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PBL3770AQNT Datasheet(PDF) 6 Page - Ericsson |
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PBL3770AQNT Datasheet(HTML) 6 Page - Ericsson |
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6 / 8 page  PBL 3770A 6 voltage across the sensing resistor is below the comparator threshold level. The sinking transistor then turns on and the motor current starts to increase again, The cycle is repeated until the current is turned off via the logic inputs. When both I 1 and I0 are high, all four transistors in the output H-bridge are turned off, which means that inductive current recirculates through two opposite free-wheeling diodes (see figure 6, arrow 3). this method of turning off the current results in a faster current decay than if only one transistor was turned off and will therefore improve speed performance in half-stepping mode. Heatsinking The junction temperature of the chip highly effects the lifetime of the circuit. In high-current applications, the heatsinking must be carefully conside- red. The Rth j-a of the PBL 3770A can be reduced by soldering the ground pins to a suitable copper ground plane on the printed circuit board (see figure 14) or by applying an external heatsink type V7 or V8, see figure 14. The diagram in figure 13 shows the maximum permissible power dissipation versus the ambient temperature in °C, for heatsinks of the type V7, V8, or a 20 cm 2 copper area respectively. Any external heatsink or printed circuit board copper must be connected to electrical ground. For motor currents higher than approx 600 mA, some form of heatsinking is recommended to assure optimal reliability. The diagrams in figures 12 and 13 can be used to determine the required heatsinking of the circuit. In some systems, forced-air cooling may be available to reduce the temperature rise of the circuit. Applications Information Motor selection Some stepper motors are not designed for continuous operation at maximum current. As the circuit drives a constant current through the motor, its tempera- ture can increase, both at low- and high- speed operation. Some stepper motors have such high core losses that they are not suited for switched-mode operation. Figure 5. Motor current (I M ), Vertical : 200 mA/div, Horizontal: 1 ms/div, expanded part 100 µs/div. Figure 6. Output stage with current paths for fast and slow current decay. Figure 7. Principal operating sequence. 0 200 mA/div 1 ms/div 100 µs/div Fast Current Decay Slow Current Decay Motor Current Time 1 2 3 3 2 1 External recirculation diodes R S I 0A I 1A Ph A Ph B I 0B I 1B I MA I MB 100% –100% 60% –60% 20% –20% 100% –100% 60% –60% Half step mode at 100 % Full step mode at 60 % Stand by mode at 20 % Full step position Half step position |
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