LMD18200_05 Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – 3A, 55V H-Bridge

Application Information (Continued)
to shut off the power device. This energy can be destructive,
particularly at higher operating voltages (>30V) so some
precautions are in order. Proper heat sink design is essential
and it is normally necessary to heat sink the V_CC supply pin
(pin 6) with 1 square inch of copper on the PCB.
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 volt-
age. 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
operating frequencies up to 1 kHz.
01056806
FIGURE 5. Internal Charge Pump Circuitry
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.
01056807
FIGURE 6. Bootstrap Circuitry
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
current 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).
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