19-3020; Rev 1; 4/10
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
The MAX6921/MAX6931 are 20-output, 76V, vacuumfluorescent 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 shutdown 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).
Features
o 5MHz Industry-Standard 4-Wire Serial Interface
o 3V to 5.5V Logic Supply Range
o 8V to 76V Grid/Anode Supply Range
o -11V to 0V Filament Bias Supply (MAX6931 Only)
o Push-Pull CMOS High-Voltage Outputs
o Outputs can Source 40mA, Sink 4mA
Continuously
o Outputs can Source 75mA Repetitive Pulses
o Outputs can be Paralleled for Higher Current Drive
o Any Output can be Used as a Grid or an Anode
Driver
o Blank Input Simplifies PWM Intensity Control
o Small 28-Pin TSSOP Package
o -40°C to +125°C Temperature Range
Ordering Information
PART
TEMP RANGE
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
+Denotes a lead-free(Pb)/RoHS-compliant package.
/V denotes an automotive qualified part.
Typical Operating Circuit
Applications
White Goods
Industrial Weighing
Gaming Machines
Security
Automotive
Telecom
Avionics
VFD Modules
Instrumentation
Industrial Control
PIN-PACKAGE
+5V
+60V
C2
100nF
C1
100nF
7
VCC
8
VBB
MAX6931
VFDOUT
VFCLK
VFLOAD
VFBLANK
6
22
23
20
20
DIN
OUT0–OUT19
CLK
VFD TUBE
µC
LOAD
BLANK
VSS
-7V
9
GND
21
C3
100nF
Pin Configurations appear at end of data sheet.
________________________________________________________________ Maxim Integrated Products
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.
1
MAX6921/MAX6931
General Description
MAX6921/MAX6931
20-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 (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
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)
MAX
UNITS
Logic Supply Voltage
PARAMETER
VCC
3
5.5
V
Tube Supply Voltage
VBB
8
76
V
Bias Supply Voltage
(MAX6931 Only)
VSS
-11
0
V
Total Supply Voltage
(MAX6931 Only)
VBB - VSS
76
V
Logic Supply Operating Current
SYMBOL
ICC
CONDITIONS
78
All outputs OUT_ high, TA = +25°C
CLK = idle
TA = -40°C to +125°C
540
IBB
All outputs OUT_ high
All outputs OUT_ low
Bias Supply Operating Current
(MAX6931 Only)
ISS
All outputs OUT_ high
2
TYP
All outputs OUT_ low, TA = +25°C
CLK = idle
TA = -40°C to +125°C
All outputs OUT_ low
Tube Supply Operating Current
MIN
170
200
900
µA
1000
TA = +25°C
1.65
3.0
0.85
1.3
TA = -40°C to +125°C
6.9
TA = +25°C
TA = -40°C to +125°C
mA
1.4
TA = +25°C
-0.8
TA = -40°C to +125°C
-1.9
TA = +25°C
-1.4
TA = -40°C to +125°C
-1.5
-0.38
-0.87
_______________________________________________________________________________________
mA
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
(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
VBB ≥ 15V
IOUT = -25mA
VBB ≥ 15V
IOUT = -40mA
High-Voltage OUT_
8V < VBB < 15V
IOUT = -25mA
VBB ≥ 15V
IOUT = 1mA
Low-Voltage OUT_
(MAX6921 Only)
VL
8V < VBB < 15V
IOUT = 1mA
VBB ≥ 15V
IOUT = 1mA
Low-Voltage OUT_
(MAX6931 Only)
VL
8V < VBB < 15V
IOUT = 1mA
MIN
TYP
TA = +25°C
VBB - 2
TA = -40°C to +85°C
VBB - 2.5
MAX
UNITS
TA = -40°C to +125°C
TA = -40°C to +85°C
VBB - 3.5
TA = -40°C to +125°C
VBB - 4.0
TA = +25°C
V
VBB - 1.2
TA = -40°C to +85°C
VBB - 2.5
TA = -40°C to +125°C
VBB - 3.0
TA = +25°C
0.75
1
TA = -40°C to +85°C
1.5
TA = -40°C to +125°C
1.9
TA = +25°C
0.8
1.1
TA = -40°C to +85°C
1.6
TA = -40°C to +125°C
2.0
TA = +25°C
V
VSS + 0.75 VSS + 1
TA = -40°C to +85°C
VSS + 1.5
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
TA = -40°C to +125°C
VSS + 2.0
V
Rise Time OUT_ (20% to 80%)
tR
VBB = 60V, CL = 50pF, RL =2.3kΩ
0.9
2
µs
Fall Time OUT_ (80% to 20%)
tF
VBB = 60V, CL = 50pF, RL =2.3kΩ
0.6
1.5
µs
µs
SERIAL INTERFACE TIMING CHARACTERISTICS
LOAD Rising to OUT_ Falling Delay
(Notes 2, 3)
0.9
1.8
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)
1.3
2.5
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
∆VI
High-Voltage DOUT
VOH
ISOURCE = -1.0mA
Low-Voltage DOUT
VOL
ISINK = 1.0mA
0.5
0.8 x
VCC
V
0.3 x
VCC
0.6
V
V
VCC 0.5
V
0.5
V
_______________________________________________________________________________________
3
MAX6921/MAX6931
ELECTRICAL CHARACTERISTICS (continued)
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
TYP
MAX
3V to 4.5V
MIN
60
100
4.5V to 5.5V
30
80
UNITS
ns
CLK Clock Period
tCP
200
ns
CLK Pulse-Width High
tCH
90
ns
90
ns
CLK Pulse-Width Low
tCL
CLK Rise to LOAD Rise Hold
tCSH
DIN Setup Time
tDS
DIN Hold Time
tDH
DOUT Propagation Delay
tDO
LOAD Pulse High
(Note 2)
100
ns
5
ns
3.0V to 4.5V
20
4.5V to 5.5V
15
CDOUT = 10pF
ns
3.0V to 4.5V
25
120
240
4.5V to 5.5V
20
75
150
tCSW
ns
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.
Typical Operating Characteristics
(VCC = 5.0V, VBB = 76V, and TA = +25°C, unless otherwise noted.)
1.4
1.2
VBB = 40V
VBB = 8V
1.0
0.8
0.6
1.4
1.2
VBB = 76V
1.0
0.8
0.6
0.4
0.2
0.2
VBB = 40V
VBB = 8V
0
20
40
60
TEMPERATURE (°C)
80
100 120
350
VCC = 5V, CLK = 5MHz
300
250
VCC = 3.3V, CLK = 5MHz
200
150
100
50
0
-40 -20
400
MAX6921/31 toc03
1.6
0.4
0
4
1.8
SUPPLY CURRENT (µA)
1.6
2.0
LOGIC SUPPLY CURRENT (ICC)
vs. TEMPERATURE (OUTPUTS LOW)
MAX6921/31 toc02
VBB = 76V
1.8
SUPPLY CURRENT (mA)
2.0
TUBE SUPPLY CURRENT (IBB)
vs. TEMPERATURE (OUTPUTS HIGH)
MAX6921/31 toc01
TUBE SUPPLY CURRENT (IBB)
vs. TEMPERATURE (OUTPUTS LOW)
SUPPLY CURRENT (mA)
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
VCC = 5V, CLK = IDLE
VCC = 3.3V, CLK = IDLE
0
-40 -20
0
20
40
60
TEMPERATURE (°C)
80
100 120
-40 -20
0
20
40
60
TEMPERATURE (°C)
_______________________________________________________________________________________
80
100 120
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
LOGIC SUPPLY CURRENT (ICC)
vs. TEMPERATURE (OUTPUTS HIGH)
OUTPUT VOLTAGE (VBB - VH)
vs. TEMPERATURE (OUTPUT HIGH)
650
600
VCC = 3.3V, CLK = 5MHz
550
VCC = 5V, CLK = IDLE
500
VBB = 8V
2.5
2.0
VBB = 40V
1.5
VBB = 76V
1.0
VCC = 3.3V, CLK = IDLE
0.5
450
400
0
-40 -20
0
20
40
60
80
100 120
-40 -20
0
20
40
60
80
100 120
TEMPERATURE (°C)
TEMPERATURE (°C)
OUTPUT VOLTAGE
vs. TEMPERATURE (OUTPUT LOW)
OUTPUT RISE AND FALL WAVEFORM
IOUT = 4mA
12
VBB = 76V
10
MAX6921/31 toc06
MAX6921/31 toc07
14
OUTPUT VOLTAGE (V)
IOUT = -40mA
3.0
OUTPUT VOLTAGE (V)
SUPPLY CURRENT (µA)
700
MAX6921/31 toc05
VCC = 5V, CLK = 5MHz
750
3.5
MAX6921/31 toc04
800
BLANK
2V/div
VBB = 40V
8
6
OUT_
20V/div
VBB = 8V
4
2
0
-40 -20
0
20
40
60
80
100 120
1µs/div
TEMPERATURE (°C)
Pin Description
PIN
WIDE
SO/PLCC
TSSOP
NAME
FUNCTION
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).
Serial-Data Input. Data is loaded into the internal shift register on CLK’s rising edge.
MAX6921
MAX6931
MAX6921
1–5
1–5
—
6
6
27
OUT4 to
OUT0
DIN
7
7
28
VCC
Logic Supply Voltage. Bypass to GND with 100nF capacitor.
8
8
1
VBB
VFD Tube Supply Voltage. Bypass to GND with 100nF capacitor.
9
—
2
DOUT
Serial-Clock Output. Data is clocked out of the internal shift register to DOUT on
CLK’s rising edge.
_______________________________________________________________________________________
5
MAX6921/MAX6931
Typical Operating Characteristics (continued)
(VCC = 5.0V, VBB = 76V, and TA = +25°C, unless otherwise noted.)
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
Pin Description (continued)
PIN
WIDE
SO/PLCC
TSSOP
MAX6921
MAX6931
MAX6921
—
9
—
NAME
VSS
FUNCTION
Filament Bias Supply Voltage. Bypass to GND with a 100nF capacitor.
10–19
10–19
—
OUT19 to VFD Anode and Grid Drivers. OUT19 to OUT10 are push-pull outputs swinging from
OUT10 VBB to GND (MAX6921 only), and from VBB to VSS (MAX6931 only).
—
—
3-12
OUT19 to VFD Anode and Grid Drivers. OUT19 to OUT10 are push-pull outputs swinging from
OUT10 VBB to GND.
20
20
13
BLANK
21
21
14
GND
22
22
15
CLK
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.
Ground
Serial-Clock Input. Data is loaded into the internal shift register on CLK’s rising edge.
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.
23
23
16
LOAD
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.
CLK
MAX6921 ONLY
SERIAL-TO-PARALLEL SHIFT REGISTER
DIN
DOUT
LATCHES
LOAD
BLANK
MAX6921
MAX6931
OUT0 OUT1 OUT2
OUT19
Figure 1. MAX6921/MAX6931 Functional Diagram
6
_______________________________________________________________________________________
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
40Ω
TYPICAL
40Ω
TYPICAL
SLEW-RATE
CONTROL
OUT_
750Ω
TYPICAL
SLEW-RATE
CONTROL
OUT_
750Ω
TYPICAL
VSS
Figure 2. MAX6921 CMOS Output Driver Structure
Detailed Description
The MAX6921/MAX6931 are VFD tube drivers comprising a 4-wire serial interface driving 20 high-voltage railto-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.
The 4-wire serial interface comprises a 20-bit shift register 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 V BB and GND
(MAX6921) or VSS (MAX6931). The output rise time is
always slower than the output fall time to avoid shootthrough 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.
Figure 3. MAX6931 CMOS Output Driver Structure
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 operation of the serial interface. Data can be shifted into the
serial interface shift register and latched regardless of
the state of BLANK.
_______________________________________________________________________________________
7
MAX6921/MAX6931
VBB
VBB
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
tCSW
LOAD
tCL
tCSH
tCH
tCP
CLK
tDH
tDS
D19
DIN
D18
D1
D0
tDO
DOUT
D19
Figure 4. 4-Wire Serial Interface Timing Diagram
Table 1. 4-Wire Serial Interface Truth Table
BLANKING
SERIAL CLOCK SHIFT REGISTER CONTENTS LOAD
LATCH CONTENTS
OUTPUT CONTENTS
INPUT
INPUT
DATA INPUT
INPUT
CLK D0 D1 D2 … Dn-1 Dn
LOAD D0 D1 D2 … Dn-1 Dn
BLANK
D0 D1 D2 … Dn-1 Dn
DIN
H
H
R0 R1 … Rn-2 Rn-1
L
L
R0 R1 … Rn-2 Rn-1
X
R0
R1 R2 … Rn-1
X
X
X
…
X
P0
P1
P2 … Pn-1
Rn
X
L
R0 R1 R2
… Rn-1 Rn
Pn
H
P0
P1
P2
… Pn-1 Pn
L
P0
X
X
X
…
H
L
X
X
P1 P2
L
L
…
Pn-1
Pn
…
L
L
L = Low logic level.
H = High logic level.
X = Don’t care.
P = Present state (shift register).
R = Previous state (latched).
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 register at DIN appear at the OUT0 to OUT19 outputs.
8
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
VH and VL electrical characteristics. Use this calculated
resistance to determine the output voltage drop at different output currents.
_______________________________________________________________________________________
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
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 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 outputs cannot sink
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 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 (P D ) for the
MAX6921/MAX6931 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 (the MAX6921/MAX6931
can drive a maximum of 20).
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 = 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)
MAX6921
MAX6931
D1
OUT0
OUTPUT
D2
OUT1
R
Figure 5. Paralleling Outputs
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.
_______________________________________________________________________________________
9
MAX6921/MAX6931
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 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:
IPEAK = (grids x 1600)1/2 mA
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 TA is given by:
TJ(MAX) = TA + (PD x TJA) = 150°C = TA + (0.122
x 78.125°C/W)
Typical Application Circuit
MAX685x
MAX6921
VFDOUT
DIN
VFCLK
CLK
VFLOAD
LOAD
VFBLANK
BLANK
DOUT
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.
MAX6921
DIN
Power-Supply Considerations
The MAX6921/MAX6931 operate with multiple powersupply voltages. Bypass the V CC , V BB , and V SS
(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.
CLK
LOAD
BLANK
DOUT
MAX6921
Power-Supply Sequencing
DIN
The order of the power-supply sequencing is not important. 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.
CLK
LOAD
BLANK
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 cascaded at the full 5MHz CLK speed when VCC ≥ 4.5V.
When VCC
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