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5962-9232501MXA Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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5962-9232501MXA Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 13 page  Application Information (Continued) INTERNAL CHARGE PUMP AND USE OF BOOTSTRAP CAPACITORS To turn on the high-side (sourcing) DMOS power devices, the gate of each device must be driven approximately 8V more positive than the supply voltage. To achieve this an in- ternal charge pump is used to provide the gate drive voltage. As shown in Figure 5, an internal capacitor is alternately switched to ground and charged to about 14V, then switched to V supply thereby providing a gate drive voltage greater than V supply. This switching action is controlled by a con- tinuously running internal 300 kHz oscillator. The rise time of this drive voltage is typically 20 µs which is suitable for oper- ating frequencies up to 1 kHz. For higher switching frequencies, the LMD18200 provides for the use of external bootstrap capacitors. The bootstrap principle is in essence a second charge pump whereby a large value capacitor is used which has enough energy to quickly charge the parasitic gate input capacitance of the power device resulting in much faster rise times. The switch- ing action is accomplished by the power switches them- selves Figure 6. External 10 nF capacitors, connected from the outputs to the bootstrap pins of each high-side switch provide typically less than 100 ns rise times allowing switch- ing frequencies up to 500 kHz. INTERNAL PROTECTION DIODES A major consideration when switching current through induc- tive loads is protection of the switching power devices from the large voltage transients that occur. Each of the four switches in the LMD18200 have a built-in protection diode to clamp transient voltages exceeding the positive supply or ground to a safe diode voltage drop across the switch. The reverse recovery characteristics of these diodes, once the transient has subsided, is important. These diodes must come out of conduction quickly and the power switches must be able to conduct the additional reverse recovery current of the diodes. The reverse recovery time of the diodes protect- ing the sourcing power devices is typically only 70 ns with a reverse recovery current of 1A when tested with a full 6A of forward current through the diode. For the sinking devices the recovery time is typically 100 ns with 4A of reverse cur- rent under the same conditions. Typical Applications FIXED OFF-TIME CONTROL This circuit controls the current through the motor by apply- ing an average voltage equal to zero to the motor terminals for a fixed period of time, whenever the current through the motor exceeds the commanded current. This action causes the motor current to vary slightly about an externally con- trolled average level. The duration of the Off-period is ad- justed by the resistor and capacitor combination of the LM555. In this circuit the Sign/Magnitude mode of operation is implemented (see Types of PWM Signals). DS010568-6 FIGURE 5. Internal Charge Pump Circuitry DS010568-7 FIGURE 6. Bootstrap Circuitry www.national.com 8 |
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