Rainbow Electronics MAX6934 - Manual (Page 10)

MAX6922/MAX6932/MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers
10 ______________________________________________________________________________________
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 dis-
charges 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 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 driver
devices, try to share the average dissipation evenly
between the drivers.
Determine the power dissipation (P
D
) for the MAX6922/
MAX6932/MAX6933/MAX6934 for static tube drivers
with the following equation:
P D = (V CC x I CC ) + (V BB x I BB ) + ((V BB - V H ) x
I ANODE x A))
where:
A = number of anodes driven (maximum of 32 with the
MAX6922/MAX6934).
I ANODE = maximum anode current.
(V
BB
- V
H
) is the output voltage drop at the given maxi-
mum anode current I OUT .
A static tube dissipation example follows:
V CC = 5V ±5%, V BB = 10V to 18V, A = 32, I OUT = 2mA
P D = (5.25V x 1.5mA)+ (18V x 2.2mA) +
((2.5V x 2mA/25mA) x 2mA x 32) = 60mW
Determine the power dissipation (P D ) for the MAX6922/
MAX6932/MAX6933/MAX6934 for multiplex tube drivers
with the following equation:
P
D
= (V
CC
x I
CC
) + (V
BB
x I
BB
) + ((V
BB
- V
H
) x
I ANODE x A) + ((V BB - V H ) x I GRID ))
where:
A = number of anodes driven.
G = number of grids driven.
I ANODE = maximum anode current.
I
GRID
= maximum grid current.
The calculation presumes all anodes are on, but only
one grid is on. The calculated P
D
is the worst case, pre-
suming one digit is always being driven with all its
anodes lit. Actual P
D can be estimated by multiplying
this P
D
figure by the actual tube drive duty cycle, taking
into account interdigit blanking and any PWM intensity
control.
A multiplexed tube dissipation example follows:
V
CC
= 5V ±5%, V
BB
= 36V to 42V, A = 20, G = 12,
I ANODE = 0.4mA, I GRID = 24mA
P
D
= (5.25V x 1.5mA)+ (42V x 2.2mA) +
((2.5V x 0.4mA/25mA) x 0.4mA x 20) +
((2.5V x 24mA/25mA) x 24mA) = 158mW
Thus, for a 44-pin PLCC package (T JA = 1 / 0.0133 =
75.188°C/W from Absolute Maximum Ratings), the maxi-
mum allowed ambient temperature T
A
is given by:
T J(MAX) = T A + (P D x T JA ) = +150°C = T A + (0.158 x
75.188°C/W)
So T A = +138°C.
This means that the driver can be operated in this
application with a PLCC package up to the +125°C
maximum operating temperature.
Power-Supply Considerations
The MAX6922/MAX6932/MAX6933/MAX6934 operate
with multiple power-supply voltages. Bypass the V
CC ,
V BB , and V SS (MAX6932/MAX6933/MAX6934 only)
power-supply pins to GND with 0.1µF capacitors close
to the device. The MAX6932/MAX6933/MAX6934 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
capacitor of 1µF or greater to the V
BB supply.
Power-Supply Sequencing
The order of the power-supply sequencing is not impor-
tant. These ICs are damaged if any combination of
V
CC , V BB , and V SS 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
logic inputs if the logic supply V
CC is not operational
because the input protection diodes clamp the signals.
Cascading Drivers
(MAX6922/MAX6932/MAX6934 Only)
Multiple driver ICs 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.