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| Communication ICs Read / Write amplifier for FDD BH6629BFS The BH6629BFS, a read / write IC designed for floppy disk drives, has a recording system that puts top priority on saddle margin. Offers multi-step switching of write current, while density and inner track / outer track edge are switched internally. FApplications Floppy disk drives (1MB and 2MB drives) FFeatures 1) Internal active filter switch. 2) Time domain filter (with internal switching based on transfer rate). 3) Internal switching of write current density and inner track / outer track. FAbsolute maximum ratings (unless otherwise noted, Ta = 25_C) FRecommended operating conditions (Ta = 25_C) 428 Communication ICs FBlock diagram BH6629BFS 429 Communication ICs FPin descriptions BH6629BFS 430 Communication ICs BH6629BFS 431 Communication ICs BH6629BFS 432 Communication ICs BH6629BFS 433 Communication ICs FElectrical characteristics (unless otherwise noted, Ta = 25_C, VCC = 5V) Supply current BH6629BFS Low level voltage detection circuit Recovery time 434 Communication ICs Preamplifier BH6629BFS Preamplifier / L.P.F / differentiator (B.P.F) 435 Communication ICs Comparator and waveform shaping BH6629BFS Write circuit Erase output 436 Communication ICs Logic input BH6629BFS FMode table Total filter peak frequency setting f0 = a / (RRCC [k +0.09) [kHz] a= 391 1M outer track 468 1M inner track 777 2M outer track 786 2M inner track 732 outer track (with OPTION 1), OPTION 2 777 inner track (with OPTION 1) TDF time constant setting 250 [kbps] : T = 939 500 [kbps] : T = 403 Write current setting Iwr = RRCC [k] +667 [ns] RRCC [k] +474 [ns] 24.0 RWCC [k] [mA] 437 Communication ICs FFilter characteristic BH6629BFS 438 Communication ICs FMeasurement circuit BH6629BFS FCircuit 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 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. 439 Communication ICs FOperation 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. BH6629BFS FElectrical characteristic curves 440 Communication ICs BH6629BFS FExternal dimensions (Units: mm) 441 |
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