19-0302; Rev 0; 9/94
Low-Cost, High-Resolution, 200MHz
Video CRT Driver
_______________General Description
The MAX445 is a high-performance, monolithic, variable-
gain transconductance amplifier with a high-voltage
open-collector output capable of directly driving a video
display (CRT cathode). A 2.5ns rise time is achieved
using a peaking network with a 200Ω load resistor and
an 8pF total load (CRT and parasitic capacitance).
Differential inputs and a linear adjustable gain stage
with an output offset adjustment make the versatile
MAX445 well suited for many video display applica-
tions. A buffered bandgap reference voltage is avail-
able for the gain (contrast) and offset adjustments
along with a TTL BLANK input to turn off the output cur-
rent, independent of signal input.
The MAX445 is available in a 24-pin power-tab DIP
package. A suitable heatsink must be attached to
maintain the junction temperature within the recom-
mended operating range.
____________________________Features
o
2.5ns Rise/Fall Time into an 8pF Load
o
200MHz Small-Signal Bandwidth
o
50Vp-p Output
o
Ground Referenced Differential Inputs
o
Linear Variable Gain for Contrast Control
o
Offset Adjustment for Black Level
o
5.5V Bandgap Reference
o
Drives 1280 x 1024 and 1530 x 1280 Displays
MAX445
______________Ordering Information
PART
MAX445CPG
MAX445C/D
TEMP. RANGE
0°C to +70°C*
0°C to +70°C**
PIN-PACKAGE
24 Power-Tab DIP
Dice
________________________Applications
CRT Driver for High-Resolution Monochrome and
Color Displays
High-Voltage, Variable-Gain Transconductance
Amplifier
* Case temperature range, T
CASE
= 0°C to +90°C. See Absolute
Maximum Ratings and Applications Information for thermal/heat
sink considerations.
**Dice are specified at T
J
= +25°C, DC parameters only.
________________Functional Diagram
V
AA
__________________Pin Configuration
TOP VIEW
R
L
V
OUT
IOUT
GND
VREF
OFFSET
CONTRAST
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
GND
GND
GND
V
EEO
N.C.
IOUT
N.C.
VCB
VCB
GND
GND
GND
CONTRAST
BLANK
MAX445
VIN+
PRE-AMP
VIN-
BAND
GAP
CURRENT
AMP
VCB
GNDA
VIN-
VIN+
V
EE
V
EE
MAX445
20
19
18
17
16
15
14
13
GNDA
V
CC
BLANK
GND
OFFSET
VREF
GND
Power-Tab DIP
________________________________________________________________
Maxim Integrated Products
1
Call toll free 1-800-998-8800 for free samples or literature.
Low-Cost, High-Resolution, 200MHz
Video CRT Driver
MAX445
ABSOLUTE MAXIMUM RATINGS
V
AA
Output Supply.................................................................80V
V
AA
Output Supply with Respect to VCB...............................70V
VCB Common-Base Supply ...................................................20V
V
CC
Positive Supply ............................................................12.5V
V
EE
Negative Supply..........................................................-12.5V
Differential Input Voltage..........................................................2V
Common-Mode Input Voltage................................................±2V
Contrast Input Voltage.................................................-1V to +6V
Offset Input Voltage.....................................................-1V to +6V
Blank Input Voltage .....................................................-1V to +6V
Bandgap-Reference Output Current ...................................-5mA
Continuous Power Dissipation
derate at 170mW/°C above T
CASE
= +90°C.......................10W
Operating Junction Temperature ......................-55°C to +150°C
Storage Temperature.........................................-55°C to +150°C
Lead Temperature (soldering, 10sec) .............................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
AA
= 20V, VCB = 10V ±0.5V, V
CC
= 10V ±0.5V, V
EE
= -10.5V ±0.5V, VIN = (VIN+) - (VIN-) = 0V, CONTRAST = 1.0V,
OFFSET = 1.0V, R
L
= 0Ω, BLANK = 0.4V, T
CASE
= +25°C, unless otherwise noted.)
PARAMETER
Output-Common-Base Supply Current
Positive Supply Current
Negative Supply Current
Power-Supply Rejection Ratio
Low Blank Input Bias Current
High Blank Input Bias Current
Contrast Input Bias Current
Offset Input Bias Current
VIN+ or VIN- Signal Input Current
Input Common-Mode Rejection Ratio
VIN+ or VIN- DC Input Impedance
VIN+ or VIN- Input Capacitance
Reference Output Voltage
Output Current (Blanked)
Output Current
Output Current Change vs. Temperature
Output Current Change vs. Contrast ADJ
Output Current Change vs. VIN, Blanked
SYMBOL
I
CB
I
CC
I
EE
PSRR
I
IL
IIH
IIC
IIB
IIS
CMRR
R
VIN
C
IN
V
REF
I
OUT
I
OUT
∆I
OUT
∆I
OUT
∆I
OUT
I
LOAD
= 2mA
BLANK = 2.4V, OFFSET = 1V, V
AA
= 75V
BLANK = 2.4V, OFFSET = 3V
OFFSET = 0V, CONTRAST = 4.0V
OFFSET = 5.0V, CONTRAST = 1V
T
C
= +25°C to +90°C
CONTRAST = 0V to 5V
BLANK = 2.4V, CONTRAST = 5.0V,
∆VIN-
= 0.3V
CONTRAST = 5.0V
Transconductance, I
OUT
to VIN
Amplifier Linearity Error (∆Gm/∆V
IN
)
Contrast Linearity Error (∆Gm/∆Contrast)
Bandwidth, 3dB
BW
Gm
CONTRAST = 1.0V
CONTRAST = 0V
CONTRAST = 4.0V, OFFSET = 1.0V
VIN = 0.2V, OFFSET = 0V
OFFSET = 0V, R
LOAD
= 100Ω
200
400
70
-25
-0.1
80
±3
±10
±1
600
120
25
±2
±3
%
%
MHz
mA/V
5.25
V
CM
= ±0.5V, CONTRAST = 5.0V
I
VEE
+ I
VEEO
V
CC
, V
EE
= ±5%, VIN = +250mV,
CONTRAST = 5.0V, referred to input
BLANK = 0.4V
BLANK = 2.4V
CONTRAST = 5.0V
OFFSET = 1.0V
-100
25
-0.6
-0.4
0
0
-50
36
10
2
5.75
±1
±1
25
140
0
0
10
10
50
CONDITIONS
MIN
TYP
MAX
40
70
UNITS
mA
mA
mA
dB
mA
mA
µA
µA
µA
dB
kΩ
pF
V
mA
mA
mA
mA
mA
2
_______________________________________________________________________________________
Low-Cost, High-Resolution, 200MHz
Video CRT Driver
ELECTRICAL CHARACTERISTICS (continued)
(V
AA
= 20V, VCB = 10V ±0.5V, V
CC
= 10V ±0.5V, V
EE
= -10.5V ±0.5V, VIN = (VIN+) - (VIN-) = 0V, CONTRAST = 1.0V,
OFFSET = 1.0V, R
L
= 0Ω, BLANK = 0.4V, T
CASE
= +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
R
L
= 200Ω,
C
L
= 8pF,
V
AA
= 75V,
t
r
(VIN) < 1ns,
No peaking,
OUTp-p = 50V
With peaking,
OUTp-p = 45V
MIN
TYP
3.6
ns
2.5
8
±2
ns
%
MAX
UNITS
MAX445
Rise/Fall Time (10% to 90%)
t
r
, t
f
Settling Time (90% to 100% ±2%)
Thermal Distortion
t
s
C
L
= 8pF, no peaking
______________________________________________________________Pin Description
PIN
1, 12, 13, 14,
15, 22, 23, 24
2
3
4
5
6
7
8, 9
10
11
16, 17
18, 20
19
21
NAME
GND
VREF
OFFSET
CONTRAST
GNDA
VIN-
VIN+
V
EE
V
CC
BLANK
VCB
N.C.
IOUT
V
EEO
FUNCTION
High-Current Ground. Connect all pins to ground plane.
Reference Output (+5.5V)
Output Voltage Offset-Adjustment Input
Output Gain-Adjustment Input
Pre-Amp Ground
Inverting Signal Input
Noninverting Signal Input
Negative Supply (-10.5V)
Positive Supply (+10V)
Blanking Input, TTL
Output Common-Base Supply (+10V)
No Connection—leave open
Open-Collector Current Output
Negative Supply for Output Stage (-10.5V)
_______________________________________________________________________________________
3
Low-Cost, High-Resolution, 200MHz
Video CRT Driver
MAX445
__________Applications 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.
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 5kΩ potentiometer between VREF and ground.
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
200Ω load resistor). An internal reference supply pin,
VREF, provides the nominal 5.5V needed to drive the
contrast input. Normally, a 5kΩ 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):
+10V
V
EE
(-10.5V)
D1
1N4152
50V
D2
1N4152
50V
D3
1N486A
100V
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 Reference
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.
BEAD*
24Ω
V
AA
(+75V)
0.1µF
10
V
CC
2
VREF
0.1µF
3
5k
D4
0.1µF
4
5k
ANALOG
INPUT
6
25Ω
7
VIN-
VIN+
NOTES:
C
L
COMBINES CRT CATHODE, AND PARASITIC C.
BLANK
GNDA
GND
5
24*
V
EE
8
V
EE
9
V
EEO
21
D4 (PHILIPS BAV20 OR HITACHI 1SS91) IS ARC PROTECTION DIODE.
SEE
APPLICATIONS INFORMATION
SECTION.
IOUT
19
L3
L1
L
S
CONTRAST
OFFSET
16
VCB
17
VCB
22µF
100V
R
L
200Ω
10W
C
R
CB
L2
1
2
10pF
L1, L2, L3, AND CB ARE
ELEMENTS OF THE PEAKING COIL.
L
S
IS THE TOTAL INDUCTANCE
TO THE CATHODE. R
S
IS A SERIES
ARC PROTECTION ELEMENT.
R
S
100Ω
W (CARBON)
C
L
MAX445
50Ω
BLANK
11
BEAD*
* STACK POLE 57-0180 OR
INDIANA GENERAL F-1650-H
0.1µF
-10.5V
0.1µF
Figure 1. Typical Connection Diagram
4
_______________________________________________________________________________________
Low-Cost, High-Resolution, 200MHz
Video CRT Driver
IOUT
The MAX445’s output is an open collector of a cascode
amplifier. This output is designed to work with nominal
output supplies of V
AA
= +75V. The high-voltage supply
must be greater than any applied VCB voltage for proper
operation. The MAX445 sinks up to 250mA. Optimum
performance into a capacitive load can be achieved
when an impedance-matching network is used.
Impedance Matching Network
For maximum speed from the MAX445, be sure to
“match” the output to the CRT. Figure 1’s typical connec-
tion diagram shows a network (including parasitic reac-
tances) associated with arc protection devices, CRT
wiring and grid structure, and load resistors. These para-
sitic reactances are all detrimental to good transient
response and should be minimized as much as possible.
C
L
is the grid-to-cathode capacitance of the CRT, plus
any parasitic capacitance to ground associated with the
cathode structure. This capacitance varies from tube-
type to tube-type over the 4pF to 12pF range.
In Figure 1, L
S
is the inductance of the lead from the
amplifier board to the CRT cathode and the return path
from the grid to circuit ground. A wire in free space has
an inductance of 20nH/inch to 25nH/inch. With care, the
total path through the CRT gun can be kept at 1.5 to 2
inches, such that L
S
ranges from 30nH to 50nH.
Excessive lead length will cause undesirable overshoot
and ringing in the transient response.
The peaking networks assume that 2pF of parasitic
capacitance is associated with the CRT arc protection
diode connected at the junction of L3 and L1.
Lr is the parasitic inductance of the load resistor, R
L
. In
some cases, C
R
may be needed to improve step
response.
R
S
is a damping resistor in series with the CRT grid.
It also provides current limiting in the event of CRT
arcing.
The equations for determining optimum peaking net-
work values are as follows:
L1 = (R
L
)
2
(C
L
) / 4
L2 = 3(R
L
)
2
(C
L
) / 4
C
B
= C
L
/ 5
R
S
= R
L
/ 2
L3 = k
3
(R
L
)
2
[2.5 x 10
-12
]
C
R
(optional) = Lr / (2R
L2
)
k
3
is an empirically determined factor increasing with
C
L
and varying from 0 for C
L
~ 2pF to 1 for C
L
~ 12pF.
However, L3 >100nH will compromise large-signal per-
formance.
Table 1 shows peaking networks for the nominal load,
R
L
= 200Ω (and R
S
= 100Ω).
Optimum peaking depends on board layout and CRT
construction. The values given by these equations
should be used as starting points for empirically deter-
mining optimum values.
MAX445
VCB
The output stage consists of a common-base, high-voltage
stage and a high-speed, low-voltage current amplifier in a
cascode arrangement. The VCB input is the base connec-
tion to the common-base device of this stage. Be sure to
provide a stable DC voltage at this pin of nominally +10V.
High-frequency compensation at this input is required to
avoid output oscillations. Use a series 24Ω resistor to sup-
ply, shunted with a 10pF capacitor to ground (Figure 1).
Smaller values of this RC combination will improve output
rise/fall times, but can cause output oscillations.
Power Supplies
+10V and -10.5V supplies are required for proper opera-
tion. These supplies can be set to ±12V for conve-
nience, however this will add additional component
power dissipation. The high-voltage supply, V
AA
, can be
any voltage between VCB + 10V and VCB + 65V.
V
EEO
(pin 21) is the negative supply to the output stage
and must be DC connected to V
EE
(pins 8 and 9), the
most negative voltage applied to the device. However,
V
EEO
must be decoupled from V
EE
to prevent output
oscillations. A ferrite bead and separate 0.1µF decou-
pling capacitors, as shown in Figure 1, will provide
appropriate decoupling.
Power-Supply Sequencing
Power-supply sequencing is important to avoid internal
device latchup. To avoid sequencing problems, external
diodes should be placed from V
EE
to ground, from
ground to V
CC
, and from V
CC
to the output supply (V
AA
),
as shown in Figure 1. With diodes used as shown, spe-
cial power-supply sequencing is not required.
CRT Arc Protection
The MAX445 must be protected from electrostatic dis-
charge (“arcs”) from the CRT. It is recommended that the
output be clamped with a low-capacitance (less than
2pF) diode to the V
AA
supply. The peak current-handling
capability required of the diode is a function of the CRT
arc characteristics, but typically should be 1A or more,
such as Philips BAV20 or Hitachi 1SS91. For additional
information regarding arc protection, contact Maxim’s
applications department.
_______________________________________________________________________________________
5
