Output, 76v, serial-interfaced vfd tube drivers, Typical application circuit chip information – Rainbow Electronics MAX6931 User Manual
Page 10
MAX6921/MAX6931
A multiplexed tube dissipation example follows:
V
CC
= 5V ±5%, V
BB
= 36V to 42V, A = 12, G = 8,
I
ANODE
= 0.4mA, I
GRID
= 24mA
P
D
= (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 (T
JA
= 1 / 0.0128
= 78.125°C/W from Absolute Maximum Ratings), the
maximum allowed ambient temperature T
A
is given by:
T
J(MAX)
= T
A
+ (P
D
x T
JA
) = 150°C = T
A
+ (0.122 x
78.125°C/W)
So T
A
= +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 V
CC
, V
BB
, and V
SS
(MAX6931 only) power-supply pins to GND with 0.1µF
capacitors close to the device. The MAX6931 may be
operated with V
SS
tied to GND if a negative bias supply
is not required. For multiplex applications, it may be
necessary to add an additional bulk electrolytic capaci-
tor of 1µF or greater to the V
BB
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 V
CC
, V
BB
, and V
SS
(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 V
CC
is not
operational because the input protection diodes clamp
the signals.
Cascading Drivers (MAX6921 Only)
Multiple MAX6921s 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 cas-
caded at the full 5MHz CLK speed when V
CC
≥ 4.5V.
When V
CC
<4.5V, the longer propagation delay (t
DO
)
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
Chip Information
TRANSISTOR COUNT: 2743
PROCESS: BiCMOS