MAX6921AUI+T Maxim Integrated Products, MAX6921AUI+T Datasheet - Page 9

• Manufacturer Part Number: MAX6921AUI+T
• Description: IC DRVR VFD TUBE SRL 28-TSSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX6921AUI+T

• Display Type: Vacuum Fluorescent (VF)
• Interface: 4-Wire Serial
• Current - Supply: 1.65mA
• Voltage - Supply: 8 V ~ 76 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Configuration: -
• Digits Or Characters: -
• Lead Free Status / Rohs Status: Details

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:
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.
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 V
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
Figure 5. Paralleling Outputs
BB
voltage of only 20V. A safe way to parallel out-
I
PEAK
MAX6921
MAX6931
_______________________________________________________________________________________
= (grids x 1600)
OUT0
OUT1
Output Current Ratings
D1
D2
Paralleling Outputs
R
1/2
20-Output, 76V, Serial-Interfaced
OUTPUT
mA
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.
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 (P
MAX6921/MAX6931 for static tube drivers with the fol-
lowing equation:
where:
A = number of anodes driven (the MAX6921/MAX6931
can drive a maximum of 20).
I
(V
mum anode current I
A static tube dissipation example follows:
Determine the power dissipation (P
MAX6931 for multiplex tube drivers with the following
equation:
where:
A = number of anodes driven.
G = number of grids driven.
I
I
The calculation presumes all anodes are on, but only
one grid is on. The calculated P
presuming one digit is always being driven with all its
anodes lit. Actual P
this P
into account interdigit blanking and any PWM intensity
control.
V
ANODE
ANODE
GRID
CC
BB
P
P
((2.5V x 2mA/25mA) x 2mA x 20) = 38mW
P
- V
= 5V ±5%, V
D
= maximum grid current.
D
D
D
figure by the actual tube drive duty cycle, taking
= maximum anode current.
= maximum anode current.
H
= (V
= (5.25V x 1mA)+ (18V x 1.4mA) +
= (V
x I
x I
) is the output voltage drop at the given maxi-
ANODE
ANODE
CC
CC
VFD Tube Drivers
x I
x I
BB
CC
CC
x A)
x A) + ((V
D
OUT
) + (V
= 10V to 18V, A = 20, I
) + (V
can be estimated by multiplying
.
BB
BB
BB
x I
x I
- V
Power Dissipation
BB
BB
H
D
) + ((V
) + ((V
D
) x I
) for the MAX6921/
is the worst case,
GRID
BB
BB
D
OUT
)
- V
- V
) for the
H
H
= 2mA
)
)
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