Datasheet

clocked in are loaded. Therefore, multiple devices can
share CLK and DIN, as long as they have unique LOAD
controls.
Determining Driver Output Voltage Drop
The outputs are CMOS drivers, and have a resistive
characteristic. The typical and maximum sink and source
output resistances can be calculated from the V
H
and V
L
electrical characteristics. Use this calculated resistance
to determine the output voltage drop at different output
currents.
Output Current Ratings
The continuous current-source capability is 40mA per
output. Outputs may 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 dissipation
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 multi-
plex ratio is given by:
I
PEAK
= (grids x 1600)
1/2
mA
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.
Paralleling Outputs
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 750W), that
resistance is low enough to dissipate 530mW when short-
ed to an opposite level output at a V
BB
voltage of only
20V. A safe way to parallel outputs is to use diodes to pre-
vent the outputs from sinking current (Figure 5). Because
the diodes also stop the outputs from sinking current
from the VFD tube, an external discharge resistor, R, is
required. For static tubes, R can be a large value such as
100kW. For multiplexed tubes, the value of the resistor
can be determined by the load capacitance and timing
Figure 5. Paralleling Outputs
L = Low logic level.
H = High logic level.
X = Don’t care.
P = Present state (shift register).
R = Previous state (latched).
Table 1. 4-Wire Serial Interface Truth Table
SERIAL
DATA
INPUT
DIN
CLOCK
INPUT
SHIFT REGISTER CONTENTS
LOAD
INPUT
LATCH CONTENTS
BLANKING
INPUT
OUTPUT CONTENTS
CLK D0 D1 D2 … Dn-2 Dn-1 LOAD D0 D1 D2 … Dn-2 Dn-1 BLANK D0 D1 D2 … Dn-2 Dn-1
H H R0 R1 … Rn-2 Rn-1
L L R0 R1 … Rn-2 Rn-1
X R0 R1 R2 … Rn-1 Rn
X X X … X X L R0 R1 R2 … Rn-1 Rn
P0 P1 P2 … Pn-1 Pn H P0 P1 P2 … Pn-1 Pn L P0 P1 P2 … Pn-1 Pn
X X X … X X H L L L … L L
MAX6922
MAX6932
MAX6933
MAX6934
OUT0
OUT1
D1
D2
R
OUTPUT
www.maximintegrated.com
Maxim Integrated │ 9
MAX6922/MAX6932/
MAX6933/MAX6934
27-, 28-, and 32-Output, 76V,
Serial-Interfaced VFD Tube Drivers