to R2 is used to set the gain of the LM2412. R1, R2
and R3 are all related when calculating the output voltage of
the CRT driver. R
Q1 and Q2 are in a cascode configuration. Q1 is a low
voltage and very fast transistor. Q2 is a higher voltage
transistor. The cascode configuration gives the equivalent of
a very fast and high voltage transistor. The two output tran-
sistors, Q3 and Q4, form a class B amplifier output stage. R4
and R5 are used to limit the current through the output stage
and set the output impedance of the LM2412. Q6, along with
R7 and R8 set the bias current through Q3 and Q4 when
there is no change in the signal level. This bias current
minimizes the crossover distortion of the output stage. With
this bias current the output stage now becomes a class AB
amplifier with a crossover distortion much lower than a class
B amplifier.
Figure 2 shows a typical test circuit for evaluation of the
LM2412. Due to the very wide bandwidth of the LM2412, it is
highly recommended that the stand alone board suplied by
NSC be used for the evaluation of the CRT driver’s perfor-
mance. The 50
series resistor in the actual application. This resistor would
be part of the arc-over protection circuit. The input signal
from the generator is AC coupled to the input of the CRT
driver.
Application Hints
INTRODUCTION
National Semiconductor (NSC) is committed to providing
application information that assists our customers in obtain-
ing the best performance possible from our products. The
following information is provided in order to support this
commitment. The reader should be aware that the optimiza-
tion 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 component 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 performance.
b
resistor is used to duplicate the required
limits the current through the base of Q2.
CC
rail, minimizing the
4
POWER SUPPLY BYPASS
Since the LM2412 is a very high bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large over-
shoot, ringing and oscillation. A 0.1 µF capacitor should be
connected from the supply pin, V
the supply and ground pins as is practical. Additionally, a
10 µF to 100 µF electrolytic capacitor should be connected
from the supply pin to ground. The electrolytic capacitor
should also be placed reasonably close to the LM2412’s
supply and ground pins. A 0.1 µF capacitor should be con-
nected from the bias pin, V
practical to the part.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. Spark gaps, in the range of 200V, connected from
the CRT cathodes to CRT ground will limit the maximum
voltage, but to a value that is much higher than allowable on
the LM2412. This fast, high voltage, high energy pulse can
damage the LM2412 output stage. The application circuit
shown in Figure 9 is designed to help clamp the voltage at
the output of the LM2412 to a safe level. The clamp diodes
should have a fast transient response, high peak current
rating, low series impedance and low shunt capacitance.
FDH400 or equivalent diodes are recommended. D1 and D2
should have short, low impedance connections to V
ground respectively. The cathode of D1 should be located
very close to a separately decoupled bypass capacitor. The
ground connection of the diode and the decoupling capacitor
should be very close to the LM2412 ground. This will signifi-
cantly reduce the high frequency voltage transients that the
LM2412 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 LM2412 as well as
the voltage stress at the outputs of the device. R2 should be
a
carbon film type resistor. Inductor L1 is critical to reduce the
initial high frequency voltage levels that the LM2412 would
be subjected to during an arc-over. Having large value resis-
tors for R1 and R2 would be desirable, but this has the effect
of increasing rise and fall times. The inductor will not only
help protect the device but it will also help optimize rise and
fall times as well as minimize EMI. For proper arc protection,
it is important to not omit any of the arc protection compo-
nents shown in Figure 9 . The values of L1 and R1 may need
to be adjusted for a particular application. The recommended
minimum value for R1 is 75 , with L1 = .049 µH.
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 9 , there are three components (R1, R2
and L1) that can be adjusted to optimize the transient re-
sponse of the application circuit. Increasing the values of R1
1
2
W solid carbon type resistor. R1 can be a
FIGURE 9. One Channel of the LM2412 with the
Recommended Arc Protection Circuit.
BB
, to ground, as close as is
CC
, to ground, as close to
1
4
W metal or
DS101298-10
CC
and

Related parts for LM2412T/NOPB