M
Low-Cost, High-Resolution, 200MHz
Video CRT Driver
4
_______________________________________________________________________________________
__________Applic ations Information
Differential Inputs
VIN+ and VIN- are differential video input pins designed
to allow DC coupling of a 0V to +1V signal into VIN+, with
respect to VIN-. For correct operation, it is recommend-
ed that the signals applied to these inputs be kept within
±1V, with respect to ground. Although large signals and
offsets can be handled safely without damage, exceed-
ing these limits may cause output linearity to suffer.
Contrast Control
The contrast control is the overall DC-gain control that
will vary the voltage gain from 0V/V to -90V/V (with a
200
load resistor). An internal reference supply pin,
VREF, provides the nominal 5.5V needed to drive the
contrast input. Normally, a 5k
potentiometer between
VREF and ground is used to vary the contrast, but an
external source can be used instead of VREF, with some
degradation of gain stability with temperature.
The contrast control is a linear relationship. Vary the
input from 0V to 5V to achieve a voltage-gain range of
0V/V to -90V/V. This yields the following relationship for
overall voltage gain of this device (for IOUT < 250mA):
V
AA
- V
O
=
[V
IN
(Gm) + V
OFFSET
(0.02)] (R
L
)
V
AA
- V
O
= [V
IN
(V
CONTRAST
) (0.09) + V
OFFSET
(0.02)] (R
L
)
The MAX445’s overall gain can vary by ±20% due to
normal process variations of internal components. Also,
if multiple devices are used in a system, all devices
must track thermally (i.e., a common heatsink).
Offset Control
The offset control is used to set the output quiescent
current from 5mA to 110mA (typ) when the control input
is adjusted from 0V to 5V. Normally, offset is adjusted
using a 5k
potentiometer between VREF and ground.
Blank Control
When asserted (BLANK = TTL high), this input will dis-
able the video signal and allow the output to rise to the
V
AA
supply independent of offset control.
Bandgap Referenc e
VREF is a bandgap bias reference for easy adjustment
of the offset and contrast inputs. This reference has a
nominal output voltage of 5.5V ±5% that can source up
to 4mA.
MAX445
0.1
μ
F
3
V
EE
(-10.5V)
5k
+10V
D1
1N4152
50V
25
24
50
0.1
μ
F
0.1
μ
F
4
IOUT
D4
7
CONTRAST
VIN+
OFFSET
5k
-10.5V
ANALOG
INPUT
BLANK
GNDA
5
GND
24*
2
10
17
16
VREF
6
VIN-
11
BLANK
V
EE
8
BEAD*
V
EE
9
V
EEO
21
0.1
μ
F
0.1
μ
F
BEAD*
10pF
19
L2
NOTES:
C
L
COMBINES CRT CATHODE, AND PARASITIC C.
D4 (PHILIPS BAV20 OR HITACHI 1SS91) IS ARC PROTECTION DIODE.
SEE APPLICATIONS INFORMATIONSECTION.
L1, L2, L3, AND CB ARE
ELEMENTS OF THE PEAKING COIL.
L
IS THE TOTAL INDUCTANCE
TOTHE CATHODE. R
IS A SERIES
ARC PROTECTION ELEMENT.
L3
L1
L
S
R
L
200
10W
R
S
100
W (CARBON)
D2
1N4152
50V
D3
1N486A
100V
22
μ
F
100V
CB
V
(+75V)
C
R
1
2
C
L
V
CC
VCB
VCB
* STACK POLE 57-0180 OR
INDIANA GENERAL F-1650-H
Figure 1. Typical Connection Diagram