MAX6850 MAXIM [Maxim Integrated Products], MAX6850 Datasheet - Page 30

• Manufacturer Part Number: MAX6850
• Description: 4-Wire Interfaced, 7-, 14-, and 16-Segment Alphanumeric Vacuum-Fluorescent Display Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

The minimum commutation time, shown at (C) in Figure
14, is set by (2/OSC)s (500ns when OSC = 4MHz) to
ensure that shoot-through currents cannot flow during
phase reversal. Otherwise, the duty cycle of the bridge
(total on time: total time) sets the RMS voltage across
the filament. This technique provides a low-cost AC fila-
ment supply when using a regulated supply higher than
the RMS voltage rating of the filament.
Figure 15 shows the external components required for
the filament driver using a FET bridge.
PHASE1 and PHASE2 can be individually programmed
as one of four output types (Tables 38, 39).
When using the filament drive, first ensure that the fila-
ment duty-cycle register 0x09 is set to the correct value
before configuring the PHASE1 and PHASE2 outputs to
4-Wire Interfaced, 7-, 14-, and 16-Segment Alpha-
numeric Vacuum-Fluorescent Display Controller
Table 37. Filament Duty-Cycle Register Format
Figure 14. Filament Bridge Driver Timing Waveforms
Figure 15. Filament Bridge Driver (MOSFET)
30
Minimum setting example (01)
Maximum setting example (199 or 0xC7)
PHASE 1
PHASE 2
______________________________________________________________________________________
PHASE 1
PHASE 2
FILAMENT DUTY CYCLE
(A)
Q1
R2
100µs MULTIPLEX TIME PERIOD (OSC = 4MHz)
PHASE1 and PHASE2 Outputs
GND
Q2
(B)
(C)
VFIL
Q3
Q4
GND
(D)
R4
VFD TUBE
COMMAND
ADDRESS
(E)
0x09
0x09
D7
0
1
be filament drives. To stop the filament drive, program
either PHASE1 or PHASE2 (or both) to be logic-low gen-
eral-purpose outputs. Both PHASE1 and PHASE2 out-
puts come out of power-on-reset in logic-low condition.
The PUMP output can be programmed as one of four
output types (Table 40).
PORT0 and PORT1 can be individually programmed as
one of eight output types (Tables 41, 42). The PORT1
choices are similar to the PORT0 choices, except that
the last four items are invert logic. PORT0 output comes
out of power-on-reset in logic-low condition, whereas
PORT1 output initializes high.
The PORT0 and PORT1 shutdown outputs allow exter-
nal hardware (for example, a DC-DC converter power
supply for VFD) to be disabled by the MAX6850 when
the MAX6850 is shut down.
The 625Hz, 1250Hz, and 2500Hz outputs can drive a
piezo sounder either from PORT0 or PORT1 alone, or
by both ports together as bridge drive. For bridge
drive, the sounder is connected between PORT0 and
PORT1, taking advantage of the PORT1 output being
inverted with respect to PORT0. Select different fre-
quencies for PORT0 and PORT1 to obtain a wider
range of sounds when bridge drive is used.
The OSC1 and OSC2 inputs set the multiplex and blink
timing for the display driver. Connect an external resis-
tor from OSC2 to GND and an external capacitor C
from OSC1 to GND to set the frequency of the internal
RC oscillator. Alternatively, overdrive OSC1 with an
external TTL or CMOS clock. If an exact blink rate or
multiplex period is required, use an external clock
ranging between 2MHz and 8MHz to drive OSC1.
The multiplex clock frequency determines the multiplex
scan rate and the blink timing. The display scan rate is
{OSC / 400 / (1 + grids register value)}. There are 400
OSC cycles per digit multiplex period. For example,
with OSC = 4MHz, each display digit is enabled for
100µs. For a 40-grid display tube (grids register value
= 39 or 0x27), the display scan rate is 250Hz.
D6
0
1
D5
0
0
REGISTER DATA
Multiplex Clock and Blink Timing
D4
0
0
PORT0 and PORT1 Outputs
D3
0
0
D2
0
1
D1
0
1
PUMP Output
D0
1
1
HEX CODE
0xC7
0x01
OSC