LM2419 National Semiconductor, LM2419 Datasheet - Page 4

• Manufacturer Part Number: LM2419
• Description: Triple 65 MHz CRT Driver
• Manufacturer: National Semiconductor

ure 3 ) will reduce the rise and fall times when driving the
Figure 6 ) the overshoot is 10% with 12 pF capacitive load
Application Hints
IMPROVING RISE AND FALL TIMES
Because of an emitter follower output stage the rise and fall
times of the LM2419 are relatively unaffected by capacitive
loading However the series resistors R1 and R2 (see Fig-
CRT’s cathode which appears as a capacitive load The ca-
pacitance at the cathode typically ranges from 8 pF to
12 pF
To improve the rise and fall times at the cathode a small
inductor is often used in series with the output of the amplifi-
er The inductor L1 in Figure 3 peaks the amplifier’s fre-
quency response at the cathode thus improving rise and fall
times The inductor value is empirically determined and is
dependent on the load An inductor value of 0 1
good starting value Note that peaking the amplifier’s fre-
quency response will increase the overshoot
REDUCING OVERSHOOT
LM2419’s overshoot is a function of both the input signal
rise and fall times and the capacitive loading The overshoot
is increased by either more capacitive loading or faster rise
and fall times of the input signal
Table I shows the overshoot for a typical device with differ-
ent capacitive loads and different input signal rise and fall
times As can be observed from Table I overshoot is large
for large capacitive loads and faster input signal rise and fall
times In an actual application the LM2419 is driven from a
preamplifier with rise and fall times of 3 ns to 7 ns When
driven from LM1203 preamplifier (see application circuit
The overshoot can be reduced by including a resistor in
series with LM2419’s output as in Figure 3 Larger value
resistors for R1 and R2 would reduce overshoot but this
also increases the rise and fall times at the output Frequen-
cy peaking using an inductor in series with the output may
restore the bandwidth
(Continued)
FIGURE 3 Typical Application Circuit (One Channel)
H is a
4
Figure 4 shows LM2419’s gain versus output DC level A
Figure 4 the amplifier’s gain is constant at approximately
ure 4 ) For optimum performance it is recommended that
GAIN VS OUTPUT DC LEVEL
100 mV
the input signal’s DC level is swept As can be seen from
15 4 (V
and 65V Thus the amplifier’s output response is linear for
output voltage between 35V and 65V If the output voltage
is less than 35V or more than 70V the amplifier’s output
response becomes non-linear (note the change in gain Fig-
LM2419’s output low voltage be at 25V or above For a
50 V
signal swing from 25V to 75V LM2419’s linearity error is
measured at 8%
Input Signal
PP
FIGURE 4 Gain vs V
1 2 ns
t
7 ns
vs Capacitive Loading for a Typical Device
OUT
r
swing the output high voltage is 75V With an output
PP
t
f
AC signal is applied at the LM2419’s input and
Table I LM2419 Output Overshoot
e
1 54 V
5 pF
4%
4%
PP
) for output DC level between 35V
OUT
8 pF
6%
5%
(DC) V
C
IN
L
11 pF
7%
6%
e
100 mV
TL H 11442– 8
TL H 11442 – 9
15 pF
PP
8%
7%