LMD18200T Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – 3A, 55V H-Bridge

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 internal
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 sup-
ply thereby providing a gate drive voltage greater than V
supply. This switching action is controlled by a continuously
running internal 300 kHz oscillator. The rise time of this drive
voltage is typically 20 μs which is suitable for operating fre-
quencies up to 1 kHz.
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FIGURE 5. Internal Charge Pump Circuitry
For higher switching frequencies, the LMD18200 provides for
the use of external bootstrap capacitors. The bootstrap prin-
ciple 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 de-
vice resulting in much faster rise times. The switching action
is accomplished by the power switches themselves 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 switching 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 protecting
the sourcing power devices is typically only 70 ns with a re-
verse 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 current
under the same conditions.
Typical Applications
FIXED OFF-TIME CONTROL
This circuit controls the current through the motor by applying
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 mo-
tor current to vary slightly about an externally controlled av-
erage level. The duration of the Off-period is adjusted 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).
FIGURE 6. Bootstrap Circuitry
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