MAX6920AWP+ Maxim Integrated Products, MAX6920AWP+ Datasheet - Page 8

• Manufacturer Part Number: MAX6920AWP+
• Description: IC DRVR VFD TUBE SRL 20-SOIC
• Manufacturer: Maxim Integrated Products

Specifications of MAX6920AWP+

• Display Type: Vacuum Fluorescent (VF)
• Interface: 4-Wire Serial
• Current - Supply: 1mA
• Voltage - Supply: 8 V ~ 76 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Configuration: -
• Digits Or Characters: -
• Lead Free Status / Rohs Status: Details

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 application
(three grids) 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 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 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
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 Rl
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
12-Output, 76V, Serial-Interfaced
VFD Tube Driver
Figure 4. Paralleling Outputs
8
_______________________________________________________________________________________
BB
voltage of only 20V. A safe way to parallel out-
(Figure
I
PEAK
MAX6920
4). Because the outputs cannot sink
= (grids x 1600)
OUT0
OUT1
D1
D2
Paralleling Outputs
Power Dissipation
R
1/2
OUTPUT
mA
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
for static tube drivers with the following equation:
where:
A = number of anodes driven (a MAX6920 can drive a
maximum of 12).
I
(V
mum anode current I
A static tube dissipation example follows:
V
Determine the power dissipation (P
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.
A multiplexed tube dissipation example follows:
V
I
Thus, for a 20-pin wide SO package (T
+100°C/W from Absolute Maximum Ratings), the maxi-
mum allowed ambient temperature T
So T
ANODE
ANODE
GRID
ANODE
P
CC
CC
T
BB
D
J(MAX)
P
P
= (V
A
P
= 5V ±5%, V
D
D
- V
D
= 5V ±5%, V
= maximum grid current
D
= +140°C.
= (5.25V x 0.7mA) + (18V x 0.9mA) + ((2.5V x
x A) + ((V
= (5.25V X 0.7mA)+ (42V x 0.9mA) + ((2.5V x
figure
= (V
= maximum anode current.
= maximum anode current
= 0.4mA, I
H
CC
) is the output voltage drop at the given maxi-
0.4mA/25mA) x 0.4mA x 6) +
((2.5V x 24mA/25mA) x 24mA) = 99mW
= T
I
2mA/25mA) x 2mA x 12) = 24.7mW
ANODE
x I
+100°C/W)
CC
A
by the actual tube drive duty cycle, taking
CC
+ (P
x I
BB
) + (V
BB
CC
GRID
x A))
D
- V
BB
D
= 10V to 18V, A = 12, I
OUT
) + (V
x T
H
BB
can be estimated by multiplying
) x I
= 36V to 42V, A = 6, G = 6,
= 24mA
JA
.
x I
BB
) = +150°C = T
GRID
BB
x I
) + ((V
BB
))
D
) + ((V
D
D
A
is the worst case,
BB
) for the MAX6920
) for the MAX6920
is given by:
JA
- V
BB
A
H
= 1 / 0.01 =
OUT
+ (0.099 x
- V
) x I
H
= 2mA
ANODE
) x