ROHM BH6626FS

Magnetic disk ICs
Read / Write amplifier for FDD
BH6626FS
The BH6626FS is a read / write IC designed for floppy disk drives and has an internal active filter for memory systems
in which saddle margins are important. Any of multiple write current settings can be selected, and both density switching
and inner track / outer track switching are done internally.
Applications
Floppy disc drives (1MB and 2MB)
Features
1) Active filter switched internally.
2) Time domain filter with internal switch set according
to transfer rate.
3) Density switching and inner track / outer track
switching are done internally.
Absolute maximum ratings (unless otherwise noted, Ta = 25C)
Recommended operating conditions
522
Magnetic disk ICs
BH6626FS
Block diagram
523
Magnetic disk ICs
Pin descriptions and Input / output circuits
524
BH6626FS
Magnetic disk ICs
BH6626FS
525
Magnetic disk ICs
526
BH6626FS
Magnetic disk ICs
BH6626FS
Electrical characteristics (unless otherwise noted, Ta = 25C, VCC = 5V)
Supply current
527
Magnetic disk ICs
Low level voltage detection circuit
Recovery time
Preamplifier
528
BH6626FS
Magnetic disk ICs
BH6626FS
Preamplifier / L.P.F / differentiator (B.P.F)
Comparator and waveform shaping
Write circuit
529
Magnetic disk ICs
BH6626FS
Erase output
Logic input
Mode table
Total filter peak frequency setting
f0 = a / (RRCC [kΩ] +0.09) [kHz]
a=
412 1M outer track
439 1M inner track
790 2M outer track
867 2M inner track
732 outer track(with OPTION 1), OPTION 2
836 inner track (with OPTION 1)
TDF time constant setting
250 [kbps] : T = 939 RRCC [kΩ] +667 [ns]
500 [kbps] : T = 403 RRCC [kΩ] +474 [ns]
Write current setting
Iwr =
530
24.0
RWCC [kΩ]
[mA]
Magnetic disk ICs
BH6626FS
Filter characteristic
531
Magnetic disk ICs
BH6626FS
Measurement circuit
Circuit operation
(1) Read
The input signal from the head coils from each side of the
disc is amplified by the preamplifier and then differentiated. The filter time constant can be set externally. After
differentiation, the differential output is input to the
comparator. The time domain filter detects zero cross,
and the output is converted to read data. The monostable
multivibrator width can be set externally, while the read
data pulse width is a constant 400ns.
(2) Write
Input write data are converted to toggle movements by
532
the internal flip-flops, operating the write driver. The write
driver current is supplied by the write current generator,
but the externally set current can be controlled according
to density and by selecting inner track/ outer track.
(3) Erase
An open collector output pin is used, and the erase current is set with a resistor between it and the head.
(4) Power supply
When the low level voltage detector detects a drop in the
supply voltage, writing and erasing are prohibited.
Magnetic disk ICs
BH6626FS
Operation notes
(1) Use a short pattern for VCC, and a sufficiently wide
AGND and DGND. Keep the impedance between VCC
and GND low by inserting a bypass capacitor.
(2) Use a pattern that will minimize interference between digital signals and the head.
Electrical characteristic curves
533
Magnetic disk ICs
External dimensions (Units: mm)
534
BH6626FS