MAX445

Low-Cost, High-Resolution, 200MHz

Video CRT Driver

________________________________________________________________

Maxim Integrated Products

CONTRAST

OFFSET

VREF

GND

TOP VIEW

VIN+

VIN-

GNDA

GND

BLANK

EEO

GND

VCB

N.C.

IOUT

N.C.

GND

VCB

MAX445

Power-Tab DIP

IOUT

OFFSET

CONTRAST

BLANK

OUT

PRE-AMP

VCB

VIN-

VIN+

CURRENT

AMP

VREF

BAND

GAP

GNDA

GND

MAX445

__________________Pin Configuration

________________Functional Diagram

Call toll free 1-800-998-8800 for free samples or literature.

_______________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 200

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.

________________________Applications

CRT Driver for High-Resolution Monochrome and
Color Displays

High-Voltage, Variable-Gain Transconductance
Amplifier

____________________________Features

2.5ns Rise/Fall Time into an 8pF Load

200MHz Small-Signal Bandwidth

50Vp-p Output

Ground Referenced Differential Inputs

Linear Variable Gain for Contrast Control

Offset Adjustment for Black Level

5.5V Bandgap Reference

Drives 1280 x 1024 and 1530 x 1280 Displays

______________Ordering Information

PART

MAX445CPG

MAX445C/D

0°C to +70°C**

0°C to +70°C*

TEMP. RANGE

PIN-PACKAGE

24 Power-Tab DIP

Dice

* 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

= +25°C, DC parameters only.

19-0302; Rev 0; 9/94

MAX445

Low-Cost, High-Resolution, 200MHz
Video CRT Driver

_______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

= 20V, VCB = 10V ±0.5V, V

= 10V ±0.5V, V

= -10.5V ±0.5V, VIN = (VIN+) - (VIN-) = 0V, CONTRAST = 1.0V,

OFFSET = 1.0V, R

= 0

, BLANK = 0.4V, T

CASE

= +25°C, unless otherwise noted.)

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.

Output Supply.................................................................80V

Output Supply with Respect to VCB...............................70V

VCB Common-Base Supply ...................................................20V
V

Positive Supply ............................................................12.5V

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

OFFSET = 0V, R

LOAD

= 100

VIN = 0.2V, OFFSET = 0V

CONTRAST = 4.0V, OFFSET = 1.0V

VEE

+ I

VEEO

CONTRAST = 0V

CONTRAST = 1.0V

CONTRAST = 5.0V

BLANK = 2.4V, CONTRAST = 5.0V,

VIN- = 0.3V

CONTRAST = 0V to 5V

= +25°C to +90°C

= ±0.5V, CONTRAST = 5.0V

BLANK = 2.4V, OFFSET = 3V

BLANK = 2.4V, OFFSET = 1V, V

= 75V

OFFSET = 1.0V

, V

= ±5%, VIN = +250mV,

CONTRAST = 5.0V, referred to input

BLANK = 0.4V

BLANK = 2.4V

LOAD

= 2mA

CONTRAST = 5.0V

CONDITIONS

MHz

200

Bandwidth, 3dB

±3

Contrast Linearity Error (

Gm/

Contrast)

±2

Amplifier Linearity Error (

Gm/

)

-25

120

mA/V

400

600

Transconductance, I

OUT

to VIN

±1

OUT

Output Current Change vs. VIN, Blanked

±10

OUT

Output Current Change vs. Contrast ADJ

±3

OUT

Output Current Change vs. Temperature

±1

OUT

Output Current (Blanked)

5.25

5.75

REF

Reference Output Voltage

VIN+ or VIN- Input Capacitance

-100

Negative Supply Current

Output-Common-Base Supply Current

Positive Supply Current

VIN

VIN+ or VIN- DC Input Impedance

CMRR

Input Common-Mode Rejection Ratio

µA

-50

IIS

VIN+ or VIN- Signal Input Current

µA

IIB

Offset Input Bias Current

PSRR

Power-Supply Rejection Ratio

-0.6

Low Blank Input Bias Current

-0.4

IIH

High Blank Input Bias Current

µA

IIC

Contrast Input Bias Current

UNITS

MIN

TYP

MAX

SYMBOL

PARAMETER

OFFSET = 5.0V, CONTRAST = 1V

OFFSET = 0V, CONTRAST = 4.0V

140

-0.1

OUT

Output Current

MAX445

Low-Cost, High-Resolution, 200MHz

Video CRT Driver

_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

= 20V, VCB = 10V ±0.5V, V

= 10V ±0.5V, V

= -10.5V ±0.5V, VIN = (VIN+) - (VIN-) = 0V, CONTRAST = 1.0V,

OFFSET = 1.0V, R

= 0

, BLANK = 0.4V, T

CASE

= +25°C, unless otherwise noted.)

CONDITIONS

= 8pF, no peaking

= 200

= 8pF,

= 75V,

(VIN) < 1ns,

UNITS

MIN

TYP

MAX

SYMBOL

PARAMETER

Settling Time (90% to 100% ±2%)

2.5

No peaking,
OUTp-p = 50V

With peaking,
OUTp-p = 45V

3.6

, t

Rise/Fall Time (10% to 90%)

±2

Thermal Distortion

______________________________________________________________Pin Description

NAME

FUNCTION

1, 12, 13, 14,

15, 22, 23, 24

GND

High-Current Ground. Connect all pins to ground plane.

VREF

Reference Output (+5.5V)

PIN

OFFSET

Output Voltage Offset-Adjustment Input

CONTRAST

Output Gain-Adjustment Input

8, 9

Negative Supply (-10.5V)

VIN+

Noninverting Signal Input

VIN-

Inverting Signal Input

GNDA

Pre-Amp Ground

IOUT

Open-Collector Current Output

18, 20

N.C.

No Connection--leave open

16, 17

VCB

Output Common-Base Supply (+10V)

BLANK

Blanking Input, TTL

Positive Supply (+10V)

EEO

Negative Supply for Output Stage (-10.5V)

MAX445

Low-Cost, High-Resolution, 200MHz
Video CRT Driver

_______________________________________________________________________________________

__________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.

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

(Gm) + V

OFFSET

(0.02)] (R

)

- V

= [V

CONTRAST

) (0.09) + V

OFFSET

(0.02)] (R

)

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

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.

MAX445

0.1

(-10.5V)

+10V

1N4152

50V

0.1

IOUT

CONTRAST

VIN+

OFFSET

-10.5V

ANALOG

INPUT

BLANK

GNDA

GND

VREF

VIN-

24*

BLANK

BEAD*

EEO

0.1

BEAD*

10pF

NOTES:
C

COMBINES CRT CATHODE, AND PARASITIC C.

D4 (PHILIPS BAV20 OR HITACHI 1SS91) IS ARC PROTECTION DIODE.
SEE

APPLICATIONS INFORMATION SECTION.

L1, L2, L3, AND CB ARE
ELEMENTS OF THE PEAKING COIL.
L

IS THE TOTAL INDUCTANCE

TO THE CATHODE. R

IS A SERIES

ARC PROTECTION ELEMENT.

200

10W

100

W (CARBON)

1N4152

50V

1N486A

100V

(+75V)

1
2

VCB

* STACK POLE 57-0180 OR
INDIANA GENERAL F-1650-H

Figure 1. Typical Connection Diagram

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

= +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.

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 24

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

, can be

any voltage between VCB + 10V and VCB + 65V.

EEO

(pin 21) is the negative supply to the output stage

and must be DC connected to V

(pins 8 and 9), the

most negative voltage applied to the device. However,
V

EEO

must be decoupled from V

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

to ground, from

ground to V

, and from V

to the output supply (V

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

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.

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.

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

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

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

. In

some cases, C

may be needed to improve step

response.

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

)

) / 4

L2 = 3(R

)

) / 4

= C

/ 5

= R

/ 2

L3 = k

)

[2.5 x 10

-12

]

(optional) = Lr / (2R

)

is an empirically determined factor increasing with

and varying from 0 for C

~ 2pF to 1 for C

~ 12pF.

However, L3 >100nH will compromise large-signal per-
formance.

Table 1 shows peaking networks for the nominal load,
R

= 200

(and R

= 100

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

Low-Cost, High-Resolution, 200MHz

Video CRT Driver

_______________________________________________________________________________________

Part Number MAX445C/D