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LMD18200 Datasheet - Page 5

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Application Information
(Continued)
It is normally necessary to bypass the supply rail with a high
quality capacitor(s) connected as close as possible to the
V
Power Supply (Pin 6) and GROUND (Pin 7) A 1 F high-
S
frequency ceramic capacitor is recommended Care should
be taken to limit the transients on the supply pin below the
Absolute Maximum Rating of the device When operating
the chip at supply voltages above 40V a voltage suppressor
(transorb) such as P6KE62A is recommended from supply
to ground Typically the ceramic capacitor can be eliminated
in the presence of the voltage suppressor Note that when
driving high load currents a greater amount of supply bypass
capacitance (in general at least 100
F per Amp of load
current) is required to absorb the recirculating currents of
the inductive loads
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 in-
crease 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
supply line) serves to reduce
CC
the magnitude of a current surge to a safe level for the
LMD18200 Once the device is shut down the control cir-
cuitry will periodically try to turn the power device back on
This feature allows the immediate return to normal opera-
tion 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
supply line and therefore proper supply bypass-
CC
ing 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 destruc-
tive particularly at higher operating voltages (
FIGURE 1 Internal Charge Pump Circuitry
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 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 1 an internal capacitor is alternate-
ly 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 s which is suitable
for operating frequencies up to 1 kHz
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 2) 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 in-
ductive 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 cur-
rent of the diodes The reverse recovery time of the diodes
protecting 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
30V) so
devices the recovery time is typically 100 ns with 4A of re-
l
verse current under the same conditions
TL H 10568 – 6
FIGURE 2 Bootstrap Circuitry
5
CC
TL H 10568 – 7
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