LMD18200_13 Datasheet, PDF (11/19 Pages) Texas Instruments – LMD18200 3A, 55V H-Bridge

LMD18200
www.ti.com
SNVS091E – MAY 2004 – REVISED FEBRUARY 2011
CURRENT LIMITING
Current limiting protection circuitry has been incorporated into the design of the LMD18200. With any power
device it is important to consider the effects of the substantial surge currents through the device that may occur
as a result of shorted loads. The protection circuitry monitors this increase in current (the threshold is set to
approximately 10 Amps) and shuts off the power device as quickly as possible in the event of an overload
condition. In a typical motor driving application the most common overload faults are caused by shorted motor
windings and locked rotors. Under these conditions the inductance of the motor (as well as any series inductance
in the V_CC supply line) serves to reduce the magnitude of a current surge to a safe level for the LMD18200. Once
the device is shut down, the control circuitry will periodically try to turn the power device back on. This feature
allows the immediate return to normal operation in the event that the fault condition has been removed. While the
fault remains however, the device will cycle in and out of thermal shutdown. This can create voltage transients on
the V_CC supply line and therefore proper supply bypassing techniques are required.
The most severe condition for any power device is a direct, hard-wired (“screwdriver”) long term short from an
output to ground. This condition can generate a surge of current through the power device on the order of 15
Amps and require the die and package to dissipate up to 500 Watts of power for the short time required for the
protection circuitry 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 voltage. As shown in Figure 14, 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 continuously 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.
Figure 14. 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 switching action is accomplished by the power switches themselves Figure 15. 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.
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