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US651 / US661 Two-Coil Low Noise Fan Driver 390109020902 Page 5 of 12 Preliminary Datasheet Rev 002 Mar/07 8 Output Behaviour vs Magnetic Pole Parameter Test Conditions OUT1 OUT2 FG North pole B < Brp Low High Low South pole B > Bop High Low High Table 5: Driver output vs magnetic pole Note : The magnetic pole is applied facing the branded side of the package 9 Detailed General Description The US651/661 is a one-chip solution for driving two-coil brushless DC fans. Based on CMOS process, the chip contains a Hall-effect sensor with dynamic offset correction, logic control and two low-ohmic open-drain output drivers. The output drivers OUT1 and OUT2 are fully protected against switching transients. So there is no need of external zener diode to cut the high voltage spikes induced by the fan coils. In case the junction temperature TJ exceeds TSD, the thermal protection stops the current flowing through the full bridge by setting the outputs OUT1 and OUT2 low and setting the output FG (RD) high. The IC stays in this state until the junction temperature decreases below TREL. Reverse voltage protection is integrated on the VDD pin. The FG/RD open drain output has an internal current limit which protects the driver in case of accidentally big current flow through the logic driver. It could occur if a low-ohmic pull-up resistor is used or if the FG/RD output is directly short connected to a supply voltage. The US651 has an open-drain tachometer FG output that follows the Hall signal, thus enabling to determine the rotation speed of the fan. In the US661, the open-drain alarm output RD is a safety signal which allows detecting if the fan rotates or not. It is active low during normal spinning of the motor. It goes high when the magnetic flux switching frequency drops below nearly: - 2Hz for 12V application (60RPM for 2 pole-pair fan) - 1Hz for 5V application (30RPM for 2 pole-pair fan) |
