MSK1900 Datasheet, PDF (3/6 Pages) M.S. Kennedy Corporation – HIGH PERFORMANCE, HIGH VOLTAGE VIDEO DISPLAY DRIVER

APPLICATION NOTES
POWER SUPPLIES
The input stage of the MSK 1900 requires power supplies of
+10V and -10.5V for optimum operation. The negative power
supply can be increased to -12V if -10.5V is not available, but
additional power dissipation will cause the internal temperature
to rise. Both low voltage power supplies sould be effectively
decoupled with tantalum capacitors (at least 1µF) connected as
close to the amplifiers pins as possible. The MSK 1900 has
internal 0.01µF capacitors that also improve high frequency per-
formance. Additionally, it is also recommended to put 0.1µF
decoupling capacitors on the +10V and -10.5V supplies as
well.
The high voltage power supply (+VHV) is connected to the
amplifiers output stage and must be kept as stable as possible.
The internal Rp is connected to +VHV and as such, the amplifi-
ers DC output is directly related to the high voltage value. The
+VHV pin of the hybrid should be decoupled to ground with as
large a capacitor as possible to improve output stability.
VCB
The VCB input is the base connection to the output stage con-
sisting of a common base, high voltage stage and a high speed,
low voltage current amplifier in a cascode arrangement. This
input requires a very stable 10V DC nominal voltage. Any AC
signals at this point will be amplified and reflected in the output.
The PSRR of the output stage is directly related to the stability
of this VCB voltage.
SUPPLY SEQUENCING
The power supply sequence is +VHV, VCC, VEE followed by
the other DC control inputs. If power supply sequencing is not
possible, the time difference between each supply should be
less than five milliseconds. If the DC control signals are being
generated from a low impedance source other than the VREF
output, reverse biased diodes should be connected from each
input (VGAIN, VOFF) to the +VCC pin. This will protect the inputs
until +VCC is turned off.
VIDEO OUTPUT
When power is first applied and VIN=VGAIN=VOFF=0V, the
output will be practically at the +VHV rail voltage. The output
voltage is a function of the value of Rp and also the VGAIN and
VOFF DC inputs. The bandwidth of the amplifier largely depends
on both Rp and Lp. With +VHV set to +70V and total
Rp=200Ω, the device is capable of approximately 62Vpp total
output swing.
The MSK 1900 is conservatively specified with low values
for external Lp which yield about 5% overshoot. Additional
peaking can be obtained by using a high self-resonant frequency
inductor in series with +VHV pin. Since this value of inductance
can be very dependent on circuit layout, it is best to determine
its value by experimentation. A good starting point is typically
0.47µH for the MSK 1900.
VIDEO INPUTS
The analog inputs (±VIN) are designed to accept RS343
signals, ±0.714VPP, and operate properly with a common mode
range of ±0.5V with respect to ground. Therefore, it is recom-
mended that the input signal be limited to ±1.3V with respect
to ground, (signal + common mode). Although large offsets of
±2V (with respect to ground, signal included) can be tolerated
without damage to the hybrid, output linearity suffers and there-
fore it is not recommended.
OUTPUT PROTECTION
The output pin of the MSK 1900 can be protected from tran-
sients by connecting reversed biased ultra-low capacitance di-
odes from the output pin to both +VHV and ground. The output
can also be protected from arc voltages by inserting a small value
(50-100Ω) resistor in series with the amplifier. This resistor will
reduce system bandwidth along with the load capacitance, but a
series inductor can reduce the problem substantially.
VGAIN CONTROL INPUT
The VGAIN control (contrast) input is designed to allow the
user to vary the video gain. By simply applying a DC voltage
from 0V to VREF, the video gain can be linearly adjusted from 0
to 100V/V. The VGAIN input should be connected to the VREF
pin through a 5kΩ pot to ground. For convenient stable gain
adjustment, a 0.1µF bypass capacitor should be connected near
the VGAIN input pin to prevent output instability due to noisy
sources. Digital gain control can be accomplished by connect-
ing a D/A converter to the VGAIN pin. However, some tempera-
ture tracking performance may be lost when using an external
DC voltage source other than VREF for gain adjustment.
The overall video output of the MSK 1900 can be charac-
terized using the following expression:
Vpp=VHV-VOUT
VHV-VOUT=(VIN)(VGAIN)(Rp)(0.08)
Here is a sample calculation for the MSK1900:
Given information
VIN=0.7V
VGAIN=1VDC
Rp=200Ω (external)
VHV=70VDC
VHV-VOUT=(0.7V)(1V)(.08)(200Ω)
VHV-VOUT=11.2Vpp Nominal
The expected video output would swing from approximately
+70V to +58.8V assuming that VOFF=0V. This calculation
should be used as a nominal result because the overall gain may
vary as much as ±10% due to internal high speed device varia-
tions. Changing ambient conditions can also effect the video
gain of the amplifier slightly. It is wise to connect all video
amplifiers to a common heat sink to maximize thermal tracking
when multiple amplifiers are used in applications such as RGB
systems. Additionally, only one of the VREF outputs should be
shared by all three amplifiers. This voltage should be buffered
with a suitable low-drift op-amp for best tracking performance.
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Rev. B 9/99