MC33198 データシートの表示（PDF） - Motorola => Freescale
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MC33198 Datasheet PDF : 11 Pages
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MC33198
Pin 1 is connected to the MOSFET source pin. Two
different cases should be considered, MOSFET on or off :
When MOSFET is on and working in normal conditions,
the Vds should be less than the voltage developed at pin 2.
So, the C2 comparator output is low and the status is high. No
current will flow through the pin 8 capacitor.
If the MOSFET encounters an over-load or if the load is shor-
ted to Gnd, the voltage at the source will cross pin 2 voltage
and go below this pin 2 voltage, thereby turning on the C2
comparator. The comparator will pull the status pin 6 low and
will enable the charge of the pin 8 capacitor. When the vol-
tage at the capacitor reaches 5.5V, the C3 comparator will
switch off the MOSFET by disabling the charge pump and
the 110µA current source. The MOSFET gate will be dis-
charged only by the 10µA current source. The MOSFET is
latched off and can be turned back on again by switching
input pin 7 to 0 and back to 1.
When MOSFET is off, we have the same scheme.
In normal condition the load should pull the source voltage
to Gnd, thus C2 comparator output is high and status pin
pulled low. If the load is shorted to V bat for instance, source pin
will be higher that pin 2, C2 output comparator is low and
status pin is high. This is summarized in the following table :
T a b l e 1 •Status Functionality
Input Condition
Source
Load
Voltage
C1
Output
Comp
Fault
Pin6
Timer
Pin8
Low
Normal
< Vpin2
High
Low
Low
Low
Short circuit > Vpin 2
Low
to V bat
High
Charge
by 10µA
source
High Normal
> Vpin 2
Low
High
Low
High
Short to
Gnd
or Over load
< Vpin2
High
Low
Charge
by 10µA
source
Operation With Inductive Loads
The device can drive MOSFET in inductive loads
switching applications. In this case, a 1kΩ resistor should be
connected between source of the MOSFET and device pin 1.
The resistor will limit the current flowing into pin 1 and prevent
MC33198D from damage during switching off of the inductive
load. The gate voltage is internally clamped at - Vbe (0.6V
typical), then the Vds is limited to Vbat + Vbe + Vgson to
prevent excessive power dissipation in the MOSFET. The
load voltage is limited to Vbe + Vgson and allows reasonable
discharge current.
Switching On
The switching on of the MOSFET is ensured by the
internal charge pump. The charge pump response time
versus the MOSFET gate capacitance is shown in table
Dynamic Electrical Characteristics table .
Switching Off
Here two cases have to be discussed ; the normal
switching off of the MOSFET and the switching off under fault
condition. The normal switching off is done by internal pull
down current sources. The value is 110µA and is in fact
composed of two current sources in parallel ; a 100µA and a
10µA source. The 10µA current is always connected to the
gate pin 4 as shown in device block diagram.
The 100µA source can be disabled. This is the case when
the MOSFET is switched off under fault conditions. The
device will disable the 100µA current source and the
MOSFET gate will be discharged only by a 10µA current. The
time required to switch off the MOSFET will be much longer in
this case and will result in a lower over-voltage at the
MOSFET, especially when the device drives high inductive
loads.
Off State Operation without Vcc Connection
When pin 7 is in low state the MOSFET is off. If Vbat is
present, the gate voltage is discharged by the 110µA current
source. In the case of Vbat disconnection, a self sustaining
5µA pull down current source is incorporated in the device to
ensure that the MOSFET gate capacitor is discharged and
tied below 0.9V. In case of Vcc disconnection, input pin 7 has
no effect on gate voltage which is maintained below 0.9V and
in this case, status pin is high. Low leakage current at pin 2
(10µA max) allows operation with MOSFET and MC33198D
pin 2 permanently connected to battery. Vcc and other
functions can be switched off from the main battery line. See
figure 15.
PWM Operation
Since the MC33198D charge pump can deliver a high
current, the MOSFET gate can be charged fast enough to
allow PWM operations. The maximum PWM frequency is
dependent on the MOSFET itself and mainly its gate to source
capacitor value. Depending on the PWM frequency, the
switching off time can be long, compared to the on-switching
time response. This is due to the 110µA gate discharge
current. To improve this parameter, a resistor can be added in
parallel with the gate of the MOSFET. See figures 13 and 14
below.
Figure13.
Vbat
Vbat
4
1K
1
Rg
LOAD
3
5V
INPUT SIGNAL PIN7
0V
Vcc + 15V typ
Figure14.
Vgate WITHOUT Rgate
Vgate WITH Rgate
0V
Toff
Toff
MC33198
MOTOROLA
8
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