Datasheet

LMD18201

SNVS092D –APRIL 1998–REVISED APRIL 2013

www.ti.com

CURRENT LIMITING

Current limiting protection circuitry has been incorporated into the design of the LMD18201. 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 the current through the upper transistors and shuts

off the power device as quickly as possible in the event of an overload condition (the threshold is set to

approximately 10A). 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

supply line) serves to reduce the magnitude of a current surge to a safe level for the

LMD18201. 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 once 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

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 500W 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

supply pin (pin 6) with 1 square inch of copper on the PC board.

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 12), an internal capacitor is alternately switched to ground and charged to

about 14V, then switched to V

thereby providing a gate drive voltage greater than V

. 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 12. Internal Charge Pump Circuitry

For higher switching frequencies, the LMD18201 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 13). 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.

Product Folder Links: LMD18201