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UC1625-SP Scheda tecnica(PDF) 8 Page - Texas Instruments |
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UC1625-SP Scheda tecnica(HTML) 8 Page - Texas Instruments |
8 / 21 page UC1625-SP SLUSAG8A – SEPTEMBER 2011 – REVISED SEPTEMBER 2011 www.ti.com Terminal Functions (continued) TERMINAL DESCRIPTION NAME NO. These outputs can drive the gates of N-channel power MOSFETs directly or they can drive the bases of power Darlingtons if some form of current limiting is used. They are meant to drive low-side power devices in high-current output stages. Current available from these pins PDA, PDB, PDC 12, 13, 14 can peak as high as 0.5 A. These outputs feature a true totem-pole output stage. Beware of exceeding device power dissipation limits when using these outputs for high continuous currents. These outputs pull high to turn a “low-side” device on (active high). These outputs are open-collector, high-voltage drivers that are meant to drive high-side power devices in high-current output stages. These are active low outputs, meaning that these outputs pull low to command a high-side device on. These outputs can drive PUA, PUB, PUC 16, 17, 18 low-voltage PNP Darlingtons and P-channel MOSFETs directly, and can drive any high-voltage device using external charge pump techniques, transformer signal coupling, cascode level-shift transistors, or opto-isolated drive (high-speed opto devices are recommended). (See applications). This supply pin carries the current sourced by the PD outputs. When connecting PD outputs directly to the bases of power Darlingtons, the PWR VCC pin can be current limited with a PWR VCC 11 resistor. Darlington outputs can also be "Baker Clamped" with diodes from collectors back to PWR VCC. (See Applications) The device can chop power devices in either of two modes, referred to as “two-quadrant” (Quad Sellow) and “four quadrant” (Quad Sel high). When two-quadrant chopping, the pulldown power devices are chopped by the output of the PWM latch while the pullup drivers remain on. The load chops into one commutation diode, and except for back-EMF, will exhibit slow discharge current and faster charge current. Two-quadrant chopping can be QUAD SEL 22 more efficient than four-quadrant. When four-quadrant chopping, all power drivers are chopped by the PWM latch, causing the load current to flow into two diodes during chopping. This mode exhibits better control of load current when current is low, and is preferred in servo systems for equal control over acceleration and deceleration. The QUAD SEL input has no effect on operation during braking. Each time the TACH-OUT pulses, the capacitor tied to RC-BRAKE discharges from approximately 3.33 V down to 1.67 V through a resistor. The tachometer pulse width is approximately T = 0.67 RT CT, where RT and CT are a resistor and capacitor from RC-BRAKE to ground. Recommended values for RT are 10 kΩ to 500 kΩ, and recommended values for CT are 1 nF to 100 nF, allowing times between 5 μs and 10 ms. Best accuracy and stability are achieved with values in the centers of those ranges. RC-BRAKE also has another function. If RC-BRAKE pin is pulled below the brake threshold, RC-BRAKE 21 the device enters brake mode. This mode consists of turning off all three high-side devices, enabling all three low-side devices, and disabling the tachometer. The only things that inhibit low-side device operation in braking are low-supply, exceeding peak current, OV-COAST command, and the PWM comparator signal. The last of these means that if current sense is implemented such that the signal in the current sense amplifier is proportional to braking current, the low-side devices will brake the motor with current control. (See applications) Simpler current sense connections results in uncontrolled braking and potential damage to the power devices. The UC1625 can regulate motor current using fixed-frequency pulse width modulation (PWM). The RC-OSC pin sets oscillator frequency by means of timing resistor ROSC from the RC-OSC pin to VREF and capacitor COSC from RC-OSC to Gnd. Resistors 10 k Ω to 100 kΩ and capacitors 1 nF to 100 nF works the best, but frequency should always be below 500 kHz. Oscillator frequency is approximately: F = 2/(ROSC x COSC ) RC-OSC 25 Additional components can be added to this device to cause it to operate as a fixed off-time PWM rather than a fixed frequency PWM, using the RC-OSC pin to select the monostable time constant. The voltage on the RC-OSC pin is normally a ramp of about 1.2 V peak-to-peak, centered at approximately 1.6 V. This ramp can be used for voltage-mode PWM control, or can be used for slope compensation in current-mode control. 8 Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated Product Folder Link(s) :UC1625-SP |
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