LMD18245_06 Datasheet, PDF (12/21 Pages) National Semiconductor (TI) – 3A, 55V DMOS Full-Bridge Motor Driver

Applications Information
POWER SUPPLY BYPASSING
Step changes in current drawn from the power supply occur
repeatedly during normal operation and may cause large
voltage spikes across inductance in the power supply line.
Care must be taken to limit voltage spikes at V_CC to less than
the 60V Absolute Maximum Rating. At a change in the
direction of the load current, the initial load current tends to
raise the voltage at the power supply rail (Figure 3) again.
Current transients caused by the reverse recovery of the
clamp diodes tend to pull down the voltage at the power
supply rail.
Bypassing the power supply line at V_CC is required to protect
the device and minimize the adverse effects of normal op-
eration on the power supply rail. Using both a 1 µF high
frequency ceramic capacitor and a large-value aluminum
electrolytic capacitor is highly recommended. A value of
100 µF per ampere of load current usually suffices for the
aluminum electrolytic capacitor. Both capacitors should have
short leads and be located within one half inch of V_CC.
OVERCURRENT PROTECTION
If the forward current in either source switch exceeds a 12A
threshold, internal circuitry disables both source switches,
forcing a rapid decay of the fault current (Figure 6). Approxi-
mately 3 µs after the fault current reaches zero, the device
restarts. Automatic restart allows an immediate return to
normal operation once the fault condition has been removed.
If the fault persists, the device will begin cycling into and out
of thermal shutdown. Switching large fault currents may
cause potentially destructive voltage spikes across induc-
tance in the power supply line; therefore, the power supply
line must be properly bypassed at V_CC for the motor driver to
survive an extended overcurrent fault.
In the case of a locked rotor, the inductance of the winding
tends to limit the rate of change of the fault current to a value
easily handled by the protection circuitry. In the case of a low
inductance short from either output to ground or between
outputs, the fault current could surge past the 12A shutdown
threshold, forcing the device to dissipate a substantial
amount of power for the brief period required to disable the
source switches. Because the fault power must be dissi-
pated by only one source switch, a short from output to
ground represents the worst case fault. Any overcurrent fault
is potentially destructive, especially while operating with high
supply voltages (≥30V), so precautions are in order. Sinking
V_CC for heat with 1 square inch of 1 ounce copper on the
printed circuit board is highly recommended. The sink
switches are not internally protected against shorts to V_CC.
THERMAL SHUTDOWN
Internal circuitry senses the junction temperature near the
power bridge and disables the bridge if the junction tempera-
ture exceeds about 155˚C. When the junction temperature
cools past the shutdown threshold (lowered by a slight hys-
teresis), the device automatically restarts.
UNDERVOLTAGE LOCKOUT
Internal circuitry disables the power bridge if the power
supply voltage drops below a rough threshold between 8V
and 5V. Should the power supply voltage then exceed the
threshold, the device automatically restarts.
Trace: Fault Current at 5A/div
Horizontal: 20 µs/div
01187815
FIGURE 6. Fault Current with V_CC = 30V, OUT 1 Shorted to OUT 2, and CS OUT Grounded
www.national.com
12