General Description
The MAX6921/MAX6931 are 20-output, 76V, vacuum-
fluorescent display (VFD) tube drivers that interface a
multiplexed VFD tube to a VFD controller, such as the
MAX6850–MAX6853, or to a microcontroller. The
MAX6921/MAX6931 are also ideal for driving static VFD
tubes or telecom relays.
Data is input using an industry standard 4-wire serial
interface (CLOCK, DATA, LOAD, BLANK), compatibile
with either Maxim’s or industry-standard VFD driver and
controller.
For easy display control, the active-high BLANK input
forces all driver outputs low, turning the display off, and
automatically puts the MAX6921/MAX6931 into shut-
down mode. Display intensity can also be controlled by
directly pulse-width modulating the BLANK input.
The MAX6921 has a serial interface data output, DOUT,
allowing any number of devices to be cascaded on the
same serial interface.
The MAX6931 has a negative supply voltage input, VSS,
allowing the drivers’ output swing to be made bipolar to
simplify filament biasing in many applications.
The MAX6921 is available in 28-pin TSSOP, SO, and
PLCC packages. The MAX6931 is available in a 28-pin
TSSOP package.
Maxim also offers 12-output VFD drivers (MAX6920) and
32-output VFD drivers (MAX6922/MAX6932).
Applications
White Goods Industrial Weighing
Gaming Machines Security
Automotive Telecom
Avionics VFD Modules
Instrumentation Industrial Control
Features
o5MHz Industry-Standard 4-Wire Serial Interface
o3V to 5.5V Logic Supply Range
o8V to 76V Grid/Anode Supply Range
o-11V to 0V Filament Bias Supply (MAX6931 Only)
oPush-Pull CMOS High-Voltage Outputs
oOutputs can Source 40mA, Sink 4mA
Continuously
oOutputs can Source 75mA Repetitive Pulses
oOutputs can be Paralleled for Higher Current Drive
oAny Output can be Used as a Grid or an Anode
Driver
oBlank Input Simplifies PWM Intensity Control
oSmall 28-Pin TSSOP Package
o-40°C to +125°C Temperature Range
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
________________________________________________________________
Maxim Integrated Products
1
Ordering Information
19-3020; Rev 1; 4/10
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
PART TEMP RANGE PIN-PACKAGE
MAX6921AUI+ -40°C to +125°C 28 TSSOP
MAX6921AWI/V+ -40°C to +125°C 28 Wide SO
MAX6921AQI+ -40°C to +125°C 28 PLCC
MAX6931AUI+ -40°C to +125°C 28 TSSOP
MAX6931
DIN
CLK
LOAD
BLANK
VFDOUT
VFCLK
VFLOAD
VFBLANK
VFD TUBE
921
GND
-7V
6
22
µC
23
20
C1
100nF
C3
100nF
+5V
7
VCC
VSS
C2
100nF
+60V
8
VBB
OUT0–OUT19
20
Typical Operating Circuit
Pin Configurations appear at end of data sheet.
+
Denotes a lead-free(Pb)/RoHS-compliant package.
/V denotes an automotive qualified part.
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
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 other-
wise noted.) (Note 1)
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.
Voltage (with respect to GND)
VBB .........................................................................-0.3V to +80V
VCC ...........................................................................-0.3V to +6V
VSS (MAX6931 only) ...............................................-12V to +0.3V
VBB - VSS (MAX6931 only) .....................................-0.3V to +80V
OUT_ (MAX6921 only) ..................(GND - -0.3V) to (VBB + 0.3V)
OUT_ (MAX6931 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 .........................-540mA
Total OUT_ Continuous Sink Current .................................90mA
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)
28-Pin TSSOP (derate 12.8mW/°C
over +70°C)................................................................1025mW
28-Pin Wide SO (derate 12.5mW/°C
over +70°C)................................................................1000mW
28-Pin PLCC (derate 10.5mW/°C
over +70°C)..................................................................842mW
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
Soldering Temperature (reflow)
Wide SO, TSSOP lead(Pb)-free ...................................+260°C
PLCC lead(Pb)-free......................................................+245°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Logic Supply Voltage VCC 3 5.5 V
Tube Supply Voltage VBB 876V
Bias Supply Voltage
(MAX6931 Only) VSS -11 0 V
Total Supply Voltage
(MAX6931 Only) VBB - VSS 76 V
TA = +25°C 78 170
All outputs OUT_ low,
CLK = idle TA = -40°C to +125°C 200
TA = +25°C 540 900
Logic Supply Operating Current ICC All outputs OUT_ high,
CLK = idle TA = -40°C to +125°C 1000
µA
TA = +25°C 1.65 3.0
All outputs OUT_ low TA = -40°C to +125°C 6.9
TA = +25°C 0.85 1.3
Tube Supply Operating Current IBB
All outputs OUT_ high TA = -40°C to +125°C 1.4
mA
TA = +25°C -0.8 -0.38
All outputs OUT_ low TA = -40°C to +125°C -1.9
TA = +25°C -1.4 -0.87
Bias Supply Operating Current
(MAX6931 Only) ISS
All outputs OUT_ high TA = -40°C to +125°C -1.5
mA
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
_______________________________________________________________________________________ 3
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 other-
wise noted.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
TA = +25°C VBB - 2
TA = -40°C to +85°C VBB - 2.5
VBB ≥ 15V
IOUT = -25mA TA = -40°C to +125°C
TA = -40°C to +85°C VBB - 3.5
VBB ≥ 15V
IOUT = -40mA TA = -40°C to +125°C VBB - 4.0
TA = +25°C VBB - 1.2
TA = -40°C to +85°C VBB - 2.5
High-Voltage OUT_
8V < VBB < 15V
IOUT = -25mA
TA = -40°C to +125°C VBB - 3.0
V
TA = +25°C 0.75 1
TA = -40°C to +85°C 1.5
VBB ≥ 15V
IOUT = 1mA TA = -40°C to +125°C 1.9
TA = +25°C 0.8 1.1
TA = -40°C to +85°C 1.6
Low-Voltage OUT_
(MAX6921 Only) VL
8V < VBB < 15V
IOUT = 1mA TA = -40°C to +125°C 2.0
V
TA = +25°C VSS + 0.75 VSS + 1
TA = -40°C to +85°C VSS + 1.5
VBB ≥ 15V
IOUT = 1mA
TA = -40°C to +125°C VSS + 1.9
TA = +25°C VSS + 0.8 VSS + 1.1
TA = -40°C to +85°C VSS + 1.6
Low-Voltage OUT_
(MAX6931 Only) VL
8V < VBB < 15V
IOUT = 1mA TA = -40°C to +125°C VSS + 2.0
V
Rise Time OUT_ (20% to 80%) tRVBB = 60V, CL = 50pF, RL =2.3kΩ0.9 2 µs
Fall Time OUT_ (80% to 20%) tFVBB = 60V, CL = 50pF, RL =2.3kΩ0.6 1.5 µs
SERIAL INTERFACE TIMING CHARACTERISTICS
LOAD Rising to OUT_ Falling Delay (Notes 2, 3) 0.9 1.8 µs
LOAD Rising to OUT_ Rising Delay (Notes 2, 3) 1.2 2.4 µs
BLANK Rising to OUT_ Falling Delay (Notes 2, 3) 0.9 1.8 µs
BLANK Falling to OUT_ Rising Delay (Notes 2, 3) 0.5 1.3 2.5 µs
Input Leakage Current
CLK, DIN, LOAD, BLANK IIH, IIL 0.05 10 µA
Logic-High Input Voltage
CLK, DIN, LOAD, BLANK VIH 0.8 x
VCC V
Logic-Low Input Voltage
CLK, DIN, LOAD, BLANK VIL 0.3 x
VCC V
Hysteresis Voltage
DIN, CLK, LOAD, BLANK ∆VI0.6 V
High-Voltage DOUT VOH ISOURCE = -1.0mA VCC -
0.5 V
Low-Voltage DOUT VOL ISINK = 1.0mA 0.5 V
Typical Operating Characteristics
(VCC = 5.0V, VBB = 76V, and TA= +25°C, unless otherwise noted.)
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
4 _______________________________________________________________________________________
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 other-
wise noted.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
3V to 4.5V 60 100
Rise and Fall Time DOUT CDOUT = 10pF
(Note 2) 4.5V to 5.5V 30 80 ns
CLK Clock Period tCP 200 ns
CLK Pulse-Width High tCH 90 ns
CLK Pulse-Width Low tCL 90 ns
CLK Rise to LOAD Rise Hold tCSH (Note 2) 100 ns
DIN Setup Time tDS 5ns
3.0V to 4.5V 20
DIN Hold Time tDH 4.5V to 5.5V 15 ns
3.0V to 4.5V 25 120 240
DOUT Propagation Delay tDO CDOUT = 10pF 4.5V to 5.5V 20 75 150 ns
LOAD Pulse High tCSW 55 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.
TUBE SUPPLY CURRENT (IBB)
vs. TEMPERATURE (OUTPUTS LOW)
MAX6921/31 toc01
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
1008040 60020-20
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0
-40 120
VBB = 76V
VBB = 40V VBB = 8V
TUBE SUPPLY CURRENT (IBB)
vs. TEMPERATURE (OUTPUTS HIGH)
MAX6921/31 toc02
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
1008040 60020-20
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0
-40 120
VBB = 76V
VBB = 40V VBB = 8V
LOGIC SUPPLY CURRENT (ICC)
vs. TEMPERATURE (OUTPUTS LOW)
MAX6921/31 toc03
TEMPERATURE (°C)
SUPPLY CURRENT (µA)
1008040 60020-20
50
100
150
200
250
300
350
400
0
-40 120
VCC = 5V, CLK = 5MHz
VCC = 3.3V, CLK = 5MHz
VCC = 5V, CLK = IDLE
VCC = 3.3V, CLK = IDLE
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
_______________________________________________________________________________________
5
OUTPUT RISE AND FALL WAVEFORM
MAX6921/31 toc07
1µs/div
BLANK
2V/div
OUT_
20V/div
Typical Operating Characteristics (continued)
(VCC = 5.0V, VBB = 76V, and TA= +25°C, unless otherwise noted.)
LOGIC SUPPLY CURRENT (ICC)
vs. TEMPERATURE (OUTPUTS HIGH)
MAX6921/31 toc04
TEMPERATURE (°C)
SUPPLY CURRENT (µA)
1008040 60020-20
450
500
550
600
650
700
750
800
400
-40 120
VCC = 5V, CLK = 5MHz
VCC = 3.3V, CLK = 5MHz
VCC = 5V, CLK = IDLE
VCC = 3.3V, CLK = IDLE
OUTPUT VOLTAGE (VBB - VH)
vs. TEMPERATURE (OUTPUT HIGH)
MAX6921/31 toc05
TEMPERATURE (°C)
OUTPUT VOLTAGE (V)
1008040 60020-20
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
-40 120
VBB = 76V
VBB = 40V
VBB = 8V
IOUT = -40mA
OUTPUT VOLTAGE
vs. TEMPERATURE (OUTPUT LOW)
MAX6921/31 toc06
TEMPERATURE (°C)
OUTPUT VOLTAGE (V)
1008040 60020-20
2
4
6
8
10
12
14
0
-40 120
VBB = 40V
VBB = 8V
VBB = 76V
IOUT = 4mA
PIN
TSSOP WIDE
SO/PLCC
MAX6921 MAX6931 MAX6921
NAME FUNCTION
1–5 1–5 — OUT4 to
OUT0
VFD Anode and Grid Drivers. OUT4 to OUT0 are push-pull outputs swinging from
VBB to GND (MAX6921 only), and from VBB to VSS (MAX6931 only).
6 6 27 DIN Serial-Data Input. Data is loaded into the internal shift register on CLK’s rising edge.
7 7 28 VCC Logic Supply Voltage. Bypass to GND with 100nF capacitor.
881V
BB VFD Tube Supply Voltage. Bypass to GND with 100nF capacitor.
9—2DOUT
Serial-Clock Output. Data is clocked out of the internal shift register to DOUT on
CLK’s rising edge.
Pin Description
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
6 _______________________________________________________________________________________
PIN
TSSOP WIDE
SO/PLCC
MAX6921 MAX6931 MAX6921
NAME FUNCTION
—9—V
SS Filament Bias Supply Voltage. Bypass to GND with a 100nF capacitor.
10–19 10–19 — OUT19 to
OUT10
VFD Anode and Grid Drivers. OUT19 to OUT10 are push-pull outputs swinging from
VBB to GND (MAX6921 only), and from VBB to VSS (MAX6931 only).
— — 3-12 OUT19 to
OUT10
VFD Anode and Grid Drivers. OUT19 to OUT10 are push-pull outputs swinging from
VBB to GND.
20 20 13 BLANK
Blanking Input. High forces outputs OUT0 to OUT19 low, without altering the
contents of the output latches. Low enables outputs OUT0 to OUT19 to follow the
state of the output latches.
21 21 14 GND Ground
22 22 15 CLK S er i al - C l ock Inp ut. D ata i s l oad ed i nto the i nter nal shi ft r eg i ster on C LK’ s r i si ng ed g e.
23 23 16 LOAD
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.
24–28 24–28 — OUT9 to
OUT5
VFD Anode and Grid Drivers. OUT9 to OUT5 are push-pull outputs swinging from
VBB to GND (MAX6921 only), and from VBB to VSS (MAX6931 only).
— — 17-26 OUT9 to
OUT0
VFD Anode and Grid Drivers. OUT9 to OUT0 are push-pull outputs swinging from
VBB to GND.
Pin Description (continued)
SERIAL-TO-PARALLEL SHIFT REGISTER
LATCHES
CLK
DIN
LOAD
BLANK
OUT0 OUT1 OUT2 OUT19
DOUT
MAX6921 ONLY
MAX6921
MAX6931
Figure 1. MAX6921/MAX6931 Functional Diagram
Detailed Description
The MAX6921/MAX6931 are VFD tube drivers compris-
ing a 4-wire serial interface driving 20 high-voltage rail-
to-rail output ports. The driver is suitable for both static
and multiplexed displays.
The output ports feature high current-sourcing capabili-
ty to drive current into grids and anodes of static or
multiplex VFDs. The ports also have active current sink-
ing for fast discharge of capacitive display electrodes
in multiplexing applications.
The 4-wire serial interface comprises a 20-bit shift reg-
ister and a 20-bit transparent latch. The shift register is
written through a clock input CLK and a data input DIN.
For the MAX6921, 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.
Each driver output is a slew-rated controlled CMOS
push-pull switch driving between VBB and GND
(MAX6921) or VSS (MAX6931). 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 of
the MAX6921/MAX6931 on power-up. All outputs OUT0
to OUT19 and the interface output DOUT (MAX6921
only) initialize low regardless of the initial logic levels of
the CLK, DIN, BLANK, and LOAD inputs.
4-Wire Serial Interface
The MAX6921/MAX6931 use 4-wire serial interface with
three inputs (DIN, CLK, LOAD) and a data output
(DOUT, MAX6921 only). This interface is used to write
output data to the MAX6921/MAX6931 (Figure 4) (Table
1). The serial interface data word length is 20 bits,
D0–D19.
The functions of the four serial interface pins are:
• CLK input is the interface clock, which shifts data
into the MAX6921/MAX6931s’ 20-bit shift register on
its rising edge.
• LOAD input passes data from the MAX6921/
MAX6931s’ 20-bit shift register to the 20-bit 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 MAX6921’s 20-bit shift register on the
rising edge of CLK. Data at DIN is propagated
through the shift register and appears at DOUT (20
CLK cycles + tDO) later.
A fifth input, BLANK, can be taken high to force outputs
OUT0 to OUT19 low, without altering the contents of the
output latches. When the BLANK input is low, outputs
OUT0 to OUT19 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 oper-
ation of the serial interface. Data can be shifted into the
serial interface shift register and latched regardless of
the state of BLANK.
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
_______________________________________________________________________________________ 7
SLEW-RATE
CONTROL
VBB
OUT_
40Ω
TYPICAL
750Ω
TYPICAL
Figure 2. MAX6921 CMOS Output Driver Structure
SLEW-RATE
CONTROL
VBB
VSS
OUT_
40Ω
TYPICAL
750Ω
TYPICAL
Figure 3. MAX6931 CMOS Output Driver Structure
MAX6921/MAX6931
Writing Device Registers Using the
4-Wire Serial Interface
The MAX6921/MAX6931 are normally written using the
following sequence:
1) Take CLK low.
2) Clock 20 bits of data in order D19 first to D0 last
into DIN, observing the data setup and hold times.
3) Load the 20 output latches with a falling edge
on LOAD.
LOAD can be high or low during a transmission. If
LOAD is high, then the data shifted into the shift regis-
ter at DIN appear at the OUT0 to OUT19 outputs.
CLK and DIN can be used to transmit data to other
peripherals. Activity on CLK always shifts data into the
MAX6921/MAX6931s’ shift register. However, the
MAX6921/MAX6931 only update their output latch on
the rising edge of LOAD, and the last 20 bits of data
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
VHand VLelectrical characteristics. Use this calculated
resistance to determine the output voltage drop at dif-
ferent output currents.
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
8 _______________________________________________________________________________________
LOAD
tCSW
tCP
tCSH
tCH
tDH
tDO
tDS
D19 D18 D1 D0
D19
tCL
CLK
DIN
DOUT
Figure 4. 4-Wire Serial Interface Timing Diagram
CLOCK
INPUT SHIFT REGISTER CONTENTS LOAD
INPUT LATCH CONTENTS BLANKING
INPUT OUTPUT CONTENTS
SERIAL
DATA
INPUT
DIN CLK D0 D1 D2 … Dn-1 Dn LOAD D0 D1 D2 … Dn-1 Dn BLANK D0 D1 D2 … Dn-1 Dn
H H R0 R1 … Rn-2 Rn-1
L L R0 R1 … Rn-2 Rn-1
X R0 R1 R2 … Rn-1 Rn
X X X … X X L R0 R1 R2 … Rn-1 Rn
P0 P1 P2 … Pn-1 Pn H P0 P1 P2 … Pn-1 Pn L P0 P1 P2 … Pn-1 Pn
XXX…X X H LLL… L L
Table 1. 4-Wire Serial Interface Truth Table
L = Low logic level.
H = High logic level.
X = Don’t care.
P = Present state (shift register).
R = Previous state (latched).
Output Current Ratings
The continuous current-source capability is 40mA per
output. Outputs can 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 dissipa-
tion 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 mul-
tiplex ratio is given by:
IPEAK = (grids x 1600)1/2 mA
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.
Paralleling Outputs
Any number of outputs within the same package can
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 dissi-
pate 530mW when shorted to an opposite level output
at a VBB voltage of only 20V. A safe way to parallel out-
puts is to use diodes to prevent the outputs from sink-
ing current (Figure 5). Because the outputs cannot sink
current from the VFD tube, an external discharge resis-
tor, 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 capaci-
tance and timing 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 dissi-
pation ratings for the chosen package are not exceed-
ed. 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 dri-
ver devices, try to share the average dissipation evenly
between the drivers.
Determine the power dissipation (PD) for the
MAX6921/MAX6931 for static tube drivers with the fol-
lowing equation:
PD= (VCC x ICC) + (VBB x IBB) + ((VBB - VH)
x IANODE x A)
where:
A = number of anodes driven (the MAX6921/MAX6931
can drive a maximum of 20).
IANODE = maximum anode current.
(VBB - VH) is the output voltage drop at the given maxi-
mum anode current IOUT.
A static tube dissipation example follows:
VCC = 5V ±5%, VBB = 10V to 18V, A = 20, IOUT = 2mA
PD= (5.25V x 1mA)+ (18V x 1.4mA) +
((2.5V x 2mA/25mA) x 2mA x 20) = 38mW
Determine the power dissipation (PD) for the MAX6921/
MAX6931 for multiplex tube drivers with the following
equation:
PD= (VCC x ICC) + (VBB x IBB) + ((VBB - VH)
x IANODE x A) + ((VBB - 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 PDis the worst case,
presuming one digit is always being driven with all its
anodes lit. Actual PDcan be estimated by multiplying
this PDfigure by the actual tube drive duty cycle, taking
into account interdigit blanking and any PWM intensity
control.
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
_______________________________________________________________________________________ 9
MAX6921
MAX6931
OUT0
OUT1
D1
D2
R
OUTPUT
Figure 5. Paralleling Outputs
MAX6921/MAX6931
A multiplexed tube dissipation example follows:
VCC = 5V ±5%, VBB = 36V to 42V, A = 12, G = 8,
IANODE = 0.4mA, IGRID = 24mA
PD= (5.25V x 1mA)+ (42V x 1.4mA)
+ ((2.5V x 0.4mA/25mA) x 0.4mA x 12)
+ ((2.5V x 24mA/25mA) x 24mA) = 122mW
Thus, for a 28-pin wide TSSOP package (TJA = 1 / 0.0128
= 78.125°C/W from
Absolute Maximum Ratings
), the
maximum allowed ambient temperature TAis given by:
TJ(MAX) = TA+ (PDx TJA) = 150°C = TA+ (0.122
x 78.125°C/W)
So TA= +140.5°C.
This means that the driver can be operated in this
application up to the MAX6921/MAX6931s’ +125°C
maximum operating temperature.
Power-Supply Considerations
The MAX6921/MAX6931 operate with multiple power-
supply voltages. Bypass the VCC, VBB, and VSS
(MAX6931 only) power-supply pins to GND with 0.1µF
capacitors close to the device. The MAX6931 can be
operated with VSS connected 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 impor-
tant. The MAX6921/MAX6931 will not be damaged if
any combination of VCC, VBB, and VSS (MAX6931 only)
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 MAX6921/
MAX6931s’ logic inputs if the logic supply VCC is not
operational because the input protection diodes clamp
the signals.
Cascading Drivers (MAX6921 Only)
Multiple MAX6921s can be cascaded, as shown in the
Typical Application Circuit
, by connecting each driver’s
DOUT to DIN of the next drivers. Devices can be cas-
caded at the full 5MHz CLK speed when VCC ≥4.5V.
When VCC <4.5V, the longer propagation delay (tDO)
limits the maximum cascaded CLK to 4MHz.
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
10 ______________________________________________________________________________________
MAX6921
DIN
CLK
LOAD
BLANK
MAX685x
VFDOUT
VFCLK
VFLOAD
VFBLANK DOUT
VFD TUBE
MAX6921
DIN
CLK
LOAD
BLANK DOUT
MAX6921
DIN
CLK
LOAD
BLANK DOUT
Typical Application Circuit
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
______________________________________________________________________________________ 11
Pin Configurations
TOP VIEW
WIDE SO TSSOP
PLCC
TSSOP
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
VCC
DIN
OUT0
OUT1
OUT2
OUT3
CLK
OUT4
OUT5
OUT6
OUT7
OUT8
OUT9
LOAD
GND
BLANK
OUT10
OUT11
OUT12
OUT13
OUT14
OUT15
OUT16
OUT17
OUT18
OUT19
DOUT
VBB
MAX6921AWI
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
OUT5
OUT6
OUT7
OUT8
OUT9
LOAD
OUT14
CLK
GND
BLANK
OUT10
OUT11
OUT12
OUT13
OUT15
OUT16
OUT17
OUT18
OUT19
DOUT
VBB
VCC
DIN
OUT0
OUT1
OUT2
OUT3
OUT4
MAX6921AUI
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
OUT5
OUT6
OUT7
OUT8
OUT9
LOAD
OUT14
CLK
GND
BLANK
OUT10
OUT11
OUT12
OUT13
OUT15
OUT16
OUT17
OUT18
OUT19
VSS
VBB
VCC
DIN
OUT0
OUT1
OUT2
OUT3
OUT4
MAX6931AUI
MAX6921AQI
OUT17
OUT16
OUT15
OUT14
OUT13
OUT12
OUT11
OUT1
OUT2
OUT3
OUT4
OUT5
OUT6
OUT7
OUT10
BLANK
GND
CLK
LOAD
OUT9
OUT8
OUT18
OUT19
DOUT
VBB
VCC
DIN
OUT0
12 13 14 15 16 17 18
1234262728
19
20
21
22
23
24
25
5
6
7
8
9
10
11
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
12 ______________________________________________________________________________________
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in
the package code indicates RoHS status only. Package draw-
ings may show a different suffix character, but the drawing per-
tains to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
28 TSSOP U28+1 21-0066
28 Wide SO W28+1 21-0042
28 PLCC Q28+1 21-0049
Chip Information
PROCESS: BiCMOS
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________
13
© 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 10/03 Initial release —
1 4/10 Added automotive and lead-free parts to Ordering Information 1
