lm2418 National Semiconductor Corporation, lm2418 Datasheet - Page 3

• Manufacturer Part Number: lm2418
• Description: Triple 30 Mhz Crt Driver
• Manufacturer: National Semiconductor Corporation

Figure 2 shows a typical test circuit for evaluation of the
LM2418 Theory of Operation
The LM2418 is a high voltage triple CRT driver suitable for
VGA display applications The LM2418 features 90V opera-
tion and low power dissipation The part is housed in the
industry standard 11-lead TO-220 molded power package
The heat sink is electrically isolated from the circuitry and
may be grounded for ease of manufacturing and RFI shield-
ing
The circuit diagram of the LM2418 is shown in Figure 1 Q1
and R2 provide a conversion of input voltage to current
while Q2 acts as a common base or cascode amplifier stage
to drive the load resistor R1 Emitter followers Q3 and Q4
isolate the impedance of R1 from the capacitance of the
CRT cathode and make the circuit relatively insensitive to
load capacitance The gain of this circuit is
fixed at
collector time constant formed by the load resistor R1 and
associated capacitance of Q2 Q3 Q4 and stray layout ca-
pacitance Diodes D1 and D2 provide forward bias to the
output stage to reduce crossover distortion at low signal
levels while R3 provides a DC bias offset to match the out-
put level characteristics of the LM1203 RGB Video Amplifier
System Proprietary transistor design allows for high band-
width with low operating power
LM2418 This circuit is designed to allow testing of the
LM2418 in a 50X environment such as a pulse generator
and a scope or a network analyzer In this test circuit two
resistors in series totaling 4 95 kX form a wideband low
b
FIGURE 4 LM2418 DC Characteristics
19 The bandwidth of the circuit is set by the
b
R1 R2 and is
TL K 11125 – 7
3
Figures 4 and 5 Power supply current increases as the in-
capacitance probe to match the output of the LM2418 to a
50X cable and load Typical AC performance of the circuit is
shown in Figure 3 The input signal is AC coupled to the
base of Q1 while a DC bias of 12V is applied to the base of
Q2 (See Figure 2 )
Thermal Considerations
The transfer characteristics of the amplifier are shown in
put signal increases and consequently power dissipation
also increases
The LM2418 cannot be used without heat sinking Figure 5
shows the power dissipated in each channel over the oper-
ating voltage range of the device Typical ‘‘average’’ power
dissipation with the device output voltage at one half the
supply voltage is 1 8W per channel for a total dissipation of
5 4W package dissipation Under white screen conditions
i e 20V output dissipation increases to 3 0W per channel
or 9W total The LM2418 case temperature must be main-
tained below 100 C If the maximum expected ambient tem-
perature is 60 C then a maximum heat sink thermal resist-
ance can be calculated
PRECAUTION
AGAINST OUTPUT SHORT CIRCUITS The minimum re-
sistance the LM2418 can drive is 800X to ground or V
FIGURE 5 LM2418 Output Swing
R
th
and Power Characteristics
e
THE LM2418 IS NOT PROTECTED
(100 C
9W
b
60 C)
e
4 4 C W
TL K 11125 – 9
a