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 can be
operated with V
SS
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 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 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 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
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