27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
General Description
The MAX6922/MAX6932/MAX6933/MAX6934 multi-output, 76V, vacuum-fluorescent display (VFD) tube drivers that interface a VFD tube to a microcontroller or a
VFD controller, such as the MAX6850–MAX6853. The
MAX6922/MAX6934 have 32 outputs, while the MAX6932
has 27 outputs, and the MAX6933 has 28 outputs. All
devices are also suitable for driving telecom relays.
Data is input using standard 4-wire serial interface
(CLOCK, DATA, LOAD, BLANK) compatible with other
VFD drivers and controllers.
For easy display control, the active-high BLANK input
forces all driver outputs low, turning the display off, and
automatically puts the IC into shutdown mode. Display
intensity may also be controlled by directly pulse-width
modulating the BLANK input.
The MAX6922/MAX6932/MAX6934 have a serial interface data output, DOUT, allowing any number of devices
to be cascaded on the same serial interface.
The MAX6932/MAX6933/MAX6934 have a negative supply voltage input, VSS, allowing the drivers’ output swing
to be made bipolar to simplify filament biasing in many
applications.
The MAX6922 is available in a 44-pin PLCC package, the
MAX6932 and MAX6933 are available in 36-pin SSOP
packages, and the MAX6934 is available in 44-pin PLCC
and TQFN packages.
Maxim also offers a 12-output VFD driver (MAX6920) and
20-output VFD drivers (MAX6921/MAX6931).
Applications
●●
●●
●●
●●
●●
White Goods
Gaming Machines
Avionics
Instrumentation
Industrial Weighing
●●
●●
●●
●●
Security
Telecom
VFD Modules
Industrial Control
Selector Guide
BIPOLAR
DOUT FOR
OUTPUT
CASCADING
SWING
MAX6922
32
No
Yes
MAX6932
27
Yes
Yes
MAX6933
28
Yes
No
MAX6934
32
Yes
Yes
Pin Configurations appear at end of data sheet.
PART
19-3224; Rev 3; 7/14
NO. OF
OUTPUTS
Features
●● 5MHz Industry-Standard 4-Wire Serial Interface
●● 3V to 5.5V Logic Supply Range
●● 8V to 76V Grid/Anode Supply Range
●● - 11V to 0V Filament Bias Supply
(MAX6932/MAX6933/MAX6934 Only)
●● Push-Pull CMOS High-Voltage Outputs
●● Outputs can Source 40mA, Sink 4mA Continuously
●● Outputs can Source 75mA Repetitive Pulses
●● Outputs can Be Paralleled for Higher Current Drive
●● A
ny Output can Be Used as a Grid or an Anode
Driver
●● BLANK Input Simplifies PWM Intensity Control
●● -40°C to +125°C Temperature Range as Standard
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX6922AQH
-40°C to +125°C
44 PLCC
MAX6932AAX
-40°C to +125°C
36 SSOP
MAX6933AAX
-40°C to +125°C
36 SSOP
MAX6934AQH
-40°C to +125°C
44 PLCC
MAX6934ATH
-40°C to +125°C
44 TQFN-EP*
*EP = Exposed pad.
Typical Operating Circuit
+5V
+60V
C2
100nF
C1
100nF
38
VCC
µC
39
VBB
MAX6934
VFDOUT
VFCLK
VFLOAD
VFBLANK
37
17
18
15
DIN
CLK
LOAD
BLANK
VSS
-7V
C3
100nF
OUT0–OUT31
12
GND
16
THIN QFN
32
VFD TUBE
MAX6922/MAX6932/
MAX6933/MAX6934
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
Absolute Maximum Ratings
(Voltage with respect to GND.)
VBB.........................................................................-0.3V to +80V
VCC...........................................................................-0.3V to +6V
VSS (MAX6932/MAX6933/MAX6934 only)............-12V to +0.3V
VBB - VSS (MAX6932/MAX6933/MAX6934 only)....-0.3V to +80V
OUT_ (MAX6922 only).................(GND - -0.3V) to (VBB + 0.3V)
OUT_ (MAX6932/MAX6933/MAX6934 only)
...................................................... (VSS - -0.3V) to (VBB + 0.3V)
All Other Pins............................................ -0.3V to (VCC + 0.3V)
OUT_ Continuous Source Current ....................................-45mA
OUT_ Pulsed (1ms max, 1/4 max duty) Source Current .....-80mA
Total OUT_ Continuous Source Current .........................-840mA
Total OUT_ Continuous Sink Current ...............................140mA
Total OUT_ Pulsed (1ms max, 1/4 max duty)
Source Current ............................................................-960mA
OUT_ Sink Current .............................................................15mA
CLK, DIN, LOAD, BLANK, DOUT Current .......................±10mA
Continuous Power Dissipation (TA = +70°C)
36-Pin SSOP (derate 11.8mW/°C
over +70°C)..................................................................941mW
44-Pin Thin QFN (derate 27mW/°C
over +70°C)................................................................2165mW
44-Pin PLCC (derate 13.3mW/°C
over +70°C)................................................................1067mW
Operating Temperature Range
(TMIN to TMAX).............................................. -40°C to +125°C
Junction Temperature.......................................................+150°C
Storage Temperature Range............................. -65°C to +150°C
Lead Temperature (soldering, 10s).................................. +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
(Typical Operating Circuit, VBB = 8V to 76V, VCC = 3V to 5.5V, VSS = -11V to 0V, VBB - VSS ≤ 76V, TA = TMIN to TMAX, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Logic Supply Voltage
VCC
3
5.5
V
Tube Supply Voltage
VBB
8
76
V
Bias Supply Voltage (MAX6932/
MAX6933/MAX6934 Only)
VSS
-11
0
V
Total Supply Voltage (MAX6932/
MAX6933/MAX6934 Only)
VBB - VSS
76
V
Logic Supply Operating Current
ICC
All outputs OUT_ low,
CLK = idle
Bias Supply Operating Current
(MAX6932/MAX6933/MAX6934
Only)
IBB
All outputs OUT_ high
All outputs OUT_ low
ISS
All outputs OUT_ high
VBB ≥ 15V,
IOUT = -25mA
High-Voltage OUT_
VH
VBB ≥ 15V,
IOUT = -40mA
8V < VBB < 15V,
IOUT = -25mA
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81
TA = -40°C to +125°C
105
125
All outputs OUT_ high, TA = +25°C
CLK = idle
TA = -40°C to +125°C
All outputs OUT_ low
Tube Supply Operating Current
TA = +25°C
813
950
1000
TA = +25°C
2.0
2.5
TA = +25°C
1.3
1.75
TA = -40°C to +125°C
TA = -40°C to +125°C
TA = +25°C
3
-1.2
TA = -40°C to +125°C
-1.8
TA = +25°C
TA = -40°C to +85°C
-0.65
-1.7
VBB - 2
VBB - 1.1
VBB - 2.5
TA = -40°C to +125°C
VBB - 4.0
TA = +25°C
TA = -40°C to +85°C
TA = -40°C to +125°C
mA
-1.5
TA = -40°C to +125°C
TA = -40°C to +85°C
mA
2.0
-1
TA = -40°C to +125°C
TA = +25°C
µA
VBB - 3.5
V
VBB - 1.2
VBB - 2.5
VBB - 3.0
Maxim Integrated │ 2
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
Electrical Characteristics (continued)
(Typical Operating Circuit, VBB = 8V to 76V, VCC = 3V to 5.5V, VSS = -11V to 0V, VBB - VSS ≤ 76V, TA = TMIN to TMAX, unless otherwise noted.) (Note 1)
PARAMETER
Low-Voltage OUT_
(MAX6932 Only)
SYMBOL
CONDITIONS
VBB ≥ 15V,
IOUT = 1mA
VL
8V < VBB < 15V,
IOUT = 1mA
VBB ≥ 15V,
IOUT = 1mA
Low-Voltage OUT_
(MAX6932/MAX6933/MAX6934
Only)
VL
8V < VBB < 15V,
IOUT = 1mA
MIN
TA = +25°C
TYP
MAX
0.75
1.2
TA = -40°C to +85°C
1.5
TA = -40°C to +125°C
2.1
TA = +25°C
0.8
1.3
TA = -40°C to +85°C
1.7
TA = -40°C to +125°C
2.2
VSS
+ 0.75
TA = +25°C
VSS + 1.5
TA = -40°C to +125°C
VSS + 2.1
TA = +25°C
TA = -40°C to +85°C
V
VSS +
1.2
TA = -40°C to +85°C
VSS +
0.8
UNITS
VSS +
1.3
V
VSS + 1.7
TA = -40°C to +125°C
VSS + 2.2
Rise Time OUT_ (20% to 80%)
tR
VBB = 60V, CL = 50pF, RL = 2.3kW
0.9
2.5
µs
Fall Time OUT_ (80% to 20%)
tF
VBB = 60V, CL = 50pF, RL = 2.3kW
0.6
1.5
µs
SERIAL INTERFACE TIMING CHARACTERISTICS
LOAD Rising to OUT_ Falling
Delay
(Notes 2, 3)
0.9
3
µs
LOAD Rising to OUT_ Rising
Delay
(Notes 2, 3)
1.2
3
µs
BLANK Rising to OUT_ Falling
Delay
(Notes 2, 3)
0.9
3
µs
BLANK Falling to OUT_ Rising
Delay
(Notes 2, 3)
1.3
3
µs
0.05
10
µA
Input Leakage Current
CLK, DIN, LOAD, BLANK
IIH, IIL
Logic-High Input Voltage
CLK, DIN, LOAD, BLANK
VIH
Logic-Low Input Voltage
CLK, DIN, LOAD, BLANK
VIL
Hysteresis Voltage
DIN, CLK, LOAD, BLANK
DVI
High-Voltage DOUT
VOH
ISOURCE = -1.0mA
Low-Voltage DOUT
VOL
ISINK = 1.0mA
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0.8 x
VCC
V
0.3 x
VCC
0.6
V
V
VCC 0.5
V
0.5
V
Maxim Integrated │ 3
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
Electrical Characteristics (continued)
(Typical Operating Circuit, VBB = 8V to 76V, VCC = 3V to 5.5V, VSS = -11V to 0V, VBB - VSS ≤ 76V, TA = TMIN to TMAX, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
CDOUT = 10pF
(Note 2)
Rise and Fall Time DOUT
MIN
TYP
MAX
3V to 4.5V
80
130
4.5V to 5.5V
50
80
UNITS
ns
CLK Clock Period
tCP
200
ns
CLK Pulse-Width High
tCH
90
ns
CLK Pulse-Width Low
tCL
90
ns
(Note 2)
100
ns
5
ns
3.0V to 4.5V
20
4.5V to 5.5V
15
CLK Rise to LOAD Rise Hold
tCSH
DIN Setup Time
tDS
DIN Hold Time
tDH
DOUT Propagation Delay
tDO
LOAD Pulse High
CDOUT = 10pF
ns
3.0V to 4.5V
25
120
240
4.5V to 5.5V
20
75
150
tCSW
ns
60
ns
Note 1: All parameters are tested at TA = +25°C. Specifications over temperature are guaranteed by design.
Note 2: Guaranteed by design.
Note 3: Delay measured from control edge to when output OUT_ changes by 1V.
Typical Operating Characteristics
(VCC = 5.0V, VBB = 76V, and TA = +25°C, unless otherwise noted.)
1.6
1.2
0.8
0.4
0
1.4
1.2
1.0
VBB = 40V
VBB = 8V
0.8
0.6
0.4
0.2
-40 -20
0
20
40
60
80
TEMPERATURE (°C)
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100 120
0
0.9
LOGIC SUPPLY CURRENT (ICC)
vs. TEMPERATURE (OUTPUTS LOW)
MAX6922 toc03
1.6
1.0
SUPPLY CURRENT (mA)
VBB = 40V
VBB = 8V
VBB = 76V
1.8
SUPPLY CURRENT (mA)
SUPPLY CURRENT (mA)
2.4
2.0
2.0
MAX6922 toc01
VBB = 76V
2.8
TUBE SUPPLY CURRENT (IBB)
vs. TEMPERATURE (OUTPUTS HIGH)
MAX6922 toc02
TUBE SUPPLY CURRENT (IBB)
vs. TEMPERATURE (OUTPUTS LOW)
VCC = 5V, CLK = 5MHz
0.8
0.7
VCC = 3.3V, CLK = 5MHz
0.6
0.5
0.4
VCC = 3.3V, CLK = IDLE
0.3
0.2
VCC = 5V, CLK = IDLE
0.1
-40 -20
0
20
40
60
80
TEMPERATURE (°C)
100 120
0
-40 -20
0
20
40
60
80
100 120
TEMPERATURE (°C)
Maxim Integrated │ 4
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
Typical Operating Characteristics (continued)
(VCC = 5.0V, VBB = 76V, and TA = +25°C, unless otherwise noted.)
0.9
0.8
VCC = 3.3V, CLK = 5MHz
VCC = 5V, CLK = IDLE
0.7
0.6
0
20
40
12
VBB = 40V
10
8
6
VBB = 8V
4
60
80
TEMPERATURE (°C)
100 120
0
IOUT = -40mA
VBB = 40V
3.0
2.5
MAX6922 toc06
3.5
OUTPUT VOLTAGE (VBB - VT)
vs. TEMPERATURE (OUTPUTS HIGH)
VBB = 76V
2.0
1.5
VBB = 8V
1.0
0.5
2
VCC = 3.3V, CLK = IDLE
-40 -20
VBB = 76V
IOUT = 4mA
OUTPUT VOLTAGE (V)
1.0
OUTPUT VOLTAGE
vs. TEMPERATURE (OUTPUTS LOW)
MAX6922 toc05
VCC = 5V, CLK = 5MHz
14
OUTPUT VOLTAGE (V)
SUPPLY CURRENT (mA)
1.1
16
MAX6922 toc04
1.2
LOGIC SUPPLY CURRENT (ICC)
vs. TEMPERATURE (OUTPUTS HIGH)
-40 -20
0
20
40
60
80
0
100 120
-40 -20
0
20
40
60
80
100 120
TEMPERATURE (°C)
TEMPERATURE (°C)
OUTPUT FALL AND RISE TIME
MAX6922 toc07
BLANK
2V/div
OUT
20V/div
1µs/div
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Maxim Integrated │ 5
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
Pin Description
PIN
MAX6922/
MAX6934
PLCC
MAX6932/
MAX6933
SSOP
MAX6934
TQFN
NAME
1
1
39
VBB
2
2
40
FUNCTION
VFD Supply Voltage
DOUT
Serial-Data Output. Data is clocked out of the internal shift register to DOUT
(MAX6932) on CLK’s falling edge. For the MAX6933 only—VFD anode and grid driver.
(OUT27)
(OUT27 is a push-pull output swinging from VBB to VSS.)
(MAX6933)
2
3, 4, 5, 7–
17, 19, 20,
25, 26, 27,
30–42
—
1–11, 13,
14, 19, 20,
21, 24–36,
41, 42, 43
OUT0 to
OUT31
VFD Anode and Grid Drivers. OUT_ are push-pull outputs swinging from
VBB to GND for the MAX6922 and from VBB to VSS for the MAX6934.
—
3 –13, 15,
16, 21–34
—
OUT0 to
OUT26
VFD Anode and Grid Drivers. OUT_ are push-pull outputs swinging from
VBB to VSS.
6, 28, 29
—
22, 23, 44
N.C.
18
—
—
N.C. (VSS)
—
14
12
VSS
21
17
15
BLANK
22
18
16
GND
Ground
23
19
17
CLK
Serial-Clock Input. Data is loaded into the internal shift register on CLK’s
rising edge. On CLK’s falling edge, data is clocked out of DOUT.
24
20
18
LOAD
43
35
37
DIN
Serial-Data Input. Data is loaded into the internal shift register on CLK’s
rising edge.
44
36
38
VCC
Logic Supply Voltage
—
—
EP
EP
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No Connection. Not internally connected.
For the MAX6922—No Connection. Not internally connected. For the
MAX6934—bias supply voltage.
Bias Supply Voltage
Blanking Input. High forces outputs OUT_ low without altering the contents
of the output latches. Low enables outputs OUT_ to follow the state of the
output latches.
Load Input. Data is loaded transparently from the internal shift register to
the output latch while LOAD is high. Data is latched into the output latch on
LOAD’s rising edge, and retained while LOAD is low.
Exposed Pad. Connect to a large ground plane to maximize thermal
performance.
Maxim Integrated │ 6
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
CLK
MAX6922/MAX6932/
MAX6934 ONLY
SERIAL-TO-PARALLEL SHIFT REGISTER
DIN
DOUT
LATCHES
LOAD
BLANK
MAX6922
MAX6932
MAX6933
MAX6934
WHERE n =
27 FOR MAX6932
28 FOR MAX6933
32 FOR MAX6922/MAX6934
OUT0 OUT1 OUT2
OUTn
Figure 1. MAX6922/MAX6932/MAX6933/MAX6934 Functional Diagram
VBB
VBB
SLEW-RATE
CONTROL
40Ω
TYPICAL
OUT_
750Ω
TYPICAL
40W
TYPICAL
SLEW-RATE
CONTROL
OUT_
750W
TYPICAL
VSS
Figure 2. MAX6922 CMOS Output Driver Structure
Figure 3. MAX6932/MAX6933/MAX6934 CMOS Output Driver
Structure
Detailed Description
The 4-wire serial interface comprises a shift register and
transparent latch with 32 bits for the MAX6922/MAX6934,
28 bits for the MAX6933, and 27 bits for the MAX6932.
The shift register is written through a clock input CLK and
a data input DIN. For the MAX6922/MAX6932/MAX6934,
the data propagates to a data output DOUT. The data output allows multiple drivers to be cascaded and operated
together. The output latch is transparent to the shift register outputs when LOAD is high, and latches the current
state on the falling edge of LOAD.
The MAX6922/MAX6932/MAX6933/MAX6934 are VFD
tube drivers comprising a 4-wire serial interface driving
high-voltage rail-to-rail output ports. The driver is suitable
for both static and multiplexed displays.
The output ports feature high current-sourcing capability
to drive current into grids and anodes of static or multiplex
VFDs. The ports also have active current sinking for fast
discharge of capacitive display electrodes in multiplexing
applications.
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Maxim Integrated │ 7
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
Each driver output is a slew-rate controlled CMOS pushpull switch driving between VBB and GND (MAX6922)
or VBB and VSS (MAX6932/MAX6933/ MAX6934). The
output rise time is always slower than the output fall time
to avoid shoot-through currents during output transitions.
The output slew rates are slow enough to minimize EMI,
yet are fast enough so as not to impact the typical 100µs
digit multiplex period and affect the display intensity.
Initial Power-Up and Operation
An internal reset circuit clears the internal registers on
power-up. All outputs and the interface output DOUT
(MAX6922/MAX6932/MAX6934 only) initialize low regardless of the initial logic levels of the CLK, DIN, BLANK, and
LOAD inputs.
4-Wire Serial Interface
These driver ICs use a 4-wire serial interface with three
inputs (DIN, CLK, LOAD) and a data output (DOUT,
MAX6922/MAX6932/MAX6934 only). This interface is
used to write data to the ICs (Figure 4) (Table 1). The serial interface data word length is 32 bits for the MAX6922/
MAX6934, 27 bits for the MAX6932, and 28 bits for the
MAX6933.
The functions of the four serial interface pins are:
●● CLK input is the interface clock, which shifts data into
the shift register on its rising edge.
●● LOAD input passes data from the shift register to the
output latch when LOAD is high (transparent latch),
and latches the data on LOAD’s falling edge.
●● DIN is the interface data input, and must be stable
when it is sampled on the rising edge of CLK.
●● DOUT is the interface data output, which shifts data
out from the shift register on the rising edge of CLK.
Data at DIN is propagated through the shift register
and appears at DOUT (n CLK cycles + tDO) later,
where n is the number of drivers in the IC.
A fifth input, BLANK, can be taken high to force the outputs low, without altering the contents of the output latches. When the BLANK input is low, the outputs follow the
state of the output latches. A common use of the BLANK
input is PWM intensity control.
The BLANK input’s function is independent of the operation of the serial interface. Data can be shifted into the
serial interface shift register and latched regardless of the
state of BLANK.
Writing Device Registers Using
the 4-Wire Serial Interface
The MAX6922/MAX6932/MAX6933/MAX6934 are normally written using the following sequence:
1) Take CLK low.
2) Clock n bits of data in order Dn-1 first to D0 last into
DIN, observing the data setup and hold times.
3) Load the n output latches with a falling edge on
LOAD, where n is 27 for the MAX6932, 28 for the
MAX6933, and 32 for the MAX6922 and MAX6934.
LOAD may be high or low during a transmission. If LOAD
is high, then the data shifted into the shift register at
DIN appears at the OUT0 to OUTn-1 outputs.
CLK and DIN may be used to transmit data to other
peripherals. Activity on CLK always shifts data into the
shift register. However, the output latches only update
on the rising edge of LOAD, and the last n bits of data
tCSW
LOAD
tCH
tCL
CLK
tDS
DIN
tCSH
tCP
tDH
Dn-1
Dn-2
D1
D0
tDO
DOUT
Dn-1
Figure 4. 4-Wire Serial Interface Timing Diagram
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Maxim Integrated │ 8
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
Table 1. 4-Wire Serial Interface Truth Table
BLANKING
SERIAL CLOCK SHIFT REGISTER CONTENTS LOAD
LATCH CONTENTS
OUTPUT CONTENTS
INPUT
INPUT
INPUT
DATA
INPUT
CLK D0 D1 D2 … Dn-2 Dn-1 LOAD D0 D1 D2 … Dn-2 Dn-1 BLANK D0 D1 D2 … Dn-2 Dn-1
DIN
H
L
X
H
R0 R1 …
Rn-2
Rn-1
L
R0 R1 …
Rn-2
Rn-1
R0 R1 R2 …
Rn-1
Rn
X
X
X
…
P0 P1 P2 …
X
X
L
R0 R1 R2
… Rn-1 Rn
Pn-1
Pn
H
P0 P1 P2
… Pn-1 Pn
L
X
…
H
X
X
X
X
P0 P1 P2 … Pn-1
L
L
L
…
L
Pn
L
L = Low logic level.
H = High logic level.
X = Don’t care.
P = Present state (shift register).
R = Previous state (latched).
clocked in are loaded. Therefore, multiple devices can
share CLK and DIN, as long as they have unique LOAD
controls.
Determining Driver Output Voltage Drop
The outputs are CMOS drivers, and have a resistive
characteristic. The typical and maximum sink and source
output resistances can be calculated from the VH and VL
electrical characteristics. Use this calculated resistance
to determine the output voltage drop at different output
currents.
Output Current Ratings
The continuous current-source capability is 40mA per
output. Outputs may drive up to 75mA as a repetitive
peak current, subject to the on-time (output high) being no
longer than 1ms, and the duty cycle being such that the
output power dissipation is no more than the dissipation
for the continuous case. The repetitive peak rating allows
outputs to drive a higher current in multiplex grid driver
applications, where only one grid is on at a time, and the
multiplex time per grid is no more than 1ms.
Since dissipation is proportional to current squared, the
maximum current that can be delivered for a given multiplex ratio is given by:
Paralleling Outputs
Any number of outputs within the same package may be
paralleled in order to raise the current drive or reduce
the output resistance. Only parallel outputs directly (by
shorting outputs together) if the interface control can be
guaranteed to set the outputs to the same level. Although
the sink output is relatively weak (typically 750Ω), that
resistance is low enough to dissipate 530mW when shorted to an opposite level output at a VBB voltage of only
20V. A safe way to parallel outputs is to use diodes to prevent the outputs from sinking current (Figure 5). Because
the diodes also stop the outputs from sinking current
from the VFD tube, an external discharge resistor, R, is
required. For static tubes, R can be a large value such as
100kΩ. For multiplexed tubes, the value of the resistor
can be determined by the load capacitance and timing
MAX6922
MAX6932
MAX6933 OUT0
MAX6934
OUT1
IPEAK = (grids x 1600)1/2 mA
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OUTPUT
D2
R
where grids is the number of grids in a multiplexed
display.
This means that a duplex application (two grids) can use
a repetitive peak current of 56.5mA, a triplex (three grids)
application can use a repetitive peak current of 69.2mA,
and higher multiplex ratios are limited to 75mA.
D1
Figure 5. Paralleling Outputs
Maxim Integrated │ 9
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
characteristics required. Resistor R discharges tube
capacitance C to 10% of the initial voltage in 2.3 x RC
seconds. So, for example, a 15kΩ value for R discharges
100pF tube grid or anode from 40V to 4V in 3.5µs, but
draws an additional 2.7mA from the driver when either
output is high.
Power Dissipation
Take care to ensure that the maximum package dissipation ratings for the chosen package are not exceeded.
Over-dissipation is unlikely to be an issue when driving
static tubes, but the peak currents are usually higher for
multiplexed tubes. When using multiple driver devices,
try to share the average dissipation evenly between the
drivers.
Determine the power dissipation (PD) for the MAX6922/
MAX6932/MAX6933/MAX6934 for static tube drivers with
the following equation:
PD = (VCC x ICC) + (VBB x IBB) + ((VBB - VH) x
IANODE x A))
where:
A = number of anodes driven (maximum of 32 with the
MAX6922/MAX6934).
IANODE = maximum anode current.
(VBB - VH) is the output voltage drop at the given maximum anode current IOUT.
A static tube dissipation example follows:
VCC = 5V ±5%, VBB = 10V to 18V, A = 32, IOUT = 2mA
PD = (5.25V x 1.5mA)+ (18V x 2.2mA) +
((2.5V x 2mA/25mA) x 2mA x 32) = 60mW
Determine the power dissipation (PD) for the MAX6922/
MAX6932/MAX6933/MAX6934 for multiplex tube drivers
with the following equation:
PD = (VCC x ICC) + (VBB x
IANODE x A) + ((VBB -
IBB) + ((VBB - VH) x
VH) x IGRID))
where:
A = number of anodes driven.
G = number of grids driven.
IANODE = maximum anode current.
IGRID = maximum grid current.
The calculation presumes all anodes are on, but only one
grid is on. The calculated PD is the worst case, presuming one digit is always being driven with all its anodes lit.
Actual PD can be estimated by multiplying this PD figure
by the actual tube drive duty cycle, taking into account
interdigit blanking and any PWM intensity control.
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A multiplexed tube dissipation example follows:
VCC = 5V ±5%, VBB = 36V to 42V, A = 20, G = 12,
IANODE = 0.4mA, IGRID = 24mA
PD = (5.25V x 1.5mA)+ (42V x 2.2mA) +
((2.5V x 0.4mA/25mA) x 0.4mA x 20) +
((2.5V x 24mA/25mA) x 24mA) = 158mW
Thus, for a 44-pin PLCC package (TJA = 1/0.0133 =
75.188°C/W from Absolute Maximum Ratings), the maximum allowed ambient temperature TA is given by:
TJ(MAX) = TA + (PD x TJA) = +150°C = TA + (0.158 x
75.188°C/W)
So TA = +138°C.
This means that the driver can be operated in this
application with a PLCC package up to the +125°C
maximum operating temperature.
Power-Supply Considerations
The MAX6922/MAX6932/MAX6933/MAX6934 operate
with multiple power-supply voltages. Bypass the VCC,
VBB, and VSS (MAX6932/MAX6933/MAX6934 only) power-supply pins to GND with 0.1µF capacitors close to the
device. The MAX6932/MAX6933/MAX6934 may be operated with VSS tied to GND if a negative bias supply is not
required. For multiplex applications, it may be necessary
to add an additional bulk electrolytic capacitor of 1µF or
greater to the VBB supply.
Power-Supply Sequencing
The order of the power-supply sequencing is not important. These ICs are damaged if any combination of VCC,
VBB, and VSS is grounded while the other supply or
supplies are maintained up to their maximum ratings.
However, as with any CMOS device, do not drive the logic
inputs if the logic supply VCC is not operational because
the input protection diodes clamp the signals.
Cascading Drivers
(MAX6922/MAX6932/MAX6934 Only)
Multiple driver ICs may be cascaded, as shown in the
Typical Application Circuit, by connecting each driver’°s
DOUT to DIN of the next drivers. Devices may be cascaded at the full 5MHz CLK speed when VCC ≥ 4.5V.
When VCC
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